Electrons dynamics control by shaping femtosecond laser pulses in micro/nanofabrication: modeling, method, measurement and application

Lan Jiang; An-Dong Wang; Bo Li; Tian-Hong Cui; Yong-Feng Lu

doi:10.1038/lsa.2017.134

Electrons dynamics control by shaping femtosecond laser pulses in micro/nanofabrication: modeling, method, measurement and application

Jiang, Lan; Wang, An-Dong; Li, Bo; Cui, Tian-Hong; Lu, Yong-Feng 2017-08-30 00:00:00 Light: Science & Applications (2018) 7, 17134; doi:10.1038/lsa.2017.134 OPEN Ofﬁcial journal of the CIOMP 2047-7538/18 www.nature.com/lsa REVIEW ARTICLE Electrons dynamics control by shaping femtosecond laser pulses in micro/nanofabrication: modeling, method, measurement and application 1 1 1 2 3 Lan Jiang , An-Dong Wang ,BoLi , Tian-Hong Cui and Yong-Feng Lu During femtosecond laser fabrication, photons are mainly absorbed by electrons, and the subsequent energy transfer from elec- trons to ions is of picosecond order. Hence, lattice motion is negligible within the femtosecond pulse duration, whereas femtose- cond photon-electron interactions dominate the entire fabrication process. Therefore, femtosecond laser fabrication must be improved by controlling localized transient electron dynamics, which poses a challenge for measuring and controlling at the elec- tron level during fabrication processes. Pump-probe spectroscopy presents a viable solution, which can be used to observe elec- tron dynamics during a chemical reaction. In fact, femtosecond pulse durations are shorter than many physical/chemical characteristic times, which permits manipulating, adjusting, or interfering with electron dynamics. Hence, we proposed to control localized transient electron dynamics by temporally or spatially shaping femtosecond pulses, and further to modify localized tran- sient materials properties, and then to adjust material phase change, and eventually to implement a novel fabrication method. This review covers our progresses over the past decade regarding electrons dynamics control (EDC) by shaping femtosecond laser pulses in micro/nanomanufacturing: (1) Theoretical models were developed to prove EDC feasibility and reveal its mechanisms; (2) on the basis of the theoretical predictions, many experiments are conducted to validate our EDC-based femtosecond laser fabrication method. Seven examples are reported, which proves that the proposed method can signiﬁcantly improve fabrication precision, quality, throughput and repeatability and effectively control micro/nanoscale structures; (3) a multiscale measurement system was proposed and developed to study the fundamentals of EDC from the femtosecond scale to the nanosecond scale and to the millisecond scale; and (4) As an example of practical applications, our method was employed to fabricate some key struc- tures in one of the 16 Chinese National S&T Major Projects, for which electron dynamics were measured using our multiscale measurement system. Light: Science & Applications (2018) 7, 17134; doi:10.1038/lsa.2017.134; published online 9 February 2018 Keywords: electrons dynamics control; femtosecond laser; micro/nano fabrication; pulse shaping INTRODUCTION the initial defects of the target materials. Hence, femtosecond laser Because of their ultrashort irradiation periods and ultrahigh inten- ablation is deterministic and reproducible , and almost any material 8–10 sities, femtosecond laser pulses in some aspects fundamentally change can be machined using a femtosecond laser, including metals , 11,12 13,14 15 the laser-material interactions mechanisms compared with long laser semiconductors , dielectrics , polymers , two-dimensional 16–20 21 22,23 pulses, which has created wide-range and exciting new possibilities in materials , ultrahard materials and biological tissues .In 1–3 micro/nanoscale fabrication . addition, ionization mechanism can be adjusted by changing femto- The ultrahigh intensity makes femtosecond laser-material interac- second laser energy and its temporal/spatial distribution to control 4,5 24 tions a strongly nonlinear process . Linear ionization generally laser-material interactions . dominates in free electron generations for continuous or long-pulse The ultrashort irradiation period of a femtosecond laser also makes laser processing of wide bandgap materials . The intensities of femtosecond laser-material interactions a strongly nonequilibrium 12 − 2 3,25 femtosecond lasers can easily exceed 10 Wcm , thus nonlinear process . The duration of a femtosecond laser pulse is much shorter 12 − 2 − 10 − 12 ionization mechanism such as avalanche ionization (~10 Wcm ), than the electron-lattice energy relaxation time (10 − 10 s). 13 14 − 2 multiphoton ionization (~10 –10 Wcm ), and tunnel ionization Therefore, laser energy absorption is completed before the lattice 15 − 2 (410 Wcm ; Ref. 3), can occur in the femtosecond laser fabrica- changes, resulting in a signiﬁcantly nonequilibrium state between tion processes. The nonlinear ionizations are almost independent on electrons and lattices. Hydrodynamic motion and heat conduction 1 2 Laser Micro/Nano-Fabrication Laboratory, School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China; Department of Mechanical Engineering, University of Minnesota, Minneapolis, MN 55455, USA and Department of Electrical Engineering, University of Nebraska-Lincoln, Lincoln, NE 68588-0511, USA Correspondence: L Jiang, Email: [email protected] Received 12 April 2017; revised 27 August 2017; accepted 28 August 2017; accepted article preview online 30 August 2017 The accepted article preview was available with the details: Light: Science & Applications (2018) 7, e17134; doi: 10.1038/lsa.2017.134. Electrons dynamics control LJiang et al Table 1 The estimations of optical and thermal properties in classical and improved two-temperature models Property Classical TTM Improved TTM ∂hi ε Electron heat capacity C = γT C ¼ n e e e e ∂T e pﬃﬃﬃﬃﬃ 3=2 3 m ðk T Þ e B pﬃﬃﬃ t ¼ 3mek e 2 2 2pðZ Þ n e lnL Electron relaxation time t ¼ e e 2 p n k T e B e fg 1 þ exp½ mðÞ n ; T =k T F e B 1=2 T 1 2 Electron heat conductivity k ¼ k k ¼ n t C e eq e e e T 3 e 2 2 2 2 o ðÞ n t o ðÞ n t e e p e p e εðÞ t; r ; z ¼ 1 þ i DR DT e 2 2 2 Optical property D 1þo t o 1þo t e ðÞ e ðÞ DR ðÞ DT max e max þ DεðÞ contribution of the interband transition ab c 14 000 14 000 Electron temperature Electron temperature Experiments Lattice temperature Lattice temperature 12 000 12 000 Existing model 0.8 Proposed model 10 000 10 000 0.6 8000 8000 6000 6000 0.4 4000 4000 0.2 2000 2000 0 0 0 2 3 4 5 –5 0 5 1015202530 –5 0 5 10 15 20 25 30 10 10 10 10 Pulse duration (fs) Time (ps) Time (ps) − 2 Figure 1 The calculated electron and phonon temperatures of 200 nm gold ﬁlm irradiated by a 140 fs, 1053 nm pulse at 0.2 J cm by (a) the classical model and (b) the improved model. (c) The predicted damage threshold ﬂuences of 200 nm gold ﬁlm processed by a 1053 nm laser at different pulse. Reproduced from Ref. 117 (with the permission of SPIE). through lattices are negligible during the femtosecond pulse duration. from experiments may contribute to theoretical comprehension and Thus, recast, thermal damage (microcracks) and heat-affected-zone prediction. are greatly reduced . Because of the nonequilibrium laser-material Many innovations in measurement technologies could also be interactions, including phase change and material removal, are converted to new methods in fabrication technologies. In the pump- 26,27 essentially determined by laser-electron interactions . Hence, fem- probe experiments, the observation of electron dynamics is achieved by analyzing the probe pulse disturbed by electrons. By increasing the tosecond laser fabrication must be improved by controlling electron energy of the probe laser, the disturbing process can be transformed dynamics during the fabrication process, which poses a challenge for measuring and controlling at the electron level during fabrication into certain control if it is purposely designed. Much more complex processes. pulses other than merely two pulses can be designed to control the Pump-probe spectroscopy, which has been widely used in ﬁelds electron dynamics. Recent developments of optical devices substan- 28–33 34–37 38–40 58–63 including physics ,chemistry and materials science , tially enhanced the capability to shape laser pulses. The amplitude 64,65 66,67 68,69 presents a viable solution for detecting electron dynamics. The basic /phase /polarization /frequency can be easily manipulated in mechanism involves splitting one femtosecond pulse into two sub- both temporal and spatial domains. By temporally/spatially shaping pulses. One sub-pulse is the pump pulse and the other is the probe the femtosecond laser, the local transient electron dynamics can be pulse, and the time delay between them can be precisely controlled by precisely controlled. Many studies have demonstrated that properties adjusting the optical path difference. On the basis of this principle, of atoms and molecules can be controlled by temporal/spatially shaped 41 70–72 Zewail and other pioneers developed the discipline of femtochem- ultrafast laser pulses . For example, using a shaped ultrafast laser 73–75 istry, which elucidates ultrafast chemical reaction processes at the pulse, atoms can be selectively ionized ; spin states can be 42–44 76–80 electron level . Their research demonstrates how chemical bonds dynamically controlled ; molecular ground state rotational 45,46 81,82 change and electrons transfer during chemical reactions . Recently, dynamics and vibrational modes can be manipulated ;chemical 83,84 attosecond laser pulses have been used to probe electron dynamics in reactions can be controlled and X-ray line emission from plasma 47–49 85–87 greater detail , and even the electronic charge distribution within under the femtosecond pulse can be signiﬁcantly enhanced .In the 34,50,51 5,88–90 molecules has been detected . In addition, electron dynamics ﬁeld of ultrafast laser fabrication, Herman and Ilday et al during laser-material interactions have been captured through the demonstrated the heat accumulation effect can be controlled by pump-probe technique. For instance, researchers have imaged electron harnessing high-repetition-rate burst trains of ultrafast laser pulses, excitation, electron spatiotemporal distribution and electron decay on achieving signiﬁcant beneﬁts compared with unshaped pulses. Xu 52–54 24,91,92 the femtosecond scale . Furthermore, the electron collision time et al revealed the conceptual signiﬁcance of controlling coherent and laser-induced plasma lifetime can be determined using pump- phonon dynamics for controlling phase change by speciﬁcally designed 55–57 3 probe measurement . The information on electrons measured ultrafast pulse trains with different pulse delays. Gamaly reported Light: Science & Applications doi:10.1038/lsa.2017.134 Temperature (K) Temperature (K) –2 Threshold fluence (J cm ) Electrons dynamics control LJiang et al a c 0.2 0.2 0.1 0.1 0.0 0.0 –0.1 –0.1 –0.2 0.8 –0.2 010 200 10 20 0.6 Time (fs) Time (fs) 0.4 0.2 0.0 0.2 0.1 0.0 –1.1e–2 1.5e–2 –1.1e–2 1.5e–2 –9e–4 9e–4 –0.1 Two pulses 1:1 Two pulses 4:1 Difference –0.2 400nm+800nm 800nm+400nm 800nm+800nm 0 510010 20 30 0 25 50 75 100 125025 50 75 100 1250 25 50 75 100 125 0 20 40 Time (fs) Time (fs) Time (fs) Time (fs) Time (fs) Time (fs) Figure 2 Schemes of electrons dynamics adjusted by shaped femtosecond laser pulses. Electric ﬁelds of the applied laser pulse and time-dependent excited 99 99 electrons of diamond with different (a) pulse delays and (b) sub-pulse numbers .(c)Electric ﬁelds of the applied laser pulse (top panel) and the electron density change of diamond from that in the ground state after laser termination with different pulse energy ratios .(d) Time-dependent electron temperature of fused silica with different pulse dual-wavelengths . Reproduced from Ref. 99 (with the permission of IOP) and Ref. 111 (with the permission of AIP publishing). controlling over the ablation rate and phase state of laser-produced domains, we are able to control photon–electron interactions, and plume by using temporal/sptial pulse shaping. Sheppard and Wilson then to control the localized transient electron dynamics (including produced the Bessel beam with an annular lens, and Marcinkevičius electron density, temperature and excited-state distribution), and 94 95–97 et al and Courvoisier et al obtained some micro/nanostructures further to modify localized transient materials properties, and then by spatially shaping ultrafast laser pulses. to adjust material phase change, and ﬁnally to implement the novel Although various results have been obtained to control the laser- fabrication method. We have devoted our efforts on using EDC to material interactions, electron dynamics have not been extensively improve femtosecond micro/nano fabrications for more than a studied or deliberately controlled, which is vital for laser micro/nano decade. This review summarizes our recent progresses based on fabrication. In femtosecond laser micro/nano fabrication process, EDC by shaping femtosecond pulses in micro/nanofabrication: photons are mainly absorbed by electrons, and the subsequent energy transfer from electrons to ions is of picosecond order 1) Theoretical modeling for EDC feasibility: four models were − 12 − 10 (10 − 10 s). Hence, lattice motion is negligible within the developed, which consists of the ab initio calculations for electron femtosecond pulse duration, and femtosecond photon − electron 98–107 dynamics , a revised molecular dynamics simulation for phase interactions are the only factor to be considered, which dominates 108,109 110–115 change , a plasma model for ionization processes and − 10 − 3 the subsequent fabrication processes (10 –10 s). Therefore, 116–118 an improved two-temperature model for energy transport . femtosecond laser fabrication must be improved by controlling 2) Experiments for EDC validation: By shaping a femtosecond pulse localized transient electron dynamics. The key challenge is to measure in temporal or spatial domains, the photon-electron interactions and control at the electron level during fabrication processes. Pump- can be controlled to adjust the localized transient electron probe spectroscopy presents a viable solution, which can be used to dynamics to implement the novel fabrication methods. Various observe electron dynamics during a chemical reaction. In the pump- experiments were conducted to validate the effectiveness of EDC. probe experiments, the observation of electron dynamics is achieved Basedon EDC, weproposedto: by analyzing the probe pulse disturbed by electrons. By increasing the energy of the probe laser, the disturbing process can be transformed ▪ control the localized transient electron density to induce reso- into certain control if it is purposely designed. Therefore, we propose nance absorption, by which microchannel processing efﬁciency the core idea of electrons dynamics control (EDC; Supplementary was increased by 56 times and the maximum aspect ratio was Movie 1, Supplementary Information): By shaping the amplitude, 119,120 extended by three times ; phase and polarization of femtosecond pulses in temporal and spatial doi:10.1038/lsa.2017.134 Light: Science & Applications Excitation energy Electric amplitude (a.u.) Electric amplitude (a.u.) Electric amplitude (a.u.) (eV per atom) Electric temperature (10 K) Electric amplitude (a.u.) 50 –25 –50 –25 –50 –25 –50 –25 –50 –25 –50 –25 –50 –25 –50 –25 –50 –25 –50 –25 –50 Depth (nm) Depth (nm) Depth (nm) Time (fs) Depth (nm) Time (fs) Time (fs) Time (fs) Time (fs) Depth (nm) Electrons dynamics control LJiang et al a b c d e Pulse delay: 0 fs Pulse delay: 100 fs Pulse delay: 200 fs Pulse delay: 300 fs Pulse delay: 400 fs 1.0 0.8 0.6 0.4 0.2 0.0 fg h i j Pulse delay: 0 fs Pulse delay: 100 fs Pulse delay: 200 fs Pulse delay: 300 fs Pulse delay: 400 fs 75.0 50.0 25.0 0.0 Figure 3 Simulation of materials properties adjusted by varying pulse delay within a femtosecond laser pulse train. (a–e)Reﬂectivity of the material surface and (f–j) the corresponding peak laser intensity distribution at different pulse delays. ▪ modify free electron density and the corresponding photon Furthermore, we revealed the forming mechanisms of the high- absorption efﬁciency, then modulate material properties, by quality and high aspect-ratio microholes using the multiscale time- which laser-assisted chemical etching rate was enhanced by 37 resolved measurement system . times ; ▪ adjust electron generation on fabricated material surfaces, through which the periods, orientations and structures of the MODELING FOR EDC FEASIBILITY 122,123 surface ripples can be effectively adjusted ; Modeling method ▪ control electron density and its distribution, through which we Femtosecond photon–electron interactions dominate the entire none- obtained controllable micro/nano hierarchical structures on quilibrium and nonlinear laser fabrication processes, including the material surfaces and enhancement factors of surface-enhanced absorption of laser energy by electrons, energy transfer from electrons 9 136–138 Raman scattering (SERS) up to 1.1 × 10 (Refs. 124–126); to lattices, plasma generation, phase change and material ▪ control electron density distribution using temporally shaped modiﬁcation. These interactions range from nanometers to milli- femtosecond laser pulse to modify chemical and physical proper- meters spatially and from femtoseconds to microseconds temporally. ties of materials, by which polymorphous Au-MoS hybrids were Although a growing body of experimental observations exists, a prepared ; comprehensive model remains unavailable. Such a model is essential ▪ adjust the phase change mechanism by changing photon-electron for revealing the fundamental science underlying ultrafast laser- interactions, which reduced the recast layer thickness by 60% material interactions. According to the applicable time and space (Ref. 128); scales of each involved process, four fundamental models were used: ▪ manipulate electron density distribution using spatially modu- lated femtosecond laser pulse, by which deep subwavelength 1) The ab initio model based on the time-dependent density func- (B1/14 of the laser wavelength) and high conductivity (B1/4 of tional theory (TDDFT) was employed to understand initial non- the bulk gold) nanowires were fabricated in the open air . linear laser radiation absorption through photon-electron 98,99 interactions . 3) Measurements for EDC fundamentals: A multiscale measurement In TDDFT, the fundamental variable is not the many-body wave system (from femtosecond scale to millisecond scale) was proposed function in quantum mechanics, but the electronic density. This and developed for understanding the electron dynamics during time-dependent electron density is determined by solving an femtosecond laser ablation. It comprises a pump-probe shadow- auxiliary set of noninteracting Kohn-Sham equations, where the 130,131 graph imaging technique , time-resolved plasma laser ﬁelds are treated as time-dependent spatial uniform external 132,133 photography , laser-induced breakdown spectroscopy vector potentials. Describing the motion of the electrons in the (LIBS) and commercialized fast imaging device (CCD). We system, the time-dependent Kohn-Sham equation for single reveal the multiple time scale fundamentals during femtosecond particle orbitals is laser-material interactions, including the femtosecond-scale propa- gation of a laser pulse, picosecond-scale generation/evolution of ! ! ! laser-induced plasma, nanosecond-scale plasma ejection/expansion, i_ c r ; t ¼ H r ; t c r ; t ð1Þ KS ∂t and millisecond-scale hole formation. 4) Applications for the EDC method: Our proposed method was used to fabricate some key structures. Using spatially-shaped femtose- ! ! cond pulses, we optimized electron density distribution in plasma nr ; t ¼ c r ; t ð2Þ at the focus point, and then manipulated plasma expansion and phase change, which was used to drill microholes with a diameter ! ! of 1.6 μm and an aspect ratio of 330:1 (Refs. 132,135). wherenr ; t is the electron density and H r ; t is the Kohn- KS Light: Science & Applications doi:10.1038/lsa.2017.134 1.0 1.0 0.8 1.0 0.8 1.0 0.6 0.8 1.0 0.6 0.8 0.4 0.6 0.8 0.4 0.6 0.2 0.4 0.2 0.6 0 0.4 0.2 0.4 0.2 0.2 80 120 60 100 40 80 Radius (μm) Radius (μm) Radius (μm) Radius (μm) Radius (μm) Radius (μm) Radius (μm) Radius (μm) Radius (μm) Radius (μm) Laser intenstiy Reflectivity Laser intenstiy Reflectivity Laser intenstiy Reflectivity Laser intenstiy Reflectivity Laser intenstiy Reflectivity Electrons dynamics control LJiang et al –2 –2 Laser fluence=0.28 J cm 1pulse 15ps Laser fluence=0.28 J cm 20pulse 15ps ae 80 80 40 40 0 0 –40 –40 –80 –80 700 350 0 –350 –700 –1050 –1400 –1750 –2100 –2450 –2800 700 350 0 –350 –700 –1050 –1400 –1750 –2100 –2450 –2800 Z (Å) Z (Å) –2 –2 Laser fluence=0.28 J cm 1pulse 50ps Laser fluence=0.28 J cm 20pulse 50ps b f 80 80 40 40 0 0 –40 –40 –80 –80 700 350 0 –350 –700 –1050 –1400 –1750 –2100 –2450 –2800 700 350 0 –350 –700 –1050 –1400 –1750 –2100 –2450 –2800 Z (Å) Z (Å) –2 –2 Laser fluence=0.28 J cm 1pulse 92ps Laser fluence=0.28 J cm 20pulse 92ps c g 80 80 40 40 0 0 –40 –40 –80 –80 700 350 0 –350 –700 –1050 –1400 –1750 –2100 –2450 –2800 700 350 0 –350 –700 –1050 –1400 –1750 –2100 –2450 –2800 Z (Å) Z (Å) –2 –2 Laser fluence=0.28 J cm 1pulse 200ps Laser fluence=0.28 J cm 20pulse 200ps d h 80 80 40 40 –40 –40 –80 –80 700 350 0 –350 –700 –1050 –1400 –1750 –2100 –2450 –2800 700 350 0 –350 –700 –1050 –1400 –1750 –2100 –2450 –2800 Z (Å) Z (Å) ik m n 1.0×10 4 4 1.2×10 1.2×10 3 15 15 ps 4 4 8.0×10 1.0×10 1.0×10 20 ps 50 ps 3 8.0×10 3 10 8.0×10 6.0×10 92 ps 15 ps 3 3 200 ps 20 ps 6.0×10 6.0×10 4.0×10 50 ps 92 ps 4.0×10 4.0×10 Binodal Binodal 3 200 ps 2.0×10 Spinodal Spinodal 3 3 2.0×10 2.0×10 Dynamic film surface z=–7 nm 0.0 –5 0.0 0.0 3 3 3 3 3 3 700 500 300 100 –100 –300 –500 –700 700 500 300 100 –100 –300 –500 –700 00 3.0×10 6.0×10 9.0×10 3.0×10 6.0×10 9.0×10 –3 –3 Density (kg m ) Density (kg m ) Z (Å) Z (Å) jl o p 4 20 1.0×10 4 4 1.2×10 1.2×10 3 4 15 4 8.0×10 15 ps 1.0×10 1.0×10 20 ps 3 3 50 ps 8.0×10 8.0×10 3 10 6.0×10 15 ps 92 ps 3 3 20 ps 200 ps 6.0×10 6.0×10 4.0×10 50 ps 3 3 92 ps 4.0×10 4.0×10 Binodal Binodal 200 ps 0 Spinodal 2.0×10 3 Spinodal 3 2.0×10 2.0×10 z=–7nm Dynamic film surface 0.0 –5 0.0 0.0 3 3 3 3 3 3 700 500 300 100 –100 –300 –500 –700 700 500 300 100 –100 –300 –500 –700 0 3.0×10 6.0×10 9.0×10 0 3.0×10 6.0×10 9.0×10 –3 –3 Z (Å) Z (Å) Density (kg m ) Density (kg m ) Figure 4 Schemes of phase change controlled by varying the pulse delay in a fs pulse train . Snapshots of nickel thin ﬁlms irradiated by femtosecond laser − 2 (a–d) single pulse and (e–h) 20 pulse trains with the total ﬂuence of 0.28 J cm ,where X is in the direction of Ni (100) surface and Z is in the direction of laser irradiance. Lattice temperature and stress distributions at different times for (i and k) the single pulse and (j and l) the 20 pulse trains. Time evolution of the system in the ρ-T plane for different regions for (m and n) the single pulse and (o and p) the 20 pulse trains. Arrows indicate the time evolution. Reproduced from Ref. 108 (with the permission of AIP publishing). Sham Hamiltonian given by 2) The molecular dynamics model was employed to reveal the phase change resulting from electron-ion interactions . In the mole- cular dynamics model, Newtonian equations of motion of a set of ! ! ! 1 e H r ; t ¼ p þ A ðÞ t þ V r ; t KS tot ion 2m c N particles are solved to describe the phase change process. ! ð3Þ n r ;t 2 0 þe dr þ V r ; t XC ! ! r r d r ∂U m ¼ f ; f ¼ ð5Þ i t t dt ∂t ! ! where V r ; t is the electron-ion potential and V r ; t is ion XC where r is the coordinate of the ith atom, and f is the force the exchange-correlation (XC) potential.The time evolution of the i i acting on the ith atom that is usually derived from potential wave function over a short period, Δt, can be approximately energy U(r) (Ref. 139): calculated as follows: hi q q ^ Dt ^ Dt 1 2 gðÞ 1r=r g=21ðÞ r=r 0 0 ! iH ðÞ tþDt iH ðÞ t ! KS KS 2 2 UrðÞ ¼ þ D e 2e ð6Þ c r ; t þ Dt ¼ e e c r ; t ð4Þ 4pεr doi:10.1038/lsa.2017.134 Light: Science & Applications Temperature (K) Temperature (K) X (Å) X (Å) X (Å) X (Å) Stress (GPa) Stress (GPa) Temperature (K) Temperature (K) X (Å) X (Å) X (Å) X (Å) Temperature (K) Temperature (K) Electrons dynamics control LJiang et al ac –2 5.0 J cm –2 4.0 J cm 75 fs 50 fs 25 fs AB CD 0 fs Energy ratio 1:2 –2 5.0 J cm –2 4.0 J cm 40 20 0 20 40 Radius (μm) 40 AB 50 CD Energy ratio 1:1 –2 5.0 J cm –2 4.0 J cm CD AB 200 80 400nm+800nm 800nm+400nm 800nm+800nm Energy ratio 2:1 1.0 0.5 0.0 0.5 1.0 1.0 0.5 0.0 0.5 1.0 Radius (μm) Radius (μm) 110–112 Figure 5 Schemes of ablation crater shape controlled by shaped femtosecond laser pulse .(a) Ablation crater shapes created by femtosecond pulse − 2 trains consisting of double pulses with different pulse delays at a total ﬂuence of 5 J cm and central wavelength of 780 nm. (b) Ablation crater shapes − 2 created by femtosecond laser pulse trains with different wavelength composition at the total ﬂuence of 5.0 J cm and the pulse delay of 50 fs. (c)Ablation crater shapes created by 800 nm femtosecond pulse trains consisting of double pulses with three different energy ratios at the pulse delay of 50 fs. Reproduced from Ref. 110 (with the permission of IOP), 111 (with the permission of AIP publishing) and 112 (with the permission of Springer). 3) We proposed the plasma model with quantum treatment to investigate plasma generation and changes (ionization and recom- 110,113–115 (μs~ms) bination) . 114,115 In the plasma model , the free electron density distribution in dielectrics under a femtosecond laser pulse is obtained by solving the Fokker − Planck Equation: ∂nðÞ t; r; z (ps~fs) e ¼ a ItðÞ ; r; z nðÞ t; r; z þ dðÞ ItðÞ ; r; z t e N ∂t nðÞ t; r; z ð7Þ where t is time, r is the distance to the Gaussian beam axis, z is the (ps~fs) depth from the surface of the bulk material, n (t,r,z) is the free electron density, τ is the free electron recombination time, α is the impact ionization constant, and δ is the cross-section of N- photon absorption. (ps~fs) The optical properties of ionized dielectrics are calculated using the Drude model. The spatial and temporal dependence of the complex dielectric function for the plasma is expressed as: 2 2 2 2 o ðÞ n t o ðÞ n t e e p e p e εðÞ t; r; z ¼ 1 þ i ð8Þ 2 2 2 2 1 þ o t o 1 þ o t e e pﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ where o ðÞ n ¼ nðÞ t; r; z e =m ε is the plasma frequency and p e e e 0 τ is the free electron relaxation time, calculated as, follows by applying formulas derived from the Boltzmann transport Equation: Figure 6 Temporally shaping femtosecond pulses: (a) a conventional pﬃﬃﬃﬃﬃﬃ 3=2 femtosecond pulse is temporally shaped into a pulse train; (b) the number of 3 mðÞ k TðÞ t; r; z e B e tðÞ t; r; z ¼ pﬃﬃﬃ sub-pulses within a train can be controlled; (c) the delay between sub-pulses e 2 2pðÞ Z nðÞ t; r; z e lnL ð9Þ can be controlled; (d) the energy ratio of sub-pulses can be controlled. ðÞ 1 þ expðÞ mðÞ n ; T =k TðÞ t; r; z F e e B e 1=2 where Z* is the ionization state, m is the electron mass, k is the e B Light: Science & Applications doi:10.1038/lsa.2017.134 Depth (nm) Depth (nm) Depth (nm) Depth (nm) Depth (nm) Electrons dynamics control LJiang et al CCD Reflector lens Grating Lens SLM Lens Grating Reflector Reflector Fs laser Polarizer Pulse shaper Shutter Dichroic mirror Half wave Attenuator plate Objective lens Sample Computer control Hexapod stage Halogen light Figure 7 Schematic of the experimental setup for temporal shaping of femtosecond pulses. Reproduced from Ref. 141(with the permission of Springer). a b c d D1: 185.6441 um 1 μm 1 μm 1 μm D1: 12.28527 um Figure 8 Holes machined using (a) ﬁve 355-nm nanosecond pulses (duration of 30 ns, energy of 0.16 μJ) ,(b) ﬁve 800-nm femtosecond pulses (duration 119 119 of 120 fs, energy of 0.12 μJ) ,(c) ﬁve femtosecond-nanosecond pulse pairs ,(d) an 800 nm femtosecond laser double-pulse train (duration of 50 fs, energy of 20 μJ) . Reproduced from Ref. 119 (with the permission of OSA) and Ref. 120 (with the permission of OSA). Boltzmann constant, F denotes the Fermi-Dirac integrals, μ is transport equation for dense plasmas; and (3) the free-electron heating 1/2 the chemical potential and ln Λ is the Coulomb logarithm.In and interband transition were taken into account using a modiﬁed addition, the wave-particle duality of photon is also considered to Drude model as Equations (8) and (9) to calculate the reﬂectivity and predict the formation mechanism of laser-induced periodic surface absorption coefﬁcient. Table 1 compares the estimates of optical and structures . thermal properties between classical and improved two-temperature 4) We proposed the improved two-temperature model with full-run models. Figure 1 demonstrates the differences in (a, b) electron and quantum treatment to predict the electron and lattice temperature phonon temperatures between the classical approach and the 116,117 distributions . In the improved two-temperature model, improved model. The improved model signiﬁcantly increases the electron and lattice temperatures are given by prediction precisions of the damage thresholds compared with the classical model, as shown in Figure 1c . ∂T C ðÞ T ¼ ∇ ½ kðÞ T ∇T GTðÞ T þ SzðÞ ; t ð10Þ e e e e e e l The four theoretical models are suitable for systems with different ∂t temporal and spatial scales and must be employed in combination to compensate for each other’s limitations. In our improved two- ∂T CðÞ T ¼ GTðÞ T ð11Þ l l e l temperature model, the electron density is assumed to be constant. ∂t Consequently, the model is not suitable when the bound electrons in where T is the electron temperature, T is the lattice temperature, e l non-metals are substantially ionized by a femtosecond laser pulse. The C is the electron heat capacity, C is the lattice heat capacity, k is e l e plasma model is proposed to consider the ionization process in the electron heat conductivity, G is the electron-lattice coupling nonmetals, but material ablation is assumed to commence when the factor, and S is the laser source term. free electron density reaches the critical density without the con- sideration of phase changes, for example, melting, vaporization, The two-temperature model was improved by us as follows :(1) Coulomb explosion and electrostatic repulsion. Moreover, the mole- using the Fermi distribution, the heat capacity of the free electrons was cular dynamics can be used to identify the relative roles of different calculated; (2) free-electron relaxation time and electron conductivity were determined using a quantum model derived from the Boltzmann phase change mechanisms. However, the interatomic potential doi:10.1038/lsa.2017.134 Light: Science & Applications Electrons dynamics control LJiang et al 0 0 (ns~ms) 0 fs 50 fs a b cd e k l –3 –100 –100 n (cm ) 2.000E+21 –200 –200 2 μm 2 μm 5 μm 10 μm Single pulse 20 μm t = 0 t = 20 t = 45 t = 90 t = 150 e e e e e –300 –300 1.500E+21 (fs~ps) f gh ij –400 –400 –1.5 –1.0 –0.5 0.0 0.5 1.0 1.5 –1.5 –1.0 –0.5 0.0 0.5 1.0 1.5 1.000E+21 0 0 100 fs 350 fs Double pulse m n 2 μm 2 μm 5 μm 10 μm 20 μm t = 0 t = 20 t = 45 t = 90 t = 150 –100 –100 =350 fs e e e e e 5.000E+20 –200 –200 0.000 –300 –300 –400 –400 –1.5 –1.0 –0.5 0.0 0.5 1.0 1.5 –1.5 –1.0 –0.5 0.0 0.5 1.0 1.5 Radius (μm) op 20 20 Original beam Original beam Transmitted beam Transmitted beam O 16 16 Si 12 12 8 8 t u 4 4 D D 1 2 3.0 0 0 0 100 200 300 400 500 600 0 100 200 300 400 500 600 Time (fs) Time (fs) 2.6 qr 20 20 Original beam Original beam 2.2 Transmitted beam Double pulse 16 Transmitted beam 16 12 12 1.8 8 8 Single pulse 1.4 200 400 600 800 1000 1200 0 500 1000 1500 2000 0 0 –1 0 100 200 300 400 500 600 0 100 200 300 400 500 600 Wave number (cm ) Pulse delay (fs) Time (fs) Time (fs) Figure 9 Morphology evolution of the sample surface exposed by (a–e) a femtosecond laser single pulse; (f–j) femtosecond laser double pulses at different stages of the etching process, the pulse delay is 350 fs. The SEM images have varying scale bars. (k–n) Simulation of the free electron density distributions and (o–r) center laser intensity distributions in fused silica irradiated by femtosecond double pulses at different pulse delays. (s) Schematic diagram of the manufacturing and Si-O bond structure. (t) Normalized Raman spectra of modiﬁed regions irradiated using femtosecond laser single and double pulses (the pulse delay is 350 fs) in fused silica. Dashed lines below the D2 peaks are baselines used in the peak area measurement in u. Inset is the schematic diagram of 4- and 3-membered ring structures. (u) Percent area of the total reduced Raman spectrum under the D2 line versus different pulse delays. The − 2 femtosecond laser with wavelength of 800 nm, duaration of 50 fs and repetition rate up to 1 KHz. The laser ﬂuence is ﬁxed at 9.46 J cm in all experiments and the energy distribution ratio is 1:1. Reproduced from Ref. 121 (with the permission of NPG). remains constant in the molecular dynamics model, even though it in validated the concept of EDC and predicted its potentials in ultrafast 88–107 laser micro/nanofabrication . fact changes dramatically during strong ionizations. The ab initio method is therefore used to describe a system with no parameteriza- Simulation results tion, which facilitates the investigation of processes whose mechanisms In this section, we report several representative simulation results. As are not fully understood. In a typical ab initio method, the potential shown in Figure 2, the time-dependent electrons dynamics in energy for a particular electronic state is deﬁned by the electronic materials irradiated by temporally shaped femtosecond laser pulse Schrödinger equation. However, the computational demand of ab train have been calculated based on TDDFT and the improved two- initio calculation is substantial, and impact ionization is not considered temperature model . The simulation results indicated that increasing because the theory is based on orbitals that interact only through the pulse delay from 10 to 30 fs markedly increased the number of excited mean ﬁeld. The exact time-dependent xc potential and the functional electrons and absorbed energy (Figure 2a). However, the excitation for physical observables are still being studied. energy decreased as the sub-pulse number per train increased from 1 On the basis of the aforementioned factors, during femtosecond to 4 (Figure 2b). Furthermore, the shaped pulse energy and laser-material interactions, the target material can be conceptually wavelength distribution were also determined to be key parameters divided into three systems: the electron system, atom or molecule for changing the electron density and temperature (Figure 2c and 2d). system and plasma system outside the bulk material. The ab initio The results indicated that localized transient electrons dynamics, model can be applied to sub-nanometers sampling areas to determine including the number of excited electrons, excitation energy, electron the ionization mechanisms and multiphoton absorption cross density distribution, and electron temperature, can be controlled by section . The revised molecular dynamics can be employed to shaping femtosecond pulses (pulse delay, sub-pulse number, pulse determine the phase change mechanisms in areas of tens to hundreds energy ratio and dual-wavelength distribution). of nanometers within an electric ﬁeld formed by the plasma system We theoretically demonstrated that localized transient materials outside the bulk material, which is calculated using our plasma properties can be effectively modulated through localized transient model . The plasma model can be used to describe the electron EDC by temporally shaping femtosecond laser pulses (Figure 3). The system for photon absorption and plasma generation (ionization) and reﬂectivity and corresponding peak laser intensity distribution were 110,114 recombination before the lattice actually changes .Our improved completely changed by varying the pulse delays. Moreover, the two-temperature model can be employed to calculate energy transport electron density was adjusted by varying the pulse delay, and 116,117 through electron-phonon interactions .Using themodels,we reﬂectivity decreased as the pulse delay increased because of the free Light: Science & Applications doi:10.1038/lsa.2017.134 Intensity (arbitrary units) Percent area of the reduced spectrum –1 under D =605 cm line Depth (nm) 13 –2 13 –2 Laser intensity (10 W cm ) Laser intensity (10 W cm ) 13 –2 13 –2 Laser intensity (10 W cm ) Laser intensity (10 W cm ) Electrons dynamics control LJiang et al 1.05 F 1.16 F th th Δt = 0 fs Δt = 50 fs Δt = 0 fs Δt = 100 fs Δt = 300 fs a b c eg 2 μm2 μm 1 μm 4 μm 1 μm 1 μm Δt = 50 fs Δt = 200 fs Δt = 400 fs df h 1 μm 1 μm 1 μm 1 μm 4 μm 1 μm 1.8F 2F 0.6F 1.2F ij k th l th th th 2 μm 2 μm 2 μm 2 μm AFM profile 2F m 2F n 2F o th th th –0.3 –0.4 LSFL(//E ) HSFL(⊥E ) –0.5 –0.6 –0.7 0.0 0.5 1.0 1.5 2.0 2 μm 2 μm 2 μm Distance (μm) Figure 10 Control of period, orientation and topology of the LIPSS on the surface of fused silica via symmetrically shaped femtosecond pulses. (a–h)LSFL changes to HSFL with different orientation under certain pulse ﬂuences and pulse delays on fused silica .(i–o) SEM images of representative HSFL (i–k), LSFL (m–o) and double-grating structure (l) on fused silica for single-pulse trains (Ns= 1), double-pulse trains (Ns= 2), triple-pulse trains (Ns= 3) and quadruple-pulse trains (Ns= 4), respectively . Reproduced from Ref. 122 (with the permission of OSA) and Ref. 123 (with the permission of OSA). electron density was lower. Thus, the original laser intensity distribu- slightly above the volumetric phase change threshold ﬂuence of single tion was considerably reshaped, providing substantial control over the pulse ablation. Therefore, the results demonstrated that the phase energy absorption process. The results demonstrated that by control- change mechanism can be controlled by carefully designing the pulse ling localized transient electron dynamics through shaping femtose- train parameters. cond pulses, the localized transient material properties, such as the The ablation shape modulated by designed femtosecond pulse trains 110–112 on fused silica was validated using our plasma model . The crater reﬂectivity and absorption coefﬁcient, can be modiﬁed. Meanwhile, we studied the phase change mechanisms in materials shapes of the fused silica and the spacing of the subwavelength ripples such as nickel thin ﬁlms under irradiation by femtosecond laser pulse differed greatly among sub-pulses with different delay times, wave- trains by using molecular dynamics simulations and the two- lengths and energy distributions. The ablation depths were approxi- temperature model . The theoretical simulation revealed that more mately 170, 145, 102 and 91 nm for pulse delays of 0, 25, 50 and 75 fs, and smaller high-quality uniform nanoparticles can be obtained on the respectively (Figure 5a). The ablation depths were approximately 186, nickel thin ﬁlm by femtosecond laser pulse trains compared with 43 and 139 nm, and the ablation radii were approximately 860, 630 conventional pulses (Figure 4a–4h). The use of pulse train reduced the and 410 nm for pulse trains of 400+800, 800+400 and 800+800 nm, − 2 electron temperature and electron thermal conductivity dramatically respectively (Figure 5b). For 4.0 J cm , the depths of the microholes because of the lower intensity of the sub-pulses or higher transient were 77, 84 and 71 nm (depth from the surface to the position of line surface temperatures, which left the absorbed energies deposited AB), and the corresponding radii of the ablation craters were 250, 160 mainly within the nanoscale layers of the dynamic ﬁlm surface and 270 nm for the energy ratios of 1:2, 1:1 and 2:1, respectively; For − 2 (Figure 4i and 4j). By designing the pulse train, smaller ﬁlm 5.0 J cm , the depths of the microholes were 80, 95 and 70 nm compressive stresses and tensile stresses can be obtained, which (depth from the surface to the position of line CD), and the reduced microcracks (Figure 4k and 4l). Furthermore, a transition corresponding radii of the ablation craters were 430, 410 and from phase explosion to critical point phase separation (Figure 4m– 440 nm for the energy ratios of 1:2, 1:1 and 2:1, respectively 4p) enabled small uniform nanoparticle generation. The modulated (Figure 5c). Calculations based on the plasma model revealed that phase change mechanism was considered the main factor for the the time-dependent free electron density differed substantially among morphology control. Both the compressive and tensile stresses can be laser pulses with different parameters. Changing the ionized electron reduced by the pulse trains, leading to the critical point phase density distributions signiﬁcantly modiﬁed the optical and thermal separation within the uppermost ﬁlms and no liquid-vapor phase properties of the material. This interaction process greatly altered separation within the subsurface ﬁlms when total laser ﬂuence was ablation shape and subwavelength ripple. The results indicated that by doi:10.1038/lsa.2017.134 Light: Science & Applications Ns=1 Ns=1 Ns=2 Ns=2 Ns=3 Ns=3 Ns=4 Depth (μm) Electrons dynamics control LJiang et al 1.0 μm 1.0 μm a b f Δt Δt ~ ms Y 2nd 1st 2nd 1st hv + + – Ag +H O Ag+H +OH Ripple Fs laser Fs laser pulse train Nanopillar array AgNO Z Water 3 Y Si Si Y STEP TWO STEP ONE Reference 0 fs Δt =0 fs 400 fs Δt =800 fs 600 fs 800 fs 500 nm 500 nm 18 1000 fs ik 60 8 0 fs 600 fs 800 fs 1000 fs Reference 0 40 80 120 160 180 240 600 1000 1400 1800 –1 Partical diameter (nm) Raman shift (cm ) 8 500 nm 500 nm h 6 –1 Raman shift (cm ) 0 fs 400 fs 600 fs 800 fs 1000 fs 610 9.2E+06 6.2E+07 4.5E+07 2.2E+07 1.9E+07 1310 7.1E+06 2.7E+07 1.9E+07 8.4E+07 1.2E+07 1360 1.1E+07 4.8E+07 3.3E+07 1.6E+08 2.0E+07 600 800 1000 1200 1400 1600 1800 1510 1.1E+07 5.2E+07 3.5E+07 1.6E+08 2.3E+07 –1 Raman shift (cm ) 1650 8.0E+06 4.7E+07 3.0E+07 1.5E+08 1.8E+07 Figure 11 (a–e) One-step method fabrication of controllable SERS substrates .(a and b) SEM images of the silicon irradiated at pulse delays of 0 fs, and 800 fs, respectively. (c) Size distribution of silver nanoparticles in a (black), and b (red). (d) SERS signals of R6G molecules on the as-prepared substrates at various pulse delay. (e) Enhancement factors with different pulse delays. (f–k) Two-step method fabrication of controllable SERS substrate . f Schematic diagram of the SERRS substrate fabrication process. (g–j) SEM images of silicon substrates irradiated at pulse delays of g 0 fs, and i 1000 fs in deionized water, (h) 0 fs and (j) 1000 fs in 10-mM silver nitrate solution. (k) SERS spectrum of substrates fabricated at different pulse delays. Reproduced from Ref. 125 (with the permission of OSA), and Ref. 126 (with the permission of OSA). controlling femtosecond pulses trains, the ﬁnal material modiﬁcation us to further modify the chemical and physical properties of the can be improved. materials . By spatially shaping a femtosecond laser, a metal The model simulations validate theoretical feasibility of EDC, nanowire with a super sub-diffraction-limit precision (1/14th of the providing a theoretical prediction and guide for practical experiments wavelength) can be achieved . and applications. Furthermore, our models and subsequent multiscale measurements can form a mutually-supporting system. According to Femtosecond laser temporal pulse shaping the theoretical prediction, a novel fabrication method based on During femtosecond laser fabrication, most of the photon energy is temporally and spatially shaped pulse trains was proposed. Localized initially absorbed by electrons. However, conventional femtose- cond laser pulses are separated by a time scale that ranges from transient electron dynamics and the corresponding material properties microseconds to milliseconds (Figure 6), which is much longer can be actively controlled based on the proposed method. We discuss than the time scale of electron-lattice coupling (typically a few several pieces of experimental evidences in detail in the following sections. picoseconds to tens of picoseconds). Temporal pulse shaping enables sub-pulse generation with a pulse delay shorter than the EXPERIMENTS: EDC-BASED NOVEL FABRICATION METHOD characteristic time scale of electron-lattice coupling so that we can To validate the proposed method of EDC-based fabrication method control femtosecond laser photon–electron–phonon interactions. through temporal and spatial shaping of femtosecond laser pulses, As Figure 6a shows, a conventional femtosecond laser pulse can be 119–128 many experiments have been conducted . By designing tempo- split into several sub-pulses with a delay, which is called the pulse rally shaped pulse trains, we can control the localized transient train. The separation between the sub-pulses occurs in the time electron density to induce resonance absorption; thus, high efﬁciency scale—tens of femtoseconds to several picoseconds—which is 119,120 fabrication can be achieved . We can adjust the phase change generally similar to the characteristic time scale of electron- mechanism from thermal phase change to nonthermal phase change; lattice coupling. Before the photon energy that is absorbed by subsequently, high-quality fabrication can be performed .In addi- the electrons transfers to lattice, the subsequent sub-pulses con- tion, we can modify the free electron density, the corresponding tinuously interact with the materials so that controlling ultrafast photon-absorption efﬁciency and the material properties, enabling us photon-electron-phonon interactions is possible. Moreover, a to further control the chemical reaction . We can adjust the electron conventional femtosecond laser single pulse can be shaped into density and its distribution so that the periods, orientations and almost any arbitrary pulse shapes; for example, (1) a pulse can be 122,123,140 structures of the surface ripples can be effectively modulated split into a pulse train with different numbers of sub-pulses and a high sensitivity of SERS substrate (controllable micro/nano (Figure 6b); (2) the delay between the sub-pulses can be controlled 124–125 hierarchical structures on materials’ surfaces) can be achieved . (Figure 6c); and (3) the energy ratio of the sub-pulses can be Furthermore, we can control the electron density distribution controlled (Figure 6d). By shaping a femtosecond laser pulse in to induce surface chemical reduction activity in materials, enabling temporal domains to obtain a speciﬁc pulse shape, we can control Light: Science & Applications doi:10.1038/lsa.2017.134 Number Intensity (×10 a.u.) Intensity (×10 a.u.) Electrons dynamics control LJiang et al ab c d (ns~ms) 500 nm Single pulse 500 nm 500 nm 500 nm t = 0 t = 20 s t = 1 min t = 30 min r r r r ef g h (fs~ps) Double pulses ( = 5ps) 500 nm 500 nm 500 nm 500 nm t = 0 t = 20 s t = 1 min t = 30 min r r r r ij k MoS modified by temporally MoS modified by conventional 2 S 2p 2 1/2 S 2p 1/2 shaped femtosecond pulses femtosecond pulses Mo/S=1:4.28 Mo/S=1:2.85 Unbound Unbound sulfur sulfur (27.78%) (27.74%) S 2p 3/2 S 2p 3/2 4+ 4+ MoS film 172 171 170 169 168 167 166 165 164 163 162 161 160 159 158 172 171 170 169 168 167 166 165 164 163 162 161 160 159 158 Binding energy (eV) Binding energy (eV) l m n 3+ Au Shape-controlled Dangling bond Au NPs Unsaturated Saturated S atoms 500 nm Fs laser treated MoS S atoms Mo S 2 Figure 12 Morphology evolution of gold NPs reduced on MoS surfaces irradiated by femtosecond (a–d) single and (e–h) double pulses, at different stages of the reduction process, where tr represents the chemical reduction time, the pulse delay is 5 ps. (i) Schematic diagram of the manufacturing and Mo-S bond structure. XPS S 2p spectra of modiﬁed regions irradiated by femtosecond laser (j) double and (k) single pulses on MoS , where the percentage value represents the content of unbound sulfur and the atomic ratio represents the relative atomic concentration ratio of Mo and S atoms. (l) AFM image and (m) atomic scale schematic of the laser-broken micro/nano MoS debris. (n) Mechanism of chemical reduction of gold cations on laser-treated MoS 2 2 (Ref. 127). Reproduced from Ref. 127 (with the permission of ACS). the photon-electron interactions, which allow us to control the Temporal shaping of the femtosecond laser pulses can be achieved localized transient electron dynamics, including electron density, by using a commercial 4f-conﬁguration-based pulse shaper (BSI temperature, excited state distribution and further modify loca- MIIPS BOX 640, Biophotonic Solutions, Inc., East Lansing, MI, lized transient material properties, adjust material phase changes USA), which allows us to split each conventional single pulse into a and ultimately implement the novel fabrication methods. pulse train and control the number of sub-pulses, delay between sub- pulses and energy ratio of the sub-pulses within a pulse train. The Experimental setup for temporal pulse shaping.The schematic of irradiation time (that is, number of pulse bursts) is precisely controlled the experimental setup for temporal shaping of femtosecond laser by using an electromechanical shutter. The sample is mounted on a pulses is shown in Figure 7 (Ref. 141). An ampliﬁed Ti: sapphire laser computer-controlled, six-axis translation stage (M-840.5DG, PI, Inc., system (Spectra Physics Inc., Santa Clara, CA, USA) is used to generate Karlsruhe, Germany) with a positioning resolution of 1 μm. The entire 35 fs (full width at half maximum) linearly polarized laser pulses on a fabrication process can be observed by using a charge-coupled device central wavelength of 800 nm with a repetition rate of 1 kHz. In (CCD) camera along with a white-light source irradiate on the sample addition, the pulse energy can be up to 3.5 W and can be continuously surface. adjusted by combining a half-wave plate with a polarizer. Pulse energy can also be reduced to the desired values according to speciﬁc High-efﬁciency fabrication by temporal pulse shaping based on experimental conditions by using a neutral density (ND) ﬁlters. EDC. By designing femtosecond laser pulse trains on the basis of doi:10.1038/lsa.2017.134 Light: Science & Applications Intensity (a.u.) Intensity (a.u.) Electrons dynamics control LJiang et al 0.40 WS CCD 0.36 BS 0.32 SLM 0.28 DM 0.24 0.20 Phase pattern 0 200 400 600 800 Lens Pulse delay (fs) bc 0 3 μm μm –200 –200 Translation stage Figure 15 Schematic diagram of the experimental setup. WS: white light nm nm source; BS: beam splitter; DM: dichroic mirror; L1, L2: two convex lenses consisting of a 4f relay system; L3: convex lens; Inset: section of the de samples and the focusing laser in the yz plane . Reproduced from Ref. 129 (with the permission of Wiley). Figure 8a, when ﬁve 355 nm nanosecond laser pulses irradiated on fused silica, no apparent damage was observed because the photon energy (B3.5 eV) of the 355 nm wavelength is lower than the bandgap (B8.9–9.3 eV) of fused silica, resulting in little absorption of laser energy .For ﬁve femtosecond laser pulses, a shallow hole was machined (Figure 8b), because of the low-efﬁcient 800 nm (B1.55 eV) photon absorption through multiphoton ionization in Figure 13 (a) Dependence between the recast ratio (recast area/ablation wide bandgap dielectrics. In Figure 8c, femtosecond-nanosecond dual- area) and pulse delay on fused silica fabrication using femtosecond laser beam laser manufacturing revealed that a much higher fabrication pulse trains consisting of two identical sub-pulses with an identical total ﬂuence. AFM proﬁles of the structures of the fused silica fabrication using efﬁciency (that is, a 50.7-fold enhancement in material removal (b) a conventional single pulse and (c) femtosecond laser pulse train with a volume) was obtained. This enhancement was attributed to the high pulse delay of 300 fs. SEM images of the structures on fused silica free electron density generated by the femtosecond laser pulses, which fabrication using a femtosecond laser pulse train with different energy ratios leading to the signiﬁcantly-increased absorption of the nanosecond between the two sub-pulses: (d)1:1 and(e) 2:1. The femtosecond laser laser pulses energy. However, the femtosecond-nanosecond dual-beam with wavelength of 800 nm, duration of 35 fs and repetition rate up to − 2 system was complex, and the quality of the as-fabricated holes could 1 KHz. The total ﬂuence of the pulse trains in all experiments is 5 J cm ; not be guaranteed. To date, temporally shaped femtosecond laser the scale bar in d and e is 500 nm. Reproduced from Ref. 128 (with the permission of OSA). double-pulse train was used to manufacture high-quality microholes with high-efﬁciency . Through temporal pulse shaping, free electron density can be adjusted to be around the critical point, at which the laser frequency equal to the plasma frequency, nearly optimizing to the resonance absorption so that the fabrication efﬁciency is enhanced 56 folds and the aspect-ratio is enhanced 3 fold (Figure 8d). Chemical etching controlled by temporal pulse shaping based on EDC. By designing femtosecond pulse trains on the basis of EDC, Flat-top beam Vortex beam the free electron density and corresponding photon-absorption efﬁciency can be modiﬁed. Thus, the material properties can be modiﬁed to improve the etching rate of fused silica (Figure 9). Compared with conventional femtosecond laser pulses, femtosecond Gaussian beam laser double-pulse trains achieved a 37-fold enhancement in the laser- assisted chemical etching rate. Simulations indicated that by optimiz- Bessel beam Airy beam ing the pulse delay between the two sub-pulses, the free-electron density can be modiﬁed, leading to the change of localized transient Figure 14 Shaping conventional Gaussian beam into different beam types. material properties, such as the physical properties (that is, reﬂectiv- ity), so that the laser ﬁeld was reshaped, then the free electron density EDC, we can control the localized transient electron density to induce distributions (Figure 9k–9n) and absorbed laser intensity distributions resonance absorption between laser and its generated plasma; thus, (Figure 9o–9r) can be controlled, contributing to the enhancement of 119,120 high efﬁciency fabrication can be achieved .As illustrated in the photon absorption efﬁciency and result in the modiﬁcation Light: Science & Applications doi:10.1038/lsa.2017.134 Recast ratio Electrons dynamics control LJiang et al ae Ablation area Metal nanowire 112 nm 190 nm 56 nm 1 μm 40 nm 20 nm 10 nm 0.0 0.5 1.0 1.5 2.0 2.5 Energy (keV) 5 μm cd f –200 –100 0 100 200 X (nm) (nm) Figure 16 Fabrication of nanowire by spatial pulse shaping. (a) Single spot fabricated by the shaped beam. (b) Scanning electron microscope (SEM) images of nanowire. (c and d) AFM images of the nanowire and its cross section. (e) EDXS measurements of the metal nanowire and the ablation area. (f) Five-ring patterns fabricated by the proposed methods . Reproduced from Ref. 129 (with the permission of Wiley). a b Electrode pads –2 220 J m –2 260 J m –2 –4 305 J m Resistor –2 345 J m –2 –8 390 J m –1000 –500 0 500 1000 Current (μA) Figure 17 Testing of the resistivity of the nanowire. (a) The SEM images of the nanowire and the electrode pads. (b) The volt-ampere characteristics of the nanowires. Reproduced from Ref. 129 (with the permission of Wiley). improvement in the irradiated zone. We also conducted micro-Raman which in turn improve the manufacturing efﬁciency. Overall, femto- spectroscopy to characterize the internal structure of the sample. As second laser temporal pulse shaping fabrication that is based on EDC Figure 9s–9u shows, by optimizing the pulse delays between the two represents a preliminary attempt to control the chemical reaction. sub-pulses, we can adjust the chemical properties (such as the Si-O bond structure) in irradiated material, which results in a higher Modulation of femtosecond laser-induced periodic surface structures based on EDC. On the basis of the aforementioned theory on EDC, number of 3- and 4-membered ring structures in double pulses we demonstrated that femtosecond laser-induced periodic surface modiﬁed regions than that in single pulse modiﬁed regions. These structures (LIPSS, also referred as ripples) can be deliberately results lead to the increases in the reactivity of the oxygen atoms, modulated by controlling the electron density and its distribution which contributes to a higher etching rate induced by femtosecond via designed femtosecond laser pulse trains. LIPSS have been studied laser double-pulse train. In short, by varying the pulse delay of pulse 142–144 extensively in various materials, including semiconductors , trains, we can control the localized transient free electron dynamics, 145 110,146,147 including bound electron ionization, free-electron density, tempera- metals and dielectrics , because of their promising 124,148–154 ture and excited state distribution, and further modify localized applications . The periodicity, orientation and structure are transient material properties, such as physical and chemical properties, the typical parameters in the study of ripples. According to its doi:10.1038/lsa.2017.134 Light: Science & Applications Height (mm) Voltage/mV Intensity/(Arb. Unt.) Fs laser system Electrons dynamics control LJiang et al Coulomb explosion Electrostatic ablation Plasma expansion Impact ionization Shockwave Radiation Electron heating Structure evolution Photoionization Vaporization/Melting Time scale (s) –15 –14 –13 –12 –11 –10 –9 –8 –7 –6 –5 –4 –3 –2 –1 0 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 Time-resolved plasma ICCD/CCD Femtosecond laser pump-probe photography and LIBS imagery Multipulse induced Plasma expansion Laser propagation Early plasma and plasma evolution shockwave expansion Ejected material composition Laser-induced free Multipulse induced elctrons evolution Energy deposition Plasma density and temperature structure evolution a b Beam splitter Half wave plate Fs laser CCD Mirror Shutter Polarizer Illuminator Shutter Half wave plate Mirror Delay Pump stage Polarizer Lens Lens Lens Objective filter BBO Filter Probe Z 20× X Y PMMA CCD PMMA c d Lens Lens ICCD CCD Illuminator Zoom lens Zoom lens Objective Objective PMMA PMMA Figure 18 Schematic of multiscale measurement of femtosecond laser drilling, including laser propagation and laser-induced material excitation, plasma and shockwave evolution and hole formation and so on. (a) Pump-probe shadowgraph imaging technique. (b) Laser-induced breakdown spectroscopy (LIBS). (c) Time-resolved plasma photography with gated intensiﬁed charge-coupled device (ICCD). (d) Industrial continuous imagery. 159,170–172 periodicity, LIPSS can be divided into low spatial frequency LIPSS state into a metallic state . Subsequently, at the interface (LSFL) and high spatial frequency LIPSS (HSFL). It is now widely between the metallic state surface and air, SPs can be excited by the accepted that the excitation and propagation of surface plasmon coupling between the surface electrons of the irradiated sample and 155–157 polaritons (SPPs) plays a crucial role in LSFL formation .The the incident ﬁeld when the real part of the dielectric function is less formation of LSFL is affected by the initial laser-SPPs interference and than − 1 (Ref. 173). The SPs are characterized by surface electro- 158,159 the subsequent grating-assisted SPPs-laser coupling effect .Up to magnetic waves so that the coupling ﬁeld is a superposition of the now, the formation mechanism of HSFL is still under investigation. incident ﬁeld and the SP ﬁeld. When the free electron density reaches Recently, Wang et al demonstrated that structure evolution of LSFL 21 − 3 the critical density (B1.74 × 10 cm for the wavelength of and HSFL is highly dependent on the localized effective laser ﬂuence, 800 nm), the SPs can be resonantly excited . The SPPs excitation which determines the instantaneous optical permittivity by the laser- and resonance can reshape the laser intensity distribution in the excited electrons creating an active plasma layer. In general, the material and affect the subsequent linear/nonlinear ionization process. 147,161 formation mechanisms include self-organization , second har- Thus, the transient free electron density and its distribution is the key 162–164 165 monic generation (SHG) third harmonic generation (THG) , factor that affects SP excitation and properties, and ultimately, the 166 167 168 excitation of SPPs ,split , Coulomb explosion and cavitation 110,147,173 corresponding ripple formation . Meanwhile, the increased instability and so on. When a femtosecond laser irradiates the localized electron density can be further affected by trapping, diffusion surface of dielectric/semiconductor materials, free electrons can be and recombination with a time scale of several hundred femtose- generated, leading to the formation of electron-hole plasma (surface conds. Therefore, the time delay within the picosecond timescale is plasma) with time scales shorter than the electron-phonon relaxation proposed to control the electron dynamics to modulate the electron time. The localized transient free electron density is rapidly increased density and distribution, thus to modulate the resulted ripple through linear and nonlinear (multiphoton and avalanche) ionization, structures. leading to the material transforming from a dielectric/semiconducting Light: Science & Applications doi:10.1038/lsa.2017.134 Electrons dynamics control LJiang et al Femtosecond-picosecond Picosecond-nanosecond Air probe 600 ps 4 ns 16 ns ab c d e fg h 1.0 SF 0.9 100 fs 300 fs 500 fs 700 fs 900 fs 0.8 Air 0.7 PMMA 60 μm 0.6 60 μm PMMA 0.5 Laser propagation and maetrial excitation Plasma and shockwave expansion Nanosecond-submillisecond Millisecond-second Air Air Air 200 μm 500 μm PMMA PMMA PMMA Air Air Air 0 ms 0.5 s 5 s 20 s 33 s 45 s 60 s PMMA PMMA PMMA Plasma images above the surface Plasma images in the hole Multiple pulse induced structure in second scale 200 μm 0 s 10 s 20 s 30 s 40 s 50 s 60 s 70 s 80 s 90 s 100 s 110 s 120 s Figure 19 Multiscale measurement results of deep-hole drilling process in PMMA. Multiscale measurement results of deep-hole drilling process in PMMA with 100 μJ pulse energy focused by plano-convex lens (f= 100 mm). The dynamics include femtosecond-picosecond electron excitation, picosecond– nanosecond plasma and shockwave evolution and multiple pulse-induced structure in second scale. Optimal EDC using suitably shaped temporal pulse trains thus orientation parallel to the laser polarization, were obtained by gives the possibility to modulate the LIPSS artiﬁcially, offering changing the pulse delay (Δt) and pulse ﬂuence (Figure 10 upper extended ﬂexibility in material processing. Studies show that for panel). Thus, three types of LIPSS under speciﬁc conditions can be femtosecond (fs) laser pulse train processing of materials, the pulse obtained. (1) LSFL with orientation parallel to the laser polarization delay between sub-pulses strongly impacts the formation of direction; (2) HSFL with orientation parallel to the laser polarization 7,176–179 176–178 nanostructures , especially the morphology of LIPSS . direction with low pulse ﬂuence; and (3) HSFL with orientation Here, we performed relevant experiments as examples on the surface perpendicular to the laser polarization direction at higher pulse 122,123 of dielectrics to illustrate the aforementioned mechanisms . ﬂuence. The experimental results indicate that: (1) at lower pulse Fused silica was used as a dielectric material in a case study on the ﬂuences, a transition from LSFL to HSFL occurred at a pulse delay of control of the LIPSS period, area and orientation. For conventional 50 fs with a decrease in area (Figure 10a and 10b,); (2) whereas, at femtosecond laser irradiation, only LSFL with an orientation parallel higher ﬂuences, LSFL were replaced by another type of HSFL with an to the laser polarization and HSFL with an orientation perpendicular orientation perpendicular to the laser polarization at Δt4100 fs to the laser polarization were obtained on fused silica depending on (Figure 10c–10h). The average periods of LSFL and HSFL were the laser ﬂuences (F) or pulse number (N). Nevertheless, compared 560± 8 nm and 255± 30 nm, respectively. with the conventional situation, both types of ripples with controllable During processing, the second sub-pulse of the doublepulse train periods, areas and orientations, especially the HSFL with an signiﬁcantly affects the free electron density and distribution generated doi:10.1038/lsa.2017.134 Light: Science & Applications Commercialized CCD imaging Plasma photography Pump and probe Electrons dynamics control LJiang et al 3 3 2 2 C swan (d Π -a Π ) CN (B Σ–X Σ) 2 g u SP DP @ 80 ps Δv =0 Δv =0 Na I Ca I Δv =+1 Ca II 2 2 CH (A Δ–X Π,Δv =0) Δv =–1 Δv =–1 380 400 420 440 460 480 500 520 540 560 580 Wavelength (nm) bc 75 8500 70 8000 65 7500 60 7000 55 6500 50 6000 45 5500 0.1 1 10 20 40 60 80 100 120 0.1 1 10 20 40 60 80 100 120 DP delay (ps) DP delay (ps) − 2 Figure 20 Typical spectra of PMMA plasma irradiated by a single pulse and a double pulse with the same total ﬂuence of 7.8 J cm . Reproduced from Ref. 134 (with the permission of SPIE). by the ﬁrst sub-pulse, thereby inﬂuencing the mechanism of LIPSS recombination, which is determined by the electron density. By formation and the surface morphology. Consequently, by controlling controlling the pulse delay, the electron occupation is adjusted , the pulse delay and pulse ﬂuence, we can control excited electron leading to the manipulation of electron density, thus facilitating production, distribution, motivation and the interaction between SHG and resulting in a 50% cut in LIPSS periods; surface plasmon (SP) and the incident laser, then to control the 3) The strong decrease in the ripple area was due to the electron periodicity, orientation and morphology of the LIPSS. decay. The free electrons in the conduction band excited by the ﬁrst sub-pulse relaxed and returned to the valence band during the 1) LSFL obtained here at a low pulse ﬂuence were oriented parallel to time interval between the two pulses, in terms of diffusion and 184–186 the laser polarization, which cannot be explained by SPs recombination , leading to the reduced electron density; 147,180 excitation . According to calculations based on the Sipe- therefore, the energy coupling by the second sub-pulse to the 180 53 Drude model and pump-probe results , femtosecond energy excited material decreased, leading to the decay of ablation, which deposition by the ﬁrst/previous pulses can occur at speciﬁc (LSFL) resulted in the reduced rippled area; spatial frequencies, reshaping the electron density distribution 4) The transition of HSFL at a higher pulse ﬂuence was mainly along the polarization direction, and then determines the forma- attributed to the periodic plasma enhancement of the incident laser tion of LSFL with an orientation parallel to the polarization ﬁeld, which is related to the excited free electron density. When the direction. Meanwhile, thermal effects also play a critical role in pulse delay was low (o100 fs), the effects of induced SPs on the subsequent material removals at the ﬂuences for LSFL incident laser were insufﬁcient due to the low absorbed intensity. 181–183 generation ; With pulse delays of 100–500 fs, however, SPs excitation can easily 2) The periods of LSFL and HSFL (with orientation parallel to the be achieved at the initial stage of the second sub-pulse due to the polarization direction) were close to the fundamental (λ/ accumulation of the ﬁrst sub-pulse (see modeling section n= 551 nm) and second-harmonic (λ/2n= 275 nm) wavelengths for details). The interaction between SPPs and the incident laser in fused silica with lower pulse ﬂuence. Thus, SHG plays a key role ﬁeld resulted in the periodic modulated intensity enhancement at in HSFL formation with an orientation parallel to the polarization the surface. The incubation effects with multiple bursts irradiation 155,156 direction at a lower pulse ﬂuence. SHG is a result of electron led to the evolution of the local intensity distribution along the Light: Science & Applications doi:10.1038/lsa.2017.134 Intensity (a.u.) 16 –3 Electron density (×10 cm ) Temperature (K) Electrons dynamics control LJiang et al 0 ps 0.2 ps 0.3 ps 0.4 ps 0.5 ps 0.6 ps 0.7 ps 0.8 ps 2 ps 6 ps 10 ps 40 ps 80 ps 120 ps 1mm –2 3.3 J cm –2 4.4 J cm –2 6.6 J cm –2 8.8 J cm –2 11 J cm bc 2.6 –2 –2 6.6 J cm 6.6 J cm –2 –2 8.8 J cm 8.8 J cm –2 2.4 –2 11 J cm 11 J cm 2.2 2.0 1.8 1.6 1.4 0 0.2 1 10 40 80 120 0 0.2 1 10 40 80 120 Interpulse delay (ps) Interpulse delay (ps) Figure 21 Characterizations of femtosecond laser double pulse induced plasma of fused silica (a) plasma images, (b) electron densities and (c)plasma temperature. electric ﬁeld direction , resulting in HSFL formation at a speciﬁc analysis of chemical and biological species because of its high 188,189 period with an orientation perpendicular to the electric ﬁeld sensitivity and ﬁngerprint-identiﬁcation features . Previous stu- direction. dies have demonstrated that surface morphologies (for example, 190,191 ripples, nanoparticles and nanopillars) play key roles in SERS More complicated morphology control of LIPSS on fused silica can enhancement in which the enhanced electromagnetic ﬁeld on surface be obtained by EDC via shaping the conventional femtosecond laser nanostructures induced by the localized surface plasmons resonance 191–193 pulses into symmetrical triple- and quadruple-pulse sequences, (LSPR) effect dominated . Consequently, tailoring the surface structures into different morphologies and sizes, in this aspect, is especially, double-grating structures and an HSFL period as small as signiﬁcant for tuning LSPR features to further improve the sensitivity 190nmwereobtained(seeFigure10i–10o) . In addition, the and push SERS devices into practical applications. As shown in geometric morphology modulation on Si can also be obtained by Figure 11d, the SERS intensity gradually reached a maximum when EDC via adjusting double pulse delay . Therefore, the aforemen- the pulse delay was increased from 0 to 800 fs, and then decreased tioned experimental studies demonstrate that by designing a femto- when the pulse delay was further increased to 1000 fs . Although the second laser pulse train, the electron dynamics can be controlled, that small changes (that is, the modulation of pulse delays) occurred in the is, electron density, distribution, thus to control the coupling between incident pulses, the obtained surface structures signiﬁcantly improved the SPP and incident laser, and ﬁnally guide the material response the signal intensity (Figure 11e). Compared with the conventional (LIPSS morphology) towards user-designed directions with various femtosecond laser ablation (Δt= 0 fs), the designed pulse train could morphologies (for example, periods, orientations, distributions and reduce more silver nanoparticles (Figure 11a and 11b) and lead to a geometric morphologies). more uniform distribution of the nanoparticles deposited on the Detection sensitivity improvement of SERS based on EDC.By subwavelength ripples (Figure 11c). Consequently, the SERS sensitivity designing femtosecond laser pulse trains on the basis of EDC, we was improved. can control the localized transient electron density and its distribution To further conﬁrm that we controlled the properties of SPs, a two- to modulate the properties of SPs, then promote the energy transfer to step experiment was conducted using a designed pulse train, as materials and control the surface structures and photochemical schematically shown in Figure 11f . The laser polarization and pulse reduction process; thus, high detection sensitivity SERS substrates delay were synergistically controlled. Compared with the ripples formed can be achieved. SERS has been recognized as the most promising by conventional femtosecond laser pulses (Figure 11g and 11h), regular trace analyte detection method for rapid and accurate label-free nanopillar arrays were generated by a double pulse train (i.e., Δt40fs) doi:10.1038/lsa.2017.134 Light: Science & Applications 16 3 Electron density (×10 cm ) Electron temperature ( °C) Electrons dynamics control LJiang et al M M Bessel beams ac 2 3 CCD2 0 1 0 0.5 –5 –5 Initial Bessel beams –10 0 500 1000 1500 –10 –5 05 10 DM y (μm) z (μm) PBS Axicon Gaussian beams 10 1 Micro-Bessel beams MO S2 HWP z 0 0.5 –5 –5 CCD1/ICCD L x 0 –10 0 0 500 1000 1500 –10 –5 05 10 Fs laser y M y (μm) z (μm) S1 b d Bessel beams Gaussian beams 1 μm 5 μm 5 μm 5 μm 3.5 2.5 5 μm 50 μm μm 10 μm 123456 μm Figure 22 (a) Schematic of the spatial shaping femtosecond laser pulses microdrilling setups; (b) Morphology images of microholes drilled with a single pulse Bessel beam and Gaussian beam, respectively; (c) Intensity distribution simulations of the Bessel beam and Gaussian beam; (d) The hollowness characterization of microholes drilled by single-pulse Bessel beam. Reproduced from Ref. 132 (with the permission of Springer). with the polarization direction rotated by 90°, as shown in Figure 11i on the basis of EDC, bound electrons can be ionized, chemical bonds and 11j. In the case of conventional femtosecond laser ablation between atoms can be interrupted, atoms can be selectively removed and material properties can be modiﬁed so that gold cations can be (Δt= 0), SP induced by the previous pulse of the subwavelength ripples would fast damp to its original state before the subsequent pulse spontaneously reduced on MoS surface , as shown in Figure 12. Compared with the conventional femtosecond laser pulses, the arrived because of the long-time interval (millisecond scale), leading to chemical reduction rate of gold cations on MoS surface modiﬁed the formation of large and non-uniformly distributed silver nanopar- by femtosecond double-pulse trains was signiﬁcantly enhanced (Figure ticles on conventional grating-like ripples. By contrast, because the 12a–12h). We conducted X-ray photoelectron spectroscopy (XPS) and pulse delay of a pulse train is shorter than the damping time of SP, the atomic force microscope (AFM) to characterize and analyze the properties of SPs would be signiﬁcantly controlled to change the energy element valence, atomic ratio and morphology of the sample, as transfer efﬁciency, which could contribute to the formation of showninFigure 12i–12m. When material was irradiated by the abundant small and uniformly distributed silver nanoparticles on the femtosecond laser pulses, electrons were excited from bonding to anti- nanopillar arrays. In addition, the larger enhancement of the incident bonding states (bound electrons were ionized), hence abundant laser electric ﬁeld on nanopillar arrays lead to generation of much more chemical bonds between atoms (such as Mo-S bonds) were instanta- silver nanoparticles, which resulted in a lager SERS signals (maximum neously weaken or even broken entirely, resulting in an integral enhancement factor up to 2.2 × 10 ). Overall, by designing a pulse train increase in the binding energy of S atoms and the appearance of on the basis of EDC, the electron density and its distribution induced unbound sulfur (unsaturated S atoms with dangling bonds). Mean- by laser irradiation can be controlled to modulate the properties of SPs, while, abundant Mo atoms were selectively removed, and the surface resulting in a more effective energy transfer and changes in the resulting lattice structure of the material was non-perfect and broken into a structures. These ﬁndings provide new insights regarding tuning LSPR large number of micro/nano debris terminating with unsaturated S features for related applications. atoms (edge active sites). These unsaturated-terminal S atoms induced on MoS can reduce gold cations to gold atoms (Figure 12n). Surface chemical reduction activity of MoS controlled by temporal 2 2 pulse shaping based on EDC. By designing femtosecond pulse trains Compared with conventional femtosecond laser pulses, more Mo Light: Science & Applications doi:10.1038/lsa.2017.134 r (μm) r (μm) x (μm) x (μm) Electrons dynamics control LJiang et al a c 0~5 ns 5~10 ns 10~15 ns 15~20 ns 20~25 ns 25~30 ns 30~35 ns 200 fs c1 400 fs 0.5 ns c2 c11 Air 600 fs c3 PMMA 800 fs 1 ns c12 c4 1 ps c5 100 μm 1st pulse 1.2 ps 3.6 ns c6 c13 1.4 ps c7 0~5 ns 5~10 ns 10~15 ns 15~20 ns 20~25 ns 25~30 ns 30~35 ns 1.6 ps 8.6 ns c8 c14 10 ps Air c9 13.6 ns PMMA 100 ps c15 c10 The final hole fs laser 18.6 ns c16 100 μm c17 200th pulse 485 μm Air PMMA Figure 23 Time-resolved images of the plasma expansion during the Gauss beam drilling for the 1st pulse (a) and 200th pulse (b), respectively ,pump- probe shadowgraph study (c) of Bessel beam drilling . c1–c16 Time-resolved images of femtosecond-picosecond-nanosceond dynamics of Bessel beam drilling. c17 The ﬁnal hole morphology drilled by Bessel beam. Reproduced from Ref. 133 (with the permission of OSA) and Ref. 131 (with the permission of OSA). atoms were selectively removed and more unbound sulfur was including melting (various phase change mechanisms coexisted during formed, which might result from the signiﬁcantly enhanced ionization the process) dominated the ablation process, which resulted in inferior of bound electrons by femtosecond double-pulse trains (ﬁrst, bound fabrication quality. While in the case of femtosecond laser pulse train electrons were ionized by the ﬁrst sub-pulse and the electron-hole with the identical total ﬂuence, a single pulse was split into two sub- pairs separated; second, the photoexcited electron-hole pairs recom- pulses with the identical ﬂuence lower than the ablation threshold of bined, which began within 500 fs and would last for more than a fused silica. Therefore, through optimizing the pulse delay between the hundred picoseconds; last, the second sub-pulse could further ionize two sub-pulses, the laser-induced electron density can be controlled to the recombining electron-hole pairs, which might further facilitate the be slightly higher than the critical density. Subsequently, the non- 38,194,195 photochemical bond breaking) and inevitably resulted in thermal phase-change mechanisms mainly dominate the fabrication stronger chemical reduction activity and higher chemical reduction process, contributing to less recast and high fabrication quality. rate on laser-treated MoS . In conclusion, by designing pulse trains Furthermore, by adjusting of the energy ratio between the two sub- based on EDC, the bound electrons ionized by laser irradiation can be pulses, the ionization processes were altered to change the pulse energy controlled to modulate chemical bond cleavage and increase chemical absorption. Consequently, the free electron distribution can be reduction ability, which can reduce gold cations to obtain metal-MoS adjusted to make the phase change process a nonthermal one. hybrids for relevant applications. Therefore, we achieved much higher fabrication quality and more controllable structures (Figure 13d and 13e). Overall, through High-quality fabrication by temporal pulse shaping based on EDC. femtosecond laser temporal pulse shaping, the localized electron By designing femtosecond laser pulse trains on the basis of EDC, the dynamics of the materials can be controlled, thus the phase-change phase change mechanism can be controlled to achieve high-quality mechanisms can be adjusted to be dominated through nonthermal fabrication . As Figure 13a shows, the recast ratio (recast area/ phase change process so that higher fabrication quality and more ablation area) of the ablation structures on the fused silica decreased as controllable structures can be achieved. the pulse delay increased within a femtosecond laser double-pulse train. Compared with a conventional single pulse, the recast height Femtosecond laser spatial pulse shaping surrounding the ablation spot decreased by 60% when fabrication was Spatial laser shaping is another key aspect of EDC. The spatial states of performed using a femtosecond laser pulse train (see Figure 13b and electron dynamics are closely related to the spatial distribution of laser 13c, for atomic force microscope (AFM) proﬁles). In this case, the − 2 energy. Conventionally, most commercial laser systems provide a total ﬂuence of the pulse train was 5 J cm (greater than the ablation fundamental Gaussian intensity proﬁle, which has a short Rayleigh threshold of fused silica). The conventional femtosecond laser single length, small beam waist of focusing spot, homogeneous phase/ pulse induced an electron density much higher than the critical density polarization state and limited numbers of laser spots. These char- and a higher Coulomb barrier, therefore leading to signiﬁcant electron acteristics of Gaussian laser cannot satisfy the increasingly higher screening effects. Consequently, the accumulation of positive charge demands for ﬂexibility, precision and efﬁciency in high-end laser during ablation was reduced, thus the electric ﬁeld was weakened so fabrication. Thus, it is essential to spatially shape the laser pulses to that the effectiveness of the nonthermal processes (Coulomb repulsion and/or electrostatic ablation) was reduced. The thermal phase change control the spatial electron density/temperature distribution. By spatial doi:10.1038/lsa.2017.134 Light: Science & Applications Electrons dynamics control LJiang et al a c 50 μm bd 2.23 μm 5 μm 20 μm Figure 24 (a and b) The image of a microhole array throughout a 1 cm × 1 cm large area using the ﬂying punch method for Bessel beam; (c) the local magniﬁed OTM image of a microhole array; (d) the local magniﬁed OTM image of microholes in the central array under 100 × microscope objective. Reproduced from Ref. 132 (with the permission of Springer). pulse shaping, the Gaussian beam can be converted into different By using a spatially modulated femtosecond beam based on spatial beam types, such as ﬂat top beam, vortex beam and Bessel beam, Airy EDC, we achieved high-resolution nanowire patterning that breaks the beam and so on (Figure 14) Some unique characteristics of these beam light diffraction limit . The initial beam had a Gaussian intensity types can lead to special functionality for laser fabrication, which has a proﬁle with an even phase wave front. The phase of the initial beam large advantage over conventional Gaussian beams. was modulated using a liquid crystal on silicon spatial light modulator. After modulation, a relative phase difference was created between two Spatial pulse shaping experimental setup. The experimental setup is equal parts of the incident beam. Subsequently, the modulated beam shown in Figure 15. An ampliﬁed Ti: sapphire laser system (Spitﬁre was focused by an objective lens, forming a dual-peak focusing spot Ace-35F Spectra Physics Inc.) provides a fundamental Gaussian mode with an intensity valley in the center because of a diffraction effect. with a central wavelength of 800 nm and a pulse duration of 35 fs. The spatially modulated beam was used to pattern a gold thin ﬁlm, Phase patterns were generated using the liquid crystal on a silicon which was deposited on a silica substrate by electron beam evaporator. spatial light modulator (SLM, Holoeye Pluto). The size of the liquid The central thin ﬁlm was preserved due to the intensity valley thus a crystal screen was 15.36 × 8.64 mm. The modulated laser beam passes nanowire was formed in the beam center. Arbitrary nanowire can be through a 4f relay system, which consisted of two plano-convex lenses generated on the substrate by dynamically adjusting the orientation of (L1, L2). The distance between the SLM and lens L1 was equal to the the intensity valley. A minimum nanowire width of approximately L1 focal length. The distance between the two lenses was the sum of their focal lengths. The laser beam can then be transmitted to the focal 56 nm (B1/14 of the laser wavelength) can be achieved (Figure 16a– plane of the second lens L2 without any distortion. The focal length 16d). The high resolution is achieved by combining the ultrashort was 100 mm (L1) and 150 mm (L2), respectively. The samples were nature of the femtosecond and the low thermal conductivity of the mounted on a nanometer-precision stage (Newport, NPXYZ100) has a thin ﬁlm. We tested the amount of Au residue using an electron resolution of 0.2 nm in the X, Y and Z planes. The fabrication process diffraction X-ray spectrum (EDXS) experiment (Thermo Scientiﬁc, was monitored using a CCD camera and white light source (WS). MA, USA; Figure 16e). By changing the direction of the phase pattern loaded on the SLM, the center nanowire changed its orientation High precision nanowire fabrication by spatial pulse shaping based accordingly. By dynamically adjusting the direction of the phase on EDC. Diffraction is a universal phenomenon in wave optics, pattern to tangent of the scanning route, arbitrary curves can be which greatly limits the resolution of laser fabrication to half of the fabricated. Figure 16f shows Olympic rings fabricated using this wavelength level. One of the greatest challenges in laser micro/nano method, which evidence its effective curve patterning ability. fabrication is ‘overcoming’ the diffraction limit. Several methods have To investigate the electronic characteristics of the nanowire, the volt- been developed to solve this problem using different mechanisms, such 196,197 198 ampere characteristics curve of the nanowire fabricated by different as near-ﬁeld fabrication , two/multi-photon polymerizations energy pulses was measured. (Figure 17) No inner nanopores and and plasmonics-based fabrications .However,these methods all have 196,197 particle intervals are generated inside the nanowire, thus endowing the different disadvantages, including the low efﬁciency ,complex 196,197 199 procedures ,weak ﬂexibility . and limited types of applicable nanowire with favorable electronic characteristics: the conductivity of 198 7 − 1 the nanowires wasashighas1.2×10 Sm ,and themaximum materials . A novel simple, repeatable, mask-free, high-throughput, 8 − 2 broad-applicability and high-ﬂexibility method is highly desired. current density was up to 1.66 × 10 Am . This approach offers a Light: Science & Applications doi:10.1038/lsa.2017.134 Electrons dynamics control LJiang et al simple, robust alternative for high-quality nanowire fabrication as a strongly depended on the pulse delay; the signal was stronger than that complementary method to conventional lithography methods. of single-pulse irradiation when the pulse delay exceeded 10 ps. The maximum enhancement value was about 7 at the pulse delay of MULTISCALE MEASUREMENT OF ELECTRON DYNAMICS approximately 80 ps. To determine the electron dynamics difference, DURING FEMTOSECOND LASER-MATERIAL INTERACTIONS the plasma temperature and electron density were calculated using the To comprehensively understand the electron dynamics during femto- Saha–Boltzmann plot and the Stark broadening method, respectively. second laser micro/nano fabrications, a multiscale measurement The plasma temperature variation was correlated with the signal system was developed to monitor the spatiotemporal electron enhancements achieved using the double-pulse delay, implying that dynamics of laser-material interactions. This system integrates the the plasma reheating effect of the second pulse was the main 198,199 widely applied ultrafast pump-probe microscopy , time-resolved mechanism of the enhancement effect. By contrast, the maximum plasma photography with a gated intensiﬁed charge-coupled device enhancements were different for molecules, atoms and ions, which 200,201 202–204 205 (ICCD) ,LIBS and industrial continuous imagery were related to the different upper excitation energy of the emission 200,201 (Figure 18). These techniques have different characteristic time transition and the ionization stage of the electrons . resolutions, ranging from femtoseconds to seconds. By virtue of the A comparison of fused silica plasma emission induced by a single multiscale ability, the electron dynamics of femtosecond laser micro/ pulse and that induced by a double pulse is presented in Figure 21a nano fabrications can be revealed at different scales. and 21b. The plasma emissions at the pulse delay below 10 ps were As a case study, the deep-hole drilling process in poly (methyl stronger than they were in the single-pulse case. This phenomenon methacrylate) (PMMA) was investigated using the multiscale measure- was explained by the existence of free electrons and self-trapped ment system, as shown in Figure 19. In the femtosecond to picosecond excitons at this pulse delay. The free electrons and self-trapped time scale, focused laser propagation in the material, as well as free excitons left by the ﬁrst pulse increased the absorption efﬁciency of electron generation, diffusion and recombination, was detected the second pulse. Moreover, extraordinarily high enhancement factors through pump-probe shadowgraphy (Supplementary Movie 2, were observed at a pulse delay above 10 ps. The maximum enhance- Supplementary Information). Nonlinear phenomena were observed ment factor of double-pulse irradiation wasB35 times at a pulse delay − 2 at a high pulse energy as a result of the strong self-focusing effect of of 120 ps for a ﬂuence of 11 J cm . The plasma consisted of a fast the intense femtosecond laser. In the picosecond to nanosecond time component (ionized atoms and ions) and a slow plume component scale, the early plasma expansion and following shockwave evolution (partially ionized nanoparticles), with the slow part contributing little in the atmosphere can also be studied through pump-probe shadow- to plasma emission . The ionization of the slow part by the second graphy (Supplementary Movie 3, Supplementary Information). Shock- pulse greatly increased the plasma quantity and was demonstrated to wave expansion properties were investigated on the basis of the Sedov- be the main cause of the high enhancement factor when the pulse Taylor solution, revealing the change of environment and its effect on delay was larger than 10 ps. The plasma temperature was also laser ablation. For larger time scales, time-resolved plasma photo- calculated to ascertain its changes with respect to the pulse delay graphy with the gated ICCD and LIBS were employed to collect the (Figure 21c). Hence, the electron dynamics (free electrons, self- plasma expansion morphology (Supplementary Movie 4, trapped excitons or nanoparticles) left by the ﬁrst pulse dominated Supplementary Information) and plasma emission spectroscopy, the plasma emission intensity of the femtosecond double pulse LIBS. respectively. These results not only provided the expansion dynamics of plasma but also the plasma intrinsic information, including the APPLICATIONS OF EDC IN HIGH QUALITY AND ASPECT-RATIO material composition, plasma density and temperature. In the milli- MICROHOLES DRILLING second to second time scale, the plasma evolution induced by multiple As discussed before, because of the signiﬁcant electron-lattice none- pulses was studied using time-resolved plasma photography, revealing quilibrium state in the femtosecond laser fabrication process, the laser- the effect of the prior structure on the plasma intensity and material interactions, including phase change and material removal, 26,27 distribution (Supplementary Movie 5, Supplementary Information). are actually determined by the initial photon–electron interactions . Furthermore, microhole formation was studied using industrial Furthermore, the electron density distribution can be manipulated by continuous imagery (Supplementary Movie 6, Supplementary modifying the laser intensity distribution. By implementing EDC Information), providing information concerning the evolution of the through spatial pulse shaping, forming an intense, long and uniform depth, diameter and quality of the high-aspect-ratio microhole. Bessel beam, to adjust the localized transient electron density By using the measurement system, we determined the nanosecond- distribution, and thus control phase change, we fabricated high quality scale electron temperature and density evolution under double pulse and high aspect-ratio microholes. The technique was applied in key irradiation. Large differences in plasma plume ejection of PMMA and structure fabrication in one of the 16 Chinese National S&T Major fused silica were demonstrated using the LIBS technique, following the Projects. different electron dynamics designed by femtosecond laser double- Microholes fabrication in the key structure faced many challenges, pulsetrain. Bychanging the pulsedelay, wecould modify the including the high aspect-ratio (420:1), small diameter (o10 μm), generation and distribution of electrons and thus effectively improve taper-free, high quality, reduced recast/ejected materials and minimized the electron density and temperature. Finally, we could improve the in-cavity residues. Although various novel methods had been proposed plasma enhancement factor and optimize the ablation accuracy for microholes fabrication, there were many challenges that limited through EDC. their application in the key structure fabrication. By using a particle The plasma spectra of PMMA induced by single pulse and double based near-ﬁeld nanostructuring to overcome the optical diffraction pulse irradiation at the same laser ﬂuence were compared to explore limit, Quentin et al achieved nanoholes with diameters below the LIBS enhancement mechanism. The plasma spectra of PMMA 200 nm, but the structure was a shallow crater and the aspect-ratio consisted of emission peaks of molecular species (CN, CH and C )as was below 1:1. He et al reported that high aspect-ratio microholes well as atomic and ionic species (Ca I, Na I and Ga II) .AsFigure 20 were fabricated in fused silica with femtosecond laser transverse direct- shows, double-pulse-induced plasma emission signal enhancement writing followed by wet chemical etching, but the cross-sections of the doi:10.1038/lsa.2017.134 Light: Science & Applications Electrons dynamics control LJiang et al microholes fabricated by this method are usually in poor shape Bessel beam and a focused Gaussian beam in the longitudinal and ascribing to the asymmetric shape of the focal spot. Gottmann transverse plane, as shown in Figure 22c. The micro-Bessel beam et al constructed a selective laser-induced etching system of 3D exhibited a large focal depth (Z = 597 μm) in comparison with the max precision quartz glass components. By the combination of a three-axis focused Gaussian beam (Raleigh range R= 18.5 μm). According to the system to move the glass sample and a fast 3D system to move the laser simulation results, the single-pulse Bessel beam exhibited an intense, focus, the selective laser-induced etching process (LightFab 3D Printer) long, consistent and stable intensity distribution without any nonlinear is suitable to produce more complex structures in a shorter time. beam distortion along the propagation direction. These unique However, the over-etching for some complex structures and the properties of the spatial shaping pulse allow for uniform energy formation of cracks during the etching process limit advanced deposition over extended propagation lengths and then adjust free applications. In addition to the aforementioned methods, percussion electron density distribution to be intense, long and uniform through 204,206,207 drilling has attracted the most interests . Nevertheless, the photo–electron interactions. aspect-ratio of microholes with percussion drilling is usually smaller Using the aforementioned multiscale time-resolved measurement than 10:1 in air because of the saturation effect and the occurrence of system developed by us, we further revealed the forming mechanisms the bending effect. We recently reduced the ambient pressure from of the high aspect-ratio and high quality microholes using a approximately 10 Pa (air) to B1 Pa (rough vacuum) to control the femtosecond laser spatial pulse shaping beam (Bessel beam) by 133,208 expansion dynamic of the ablated plasma/material .The aspect comparing it with the Gaussian beam percussion drilling. Figure 23a ratio of the microholes was signiﬁcantly improved from ~ 40:1 (in air) shows the time-resolved plasma photography of a Gaussian beam to approximately 100:1 (in vacuum), and the bending effect was drilling with a single-pulse, the ablation plume was detected only simultaneously eliminated. Nonetheless, conventional Gaussian beam above the PMMA surface. As shown in Figure 22c, the focused drilling could barely achieve a much higher aspect-ratio of taper-free Gaussian beam was condensed, thus, the laser energy could only be microholes with diameters below 10 μm. localized within several microns below the surface. Such a high-energy To overcome the disadvantages of the traditional Gaussian beam concentration would make free electron density much higher than the drilling, we proposed a new processing method for high aspect-ratio critical density. Therefore, the Coulomb explosion and electrostatic and high-quality microdrilling by optimizing the localized transient ablation are weakened, and the material removal is mainly attributed electron density distribution in plasma in the focal spot through spatial to melting and evaporation, resulting in a large heat-affected zone. pulse shaping . Taper-free microholes with a diameter of approxi- Meanwhile, the shielding-effect induced by the ultrahigh free electron mately 1.6 μm and an aspect ratio of up to 330:1 using a single spatial density suppresses the laser propagation, leading to a short optical shaping pulse (Bessel beam). The aspect ratio of these fabricated penetration depth; consequently, only a shallow crater can be microholes was 52 times larger than those formed by using a Gaussian fabricated with a single-pulse Gaussian beam. Furthermore, during beam in similar focusing conditions. The formation of these high the multipulses percussion drilling process (Figure 23b), the subse- aspect-ratio microholes is attributed to the intense, long and uniform quent pulses energy was not sufﬁciently absorbed by the material transient localized electron density distribution, which is adjusted on because of the strong reﬂection of the generated dense ablated plasma. the basis of the unique intensity distribution and propagation stability The generated plasma would disturb the propagation of the subse- of the Bessel beam through spatial pulse shaping. quent laser pulses so that leads to an unstable ﬁlament bending slightly In our experiments, we normally used an axicon (Edmund Inc., deep inside the hole, then the laser beam energy was deposited along a Barrington, NJ, USA, base angle α= 2°, refractive index= 1.45) to bent direction. Moreover, for the deeper microholes, the plasma transform a Gaussian beam into a Bessel beam for high aspect-ratio, would cool down gradually, then adhered to the microholes sidewalls, high quality and high efﬁciency microdrilling. This is a convenient and which negatively affected the quality of the microholes. effective method for spatial pulse shaping. Figure 22a shows the By contrast, spatial shaping pulse (Bessel beam) drilling exhibits a schematic of the experimental setup used for femtosecond laser completely different fabrication mechanism. Figure 23c displays the microhole drilling . Subsequently, we performed the experiments time-resolved images of Bessel beam drilling in PMMA. As the Bessel to prove that high aspect-ratio and high quality microholes can be beam entered into the PMMA, the rising edge induced electron obtained by using a single spatial shaping pulse. As Figure 22b shows, excitation, which formed a dark strip along the light path that a microhole with a mean diameter of 1.5 μmand adepth of indicated the plasma channel dynamics. The maximum length (optical approximately 523 μm can be drilled using a single-pulse Bessel beam penetration depth) of the Bessel plasma channel was reached at 1.6 ps at a pulse energy (E) of 20 μJ in PMMA. Under identical processing and effectively coincided well with the ﬁnal hole-depth, indicating the conditions, the diameter and depth of a microhole fabricated using the dominant role of initial electron excitation and the free electron Gaussian beam were 7.2 and 41 μm, respectively. Subsequently, to distribution on the ﬁnal structure formation. In the following tens of further demonstrate the hollowness of the microholes through single picoseconds, there existed electron-ion energy transfer occurred that spatial shaping pulse drilling, the two different methods were mainly induced an extremely high pressure and temperature in the focal area, adopted, namely the liquid inﬁltration method and the cross-section resulting in an explosive and supersonic expansion of the material in proﬁle test, as shown in Figure 22d. The microholes drilled using this the nanosecond domain, as shown in Figure 23c11–23c16. In contrast method were small in diameter and exhibited a high aspect-ratio, a to the pressure waves in air and some other cases, the pressure wave taper-free sidewall, a highly circular entrance and fewer surface-ejected induced by the Bessel beam in PMMA was a cylindrical shockwave and materials. gradually expanded outward along the radial direction. This phenom- Moreover, we performed a series of theoretical investigations and enon suggests that the formation of microholes is an extrusion effect, simulations on the spatial intensity distribution of Bessel beams and leading to the formation of high quality microholes with taper-free, Gaussian beams to more thoroughly understand the physical mechan- reduced recast/ejected materials and minimized in-cavity residues. isms of pulse microdrilling with spatial shaping. On the basis of the After the understanding of the processing mechanisms of the Bessel theory of spatial pulse shaping using the axicon, we respectively beam, we proposed the ﬂying punch method for machining large-area simulated the spatial intensity distribution of single-pulse, micro- microhole arrays in PMMA by using a Bessel beam, as shown in Light: Science & Applications doi:10.1038/lsa.2017.134 Electrons dynamics control LJiang et al Figure 24. A 1 cm × 1 cm microhole array (with 251 001 ultrahigh- system limited the spatial/temporal resolution and the sensitivity aspect-ratio holes in total, at a processing speed of 100 holes of the measurement, which shall be improved to monitor the per second) was fabricated within 42 min, indicating the high- electron dynamics in more detail. In addition, more time-resolved efﬁciency and high repeatability of the fabrication process using the measurement techniques should be integrated into the multiscale ﬂying punch method . system, so that it can provide more information of electrons from different aspects. Furthermore, the measurement results should CONCLUSIONS AND OUTLOOKS correspond with the theoretical models, such as providing the In this paper, we comprehensively reviewed our decade-long efforts on characteristic values of electrons for theoretical calculations. four parts of EDC in femtosecond laser micro/nano fabrications: the (4) Broader applications: High-aspect-ratio and high-quality micro- theoretical fundamentals, experiments, multiscale measurements and holes by using spatial shaping pulses have been applied in applications. Theoretically, based on the four models with different fabricating some key structure fabrications. However, the current − 3 − 15 − 3 − 10 time scales (10 –10 s) and space scales (10 –10 m), we method is limited to a few transparent materials. We will extend demonstrated that the localized transient electron dynamics (including the range of materials and then explore novel applications of electron density, temperature and excited state distribution), and microholes, such as for microﬂuidic devices and three- subsequent phase change can be controlled by temporally/spatially dimensional integrated chip packaging. In addition, we will shaping femtosecond laser pulses. Experimentally, seven experiments substantially expand the applications of the novel method, such were reported as examples to validate the feasibility of EDC by as for the adjustment of chemical reaction pathways by ultrafast temporally/spatially shaping femtosecond pulses in micro/nanofabri- laser EDC, transient or permanent adjustment of material proper- cation. The experiments revealed that the precisions, efﬁciencies and ties through ultrafast laser EDC. qualities can be signiﬁcantly improved and that various surface micro/ nano-structures can be effectively modulated by the proposed EDC- based methods. Additionally, multiscale measurements further directly CONFLICT OF INTEREST demonstrated the fundamentals of EDC from femtosecond scale to The authors declare no conﬂict of interest. nanosecond scale and to millisecond scale. Finally, EDC was applied in high aspect-ratio (330:1) and high-quality microholes drilling at the ACKNOWLEDGEMENTS speed of 100 holes per second (251 001 holes fabricated in 1 cm × 1 cm This research was supported by the National Natural Science Foundation of area within 42 min), in which multiscale measurements were used to China (NSFC) (Grant Nos. 90923039, 91323301, 50705009, 51105037, analyze and optimize the electron dynamics. The high aspect-ratio 51322511 and 51025521), National Basic Research Program of China (973 microholes drilling was applied to key structure fabrication in one of Program) (Grant No. 2011CB013000), the 863 Project of China under Grant the 16 Chinese National S&T Major Projects. No. 2008AA03Z301, the Cultivation Fund of the Key Scientiﬁc and Technical Innovation Project, Ministry of Education of China (No. 708018), the 111 We have devoted the past ten years to studying the mechanisms, Project of China (Grant No. B08043), Multidisciplinary University Research methodologies and applications of femtosecond laser micro/nano Initiative (MURI) program of USA under Grant No. N00014-05-1-0432 and fabrications. However, many challenges still remain, especially on National Science Foundation of USA under Grant No. 0423233. We thank Prof the following topics: Xin Li, Prof Jie Hu, Prof Sumei Wang, Prof Xiaowei Li, Prof Jingya Sun, Prof Jianfeng Yan, Dr Xiaoxing Su, Dr Weina Han, Dr Qingsong Wang, Dr Zhitao (1) Comprehensive models: Femtosecond laser-material interactions Cao, Mr Zhi Wang, Dr Changji Pan, Dr Peng Ran and Dr Pei Zuo for dis- are a comprehensive nonlinear, nonequilibrium process ranging cussing, revising and proofreading this manuscript. from a nanometer scale to a millimeter scale and from a femtosecond scale to a millisecond scale. However, our present models, including the plasma and improved two-temperature models, are not applicable to some materials. Furthermore, a 1 Malinauskas M, Žukauskas A, Hasegawa S, Hayasaki Y, Mizeikis V et al. Ultrafast laser comprehensive, integrated multiscale physical-chemical modeling, processing of materials: from science to industry. 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Opt Lett article are included in the article’s Creative Commons license, unless indicated otherwise 2010; 35:282–284. in the credit line; if the material is not included under the Creative Commons license, users 205 Gottmann J, Hermans M, Repiev N, Ortmann J. Selective laser-induced etching of 3D will need to obtain permission from the license holder to reproduce the material. To view a precision quartz glass components for microﬂuidic applications—up-scaling of copy of this license, visit http://creativecommons.org/licenses/by/4.0/ complexity and speed. Micromachines 2017; 8: 110. 206 Zhao X, Shin YC. Femtosecond laser drilling of high-aspect ratio microchannels in glass. Appl Phys A 2011; 104:713–719. r The Author(s) 2018 Supplementary Information for this article can be found on the Light: Science & Applications’ website (http://www.nature.com/lsa). doi:10.1038/lsa.2017.134 Light: Science & Applications http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Light Science & Applications Springer Journals http://www.deepdyve.com/lp/springer-journals/electrons-dynamics-control-by-shaping-femtosecond-laser-pulses-in-Rm9Pb6Y7Zc

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S Haessler, J Caillat, W Boutu, C Giovanetti-Teixeira, T Ruchon (2010)
Attosecond imaging of molecular electronic wavepackets
Nat Phys, 6

Publisher: Springer Journals
Copyright: Copyright © 2018 by The Author(s)
Subject: Physics; Physics, general; Applied and Technical Physics; Atomic, Molecular, Optical and Plasma Physics; Classical and Continuum Physics; Optics, Lasers, Photonics, Optical Devices
ISSN: 2047-7538
eISSN: 2047-7538
DOI: 10.1038/lsa.2017.134
Publisher site: See Article on Publisher Site

Abstract

Light: Science & Applications (2018) 7, 17134; doi:10.1038/lsa.2017.134 OPEN Ofﬁcial journal of the CIOMP 2047-7538/18 www.nature.com/lsa REVIEW ARTICLE Electrons dynamics control by shaping femtosecond laser pulses in micro/nanofabrication: modeling, method, measurement and application 1 1 1 2 3 Lan Jiang , An-Dong Wang ,BoLi , Tian-Hong Cui and Yong-Feng Lu During femtosecond laser fabrication, photons are mainly absorbed by electrons, and the subsequent energy transfer from elec- trons to ions is of picosecond order. Hence, lattice motion is negligible within the femtosecond pulse duration, whereas femtose- cond photon-electron interactions dominate the entire fabrication process. Therefore, femtosecond laser fabrication must be improved by controlling localized transient electron dynamics, which poses a challenge for measuring and controlling at the elec- tron level during fabrication processes. Pump-probe spectroscopy presents a viable solution, which can be used to observe elec- tron dynamics during a chemical reaction. In fact, femtosecond pulse durations are shorter than many physical/chemical characteristic times, which permits manipulating, adjusting, or interfering with electron dynamics. Hence, we proposed to control localized transient electron dynamics by temporally or spatially shaping femtosecond pulses, and further to modify localized tran- sient materials properties, and then to adjust material phase change, and eventually to implement a novel fabrication method. This review covers our progresses over the past decade regarding electrons dynamics control (EDC) by shaping femtosecond laser pulses in micro/nanomanufacturing: (1) Theoretical models were developed to prove EDC feasibility and reveal its mechanisms; (2) on the basis of the theoretical predictions, many experiments are conducted to validate our EDC-based femtosecond laser fabrication method. Seven examples are reported, which proves that the proposed method can signiﬁcantly improve fabrication precision, quality, throughput and repeatability and effectively control micro/nanoscale structures; (3) a multiscale measurement system was proposed and developed to study the fundamentals of EDC from the femtosecond scale to the nanosecond scale and to the millisecond scale; and (4) As an example of practical applications, our method was employed to fabricate some key struc- tures in one of the 16 Chinese National S&T Major Projects, for which electron dynamics were measured using our multiscale measurement system. Light: Science & Applications (2018) 7, 17134; doi:10.1038/lsa.2017.134; published online 9 February 2018 Keywords: electrons dynamics control; femtosecond laser; micro/nano fabrication; pulse shaping INTRODUCTION the initial defects of the target materials. Hence, femtosecond laser Because of their ultrashort irradiation periods and ultrahigh inten- ablation is deterministic and reproducible , and almost any material 8–10 sities, femtosecond laser pulses in some aspects fundamentally change can be machined using a femtosecond laser, including metals , 11,12 13,14 15 the laser-material interactions mechanisms compared with long laser semiconductors , dielectrics , polymers , two-dimensional 16–20 21 22,23 pulses, which has created wide-range and exciting new possibilities in materials , ultrahard materials and biological tissues .In 1–3 micro/nanoscale fabrication . addition, ionization mechanism can be adjusted by changing femto- The ultrahigh intensity makes femtosecond laser-material interac- second laser energy and its temporal/spatial distribution to control 4,5 24 tions a strongly nonlinear process . Linear ionization generally laser-material interactions . dominates in free electron generations for continuous or long-pulse The ultrashort irradiation period of a femtosecond laser also makes laser processing of wide bandgap materials . The intensities of femtosecond laser-material interactions a strongly nonequilibrium 12 − 2 3,25 femtosecond lasers can easily exceed 10 Wcm , thus nonlinear process . The duration of a femtosecond laser pulse is much shorter 12 − 2 − 10 − 12 ionization mechanism such as avalanche ionization (~10 Wcm ), than the electron-lattice energy relaxation time (10 − 10 s). 13 14 − 2 multiphoton ionization (~10 –10 Wcm ), and tunnel ionization Therefore, laser energy absorption is completed before the lattice 15 − 2 (410 Wcm ; Ref. 3), can occur in the femtosecond laser fabrica- changes, resulting in a signiﬁcantly nonequilibrium state between tion processes. The nonlinear ionizations are almost independent on electrons and lattices. Hydrodynamic motion and heat conduction 1 2 Laser Micro/Nano-Fabrication Laboratory, School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China; Department of Mechanical Engineering, University of Minnesota, Minneapolis, MN 55455, USA and Department of Electrical Engineering, University of Nebraska-Lincoln, Lincoln, NE 68588-0511, USA Correspondence: L Jiang, Email: [email protected] Received 12 April 2017; revised 27 August 2017; accepted 28 August 2017; accepted article preview online 30 August 2017 The accepted article preview was available with the details: Light: Science & Applications (2018) 7, e17134; doi: 10.1038/lsa.2017.134. Electrons dynamics control LJiang et al Table 1 The estimations of optical and thermal properties in classical and improved two-temperature models Property Classical TTM Improved TTM ∂hi ε Electron heat capacity C = γT C ¼ n e e e e ∂T e pﬃﬃﬃﬃﬃ 3=2 3 m ðk T Þ e B pﬃﬃﬃ t ¼ 3mek e 2 2 2pðZ Þ n e lnL Electron relaxation time t ¼ e e 2 p n k T e B e fg 1 þ exp½ mðÞ n ; T =k T F e B 1=2 T 1 2 Electron heat conductivity k ¼ k k ¼ n t C e eq e e e T 3 e 2 2 2 2 o ðÞ n t o ðÞ n t e e p e p e εðÞ t; r ; z ¼ 1 þ i DR DT e 2 2 2 Optical property D 1þo t o 1þo t e ðÞ e ðÞ DR ðÞ DT max e max þ DεðÞ contribution of the interband transition ab c 14 000 14 000 Electron temperature Electron temperature Experiments Lattice temperature Lattice temperature 12 000 12 000 Existing model 0.8 Proposed model 10 000 10 000 0.6 8000 8000 6000 6000 0.4 4000 4000 0.2 2000 2000 0 0 0 2 3 4 5 –5 0 5 1015202530 –5 0 5 10 15 20 25 30 10 10 10 10 Pulse duration (fs) Time (ps) Time (ps) − 2 Figure 1 The calculated electron and phonon temperatures of 200 nm gold ﬁlm irradiated by a 140 fs, 1053 nm pulse at 0.2 J cm by (a) the classical model and (b) the improved model. (c) The predicted damage threshold ﬂuences of 200 nm gold ﬁlm processed by a 1053 nm laser at different pulse. Reproduced from Ref. 117 (with the permission of SPIE). through lattices are negligible during the femtosecond pulse duration. from experiments may contribute to theoretical comprehension and Thus, recast, thermal damage (microcracks) and heat-affected-zone prediction. are greatly reduced . Because of the nonequilibrium laser-material Many innovations in measurement technologies could also be interactions, including phase change and material removal, are converted to new methods in fabrication technologies. In the pump- 26,27 essentially determined by laser-electron interactions . Hence, fem- probe experiments, the observation of electron dynamics is achieved by analyzing the probe pulse disturbed by electrons. By increasing the tosecond laser fabrication must be improved by controlling electron energy of the probe laser, the disturbing process can be transformed dynamics during the fabrication process, which poses a challenge for measuring and controlling at the electron level during fabrication into certain control if it is purposely designed. Much more complex processes. pulses other than merely two pulses can be designed to control the Pump-probe spectroscopy, which has been widely used in ﬁelds electron dynamics. Recent developments of optical devices substan- 28–33 34–37 38–40 58–63 including physics ,chemistry and materials science , tially enhanced the capability to shape laser pulses. The amplitude 64,65 66,67 68,69 presents a viable solution for detecting electron dynamics. The basic /phase /polarization /frequency can be easily manipulated in mechanism involves splitting one femtosecond pulse into two sub- both temporal and spatial domains. By temporally/spatially shaping pulses. One sub-pulse is the pump pulse and the other is the probe the femtosecond laser, the local transient electron dynamics can be pulse, and the time delay between them can be precisely controlled by precisely controlled. Many studies have demonstrated that properties adjusting the optical path difference. On the basis of this principle, of atoms and molecules can be controlled by temporal/spatially shaped 41 70–72 Zewail and other pioneers developed the discipline of femtochem- ultrafast laser pulses . For example, using a shaped ultrafast laser 73–75 istry, which elucidates ultrafast chemical reaction processes at the pulse, atoms can be selectively ionized ; spin states can be 42–44 76–80 electron level . Their research demonstrates how chemical bonds dynamically controlled ; molecular ground state rotational 45,46 81,82 change and electrons transfer during chemical reactions . Recently, dynamics and vibrational modes can be manipulated ;chemical 83,84 attosecond laser pulses have been used to probe electron dynamics in reactions can be controlled and X-ray line emission from plasma 47–49 85–87 greater detail , and even the electronic charge distribution within under the femtosecond pulse can be signiﬁcantly enhanced .In the 34,50,51 5,88–90 molecules has been detected . In addition, electron dynamics ﬁeld of ultrafast laser fabrication, Herman and Ilday et al during laser-material interactions have been captured through the demonstrated the heat accumulation effect can be controlled by pump-probe technique. For instance, researchers have imaged electron harnessing high-repetition-rate burst trains of ultrafast laser pulses, excitation, electron spatiotemporal distribution and electron decay on achieving signiﬁcant beneﬁts compared with unshaped pulses. Xu 52–54 24,91,92 the femtosecond scale . Furthermore, the electron collision time et al revealed the conceptual signiﬁcance of controlling coherent and laser-induced plasma lifetime can be determined using pump- phonon dynamics for controlling phase change by speciﬁcally designed 55–57 3 probe measurement . The information on electrons measured ultrafast pulse trains with different pulse delays. Gamaly reported Light: Science & Applications doi:10.1038/lsa.2017.134 Temperature (K) Temperature (K) –2 Threshold fluence (J cm ) Electrons dynamics control LJiang et al a c 0.2 0.2 0.1 0.1 0.0 0.0 –0.1 –0.1 –0.2 0.8 –0.2 010 200 10 20 0.6 Time (fs) Time (fs) 0.4 0.2 0.0 0.2 0.1 0.0 –1.1e–2 1.5e–2 –1.1e–2 1.5e–2 –9e–4 9e–4 –0.1 Two pulses 1:1 Two pulses 4:1 Difference –0.2 400nm+800nm 800nm+400nm 800nm+800nm 0 510010 20 30 0 25 50 75 100 125025 50 75 100 1250 25 50 75 100 125 0 20 40 Time (fs) Time (fs) Time (fs) Time (fs) Time (fs) Time (fs) Figure 2 Schemes of electrons dynamics adjusted by shaped femtosecond laser pulses. Electric ﬁelds of the applied laser pulse and time-dependent excited 99 99 electrons of diamond with different (a) pulse delays and (b) sub-pulse numbers .(c)Electric ﬁelds of the applied laser pulse (top panel) and the electron density change of diamond from that in the ground state after laser termination with different pulse energy ratios .(d) Time-dependent electron temperature of fused silica with different pulse dual-wavelengths . Reproduced from Ref. 99 (with the permission of IOP) and Ref. 111 (with the permission of AIP publishing). controlling over the ablation rate and phase state of laser-produced domains, we are able to control photon–electron interactions, and plume by using temporal/sptial pulse shaping. Sheppard and Wilson then to control the localized transient electron dynamics (including produced the Bessel beam with an annular lens, and Marcinkevičius electron density, temperature and excited-state distribution), and 94 95–97 et al and Courvoisier et al obtained some micro/nanostructures further to modify localized transient materials properties, and then by spatially shaping ultrafast laser pulses. to adjust material phase change, and ﬁnally to implement the novel Although various results have been obtained to control the laser- fabrication method. We have devoted our efforts on using EDC to material interactions, electron dynamics have not been extensively improve femtosecond micro/nano fabrications for more than a studied or deliberately controlled, which is vital for laser micro/nano decade. This review summarizes our recent progresses based on fabrication. In femtosecond laser micro/nano fabrication process, EDC by shaping femtosecond pulses in micro/nanofabrication: photons are mainly absorbed by electrons, and the subsequent energy transfer from electrons to ions is of picosecond order 1) Theoretical modeling for EDC feasibility: four models were − 12 − 10 (10 − 10 s). Hence, lattice motion is negligible within the developed, which consists of the ab initio calculations for electron femtosecond pulse duration, and femtosecond photon − electron 98–107 dynamics , a revised molecular dynamics simulation for phase interactions are the only factor to be considered, which dominates 108,109 110–115 change , a plasma model for ionization processes and − 10 − 3 the subsequent fabrication processes (10 –10 s). Therefore, 116–118 an improved two-temperature model for energy transport . femtosecond laser fabrication must be improved by controlling 2) Experiments for EDC validation: By shaping a femtosecond pulse localized transient electron dynamics. The key challenge is to measure in temporal or spatial domains, the photon-electron interactions and control at the electron level during fabrication processes. Pump- can be controlled to adjust the localized transient electron probe spectroscopy presents a viable solution, which can be used to dynamics to implement the novel fabrication methods. Various observe electron dynamics during a chemical reaction. In the pump- experiments were conducted to validate the effectiveness of EDC. probe experiments, the observation of electron dynamics is achieved Basedon EDC, weproposedto: by analyzing the probe pulse disturbed by electrons. By increasing the energy of the probe laser, the disturbing process can be transformed ▪ control the localized transient electron density to induce reso- into certain control if it is purposely designed. Therefore, we propose nance absorption, by which microchannel processing efﬁciency the core idea of electrons dynamics control (EDC; Supplementary was increased by 56 times and the maximum aspect ratio was Movie 1, Supplementary Information): By shaping the amplitude, 119,120 extended by three times ; phase and polarization of femtosecond pulses in temporal and spatial doi:10.1038/lsa.2017.134 Light: Science & Applications Excitation energy Electric amplitude (a.u.) Electric amplitude (a.u.) Electric amplitude (a.u.) (eV per atom) Electric temperature (10 K) Electric amplitude (a.u.) 50 –25 –50 –25 –50 –25 –50 –25 –50 –25 –50 –25 –50 –25 –50 –25 –50 –25 –50 –25 –50 Depth (nm) Depth (nm) Depth (nm) Time (fs) Depth (nm) Time (fs) Time (fs) Time (fs) Time (fs) Depth (nm) Electrons dynamics control LJiang et al a b c d e Pulse delay: 0 fs Pulse delay: 100 fs Pulse delay: 200 fs Pulse delay: 300 fs Pulse delay: 400 fs 1.0 0.8 0.6 0.4 0.2 0.0 fg h i j Pulse delay: 0 fs Pulse delay: 100 fs Pulse delay: 200 fs Pulse delay: 300 fs Pulse delay: 400 fs 75.0 50.0 25.0 0.0 Figure 3 Simulation of materials properties adjusted by varying pulse delay within a femtosecond laser pulse train. (a–e)Reﬂectivity of the material surface and (f–j) the corresponding peak laser intensity distribution at different pulse delays. ▪ modify free electron density and the corresponding photon Furthermore, we revealed the forming mechanisms of the high- absorption efﬁciency, then modulate material properties, by quality and high aspect-ratio microholes using the multiscale time- which laser-assisted chemical etching rate was enhanced by 37 resolved measurement system . times ; ▪ adjust electron generation on fabricated material surfaces, through which the periods, orientations and structures of the MODELING FOR EDC FEASIBILITY 122,123 surface ripples can be effectively adjusted ; Modeling method ▪ control electron density and its distribution, through which we Femtosecond photon–electron interactions dominate the entire none- obtained controllable micro/nano hierarchical structures on quilibrium and nonlinear laser fabrication processes, including the material surfaces and enhancement factors of surface-enhanced absorption of laser energy by electrons, energy transfer from electrons 9 136–138 Raman scattering (SERS) up to 1.1 × 10 (Refs. 124–126); to lattices, plasma generation, phase change and material ▪ control electron density distribution using temporally shaped modiﬁcation. These interactions range from nanometers to milli- femtosecond laser pulse to modify chemical and physical proper- meters spatially and from femtoseconds to microseconds temporally. ties of materials, by which polymorphous Au-MoS hybrids were Although a growing body of experimental observations exists, a prepared ; comprehensive model remains unavailable. Such a model is essential ▪ adjust the phase change mechanism by changing photon-electron for revealing the fundamental science underlying ultrafast laser- interactions, which reduced the recast layer thickness by 60% material interactions. According to the applicable time and space (Ref. 128); scales of each involved process, four fundamental models were used: ▪ manipulate electron density distribution using spatially modu- lated femtosecond laser pulse, by which deep subwavelength 1) The ab initio model based on the time-dependent density func- (B1/14 of the laser wavelength) and high conductivity (B1/4 of tional theory (TDDFT) was employed to understand initial non- the bulk gold) nanowires were fabricated in the open air . linear laser radiation absorption through photon-electron 98,99 interactions . 3) Measurements for EDC fundamentals: A multiscale measurement In TDDFT, the fundamental variable is not the many-body wave system (from femtosecond scale to millisecond scale) was proposed function in quantum mechanics, but the electronic density. This and developed for understanding the electron dynamics during time-dependent electron density is determined by solving an femtosecond laser ablation. It comprises a pump-probe shadow- auxiliary set of noninteracting Kohn-Sham equations, where the 130,131 graph imaging technique , time-resolved plasma laser ﬁelds are treated as time-dependent spatial uniform external 132,133 photography , laser-induced breakdown spectroscopy vector potentials. Describing the motion of the electrons in the (LIBS) and commercialized fast imaging device (CCD). We system, the time-dependent Kohn-Sham equation for single reveal the multiple time scale fundamentals during femtosecond particle orbitals is laser-material interactions, including the femtosecond-scale propa- gation of a laser pulse, picosecond-scale generation/evolution of ! ! ! laser-induced plasma, nanosecond-scale plasma ejection/expansion, i_ c r ; t ¼ H r ; t c r ; t ð1Þ KS ∂t and millisecond-scale hole formation. 4) Applications for the EDC method: Our proposed method was used to fabricate some key structures. Using spatially-shaped femtose- ! ! cond pulses, we optimized electron density distribution in plasma nr ; t ¼ c r ; t ð2Þ at the focus point, and then manipulated plasma expansion and phase change, which was used to drill microholes with a diameter ! ! of 1.6 μm and an aspect ratio of 330:1 (Refs. 132,135). wherenr ; t is the electron density and H r ; t is the Kohn- KS Light: Science & Applications doi:10.1038/lsa.2017.134 1.0 1.0 0.8 1.0 0.8 1.0 0.6 0.8 1.0 0.6 0.8 0.4 0.6 0.8 0.4 0.6 0.2 0.4 0.2 0.6 0 0.4 0.2 0.4 0.2 0.2 80 120 60 100 40 80 Radius (μm) Radius (μm) Radius (μm) Radius (μm) Radius (μm) Radius (μm) Radius (μm) Radius (μm) Radius (μm) Radius (μm) Laser intenstiy Reflectivity Laser intenstiy Reflectivity Laser intenstiy Reflectivity Laser intenstiy Reflectivity Laser intenstiy Reflectivity Electrons dynamics control LJiang et al –2 –2 Laser fluence=0.28 J cm 1pulse 15ps Laser fluence=0.28 J cm 20pulse 15ps ae 80 80 40 40 0 0 –40 –40 –80 –80 700 350 0 –350 –700 –1050 –1400 –1750 –2100 –2450 –2800 700 350 0 –350 –700 –1050 –1400 –1750 –2100 –2450 –2800 Z (Å) Z (Å) –2 –2 Laser fluence=0.28 J cm 1pulse 50ps Laser fluence=0.28 J cm 20pulse 50ps b f 80 80 40 40 0 0 –40 –40 –80 –80 700 350 0 –350 –700 –1050 –1400 –1750 –2100 –2450 –2800 700 350 0 –350 –700 –1050 –1400 –1750 –2100 –2450 –2800 Z (Å) Z (Å) –2 –2 Laser fluence=0.28 J cm 1pulse 92ps Laser fluence=0.28 J cm 20pulse 92ps c g 80 80 40 40 0 0 –40 –40 –80 –80 700 350 0 –350 –700 –1050 –1400 –1750 –2100 –2450 –2800 700 350 0 –350 –700 –1050 –1400 –1750 –2100 –2450 –2800 Z (Å) Z (Å) –2 –2 Laser fluence=0.28 J cm 1pulse 200ps Laser fluence=0.28 J cm 20pulse 200ps d h 80 80 40 40 –40 –40 –80 –80 700 350 0 –350 –700 –1050 –1400 –1750 –2100 –2450 –2800 700 350 0 –350 –700 –1050 –1400 –1750 –2100 –2450 –2800 Z (Å) Z (Å) ik m n 1.0×10 4 4 1.2×10 1.2×10 3 15 15 ps 4 4 8.0×10 1.0×10 1.0×10 20 ps 50 ps 3 8.0×10 3 10 8.0×10 6.0×10 92 ps 15 ps 3 3 200 ps 20 ps 6.0×10 6.0×10 4.0×10 50 ps 92 ps 4.0×10 4.0×10 Binodal Binodal 3 200 ps 2.0×10 Spinodal Spinodal 3 3 2.0×10 2.0×10 Dynamic film surface z=–7 nm 0.0 –5 0.0 0.0 3 3 3 3 3 3 700 500 300 100 –100 –300 –500 –700 700 500 300 100 –100 –300 –500 –700 00 3.0×10 6.0×10 9.0×10 3.0×10 6.0×10 9.0×10 –3 –3 Density (kg m ) Density (kg m ) Z (Å) Z (Å) jl o p 4 20 1.0×10 4 4 1.2×10 1.2×10 3 4 15 4 8.0×10 15 ps 1.0×10 1.0×10 20 ps 3 3 50 ps 8.0×10 8.0×10 3 10 6.0×10 15 ps 92 ps 3 3 20 ps 200 ps 6.0×10 6.0×10 4.0×10 50 ps 3 3 92 ps 4.0×10 4.0×10 Binodal Binodal 200 ps 0 Spinodal 2.0×10 3 Spinodal 3 2.0×10 2.0×10 z=–7nm Dynamic film surface 0.0 –5 0.0 0.0 3 3 3 3 3 3 700 500 300 100 –100 –300 –500 –700 700 500 300 100 –100 –300 –500 –700 0 3.0×10 6.0×10 9.0×10 0 3.0×10 6.0×10 9.0×10 –3 –3 Z (Å) Z (Å) Density (kg m ) Density (kg m ) Figure 4 Schemes of phase change controlled by varying the pulse delay in a fs pulse train . Snapshots of nickel thin ﬁlms irradiated by femtosecond laser − 2 (a–d) single pulse and (e–h) 20 pulse trains with the total ﬂuence of 0.28 J cm ,where X is in the direction of Ni (100) surface and Z is in the direction of laser irradiance. Lattice temperature and stress distributions at different times for (i and k) the single pulse and (j and l) the 20 pulse trains. Time evolution of the system in the ρ-T plane for different regions for (m and n) the single pulse and (o and p) the 20 pulse trains. Arrows indicate the time evolution. Reproduced from Ref. 108 (with the permission of AIP publishing). Sham Hamiltonian given by 2) The molecular dynamics model was employed to reveal the phase change resulting from electron-ion interactions . In the mole- cular dynamics model, Newtonian equations of motion of a set of ! ! ! 1 e H r ; t ¼ p þ A ðÞ t þ V r ; t KS tot ion 2m c N particles are solved to describe the phase change process. ! ð3Þ n r ;t 2 0 þe dr þ V r ; t XC ! ! r r d r ∂U m ¼ f ; f ¼ ð5Þ i t t dt ∂t ! ! where V r ; t is the electron-ion potential and V r ; t is ion XC where r is the coordinate of the ith atom, and f is the force the exchange-correlation (XC) potential.The time evolution of the i i acting on the ith atom that is usually derived from potential wave function over a short period, Δt, can be approximately energy U(r) (Ref. 139): calculated as follows: hi q q ^ Dt ^ Dt 1 2 gðÞ 1r=r g=21ðÞ r=r 0 0 ! iH ðÞ tþDt iH ðÞ t ! KS KS 2 2 UrðÞ ¼ þ D e 2e ð6Þ c r ; t þ Dt ¼ e e c r ; t ð4Þ 4pεr doi:10.1038/lsa.2017.134 Light: Science & Applications Temperature (K) Temperature (K) X (Å) X (Å) X (Å) X (Å) Stress (GPa) Stress (GPa) Temperature (K) Temperature (K) X (Å) X (Å) X (Å) X (Å) Temperature (K) Temperature (K) Electrons dynamics control LJiang et al ac –2 5.0 J cm –2 4.0 J cm 75 fs 50 fs 25 fs AB CD 0 fs Energy ratio 1:2 –2 5.0 J cm –2 4.0 J cm 40 20 0 20 40 Radius (μm) 40 AB 50 CD Energy ratio 1:1 –2 5.0 J cm –2 4.0 J cm CD AB 200 80 400nm+800nm 800nm+400nm 800nm+800nm Energy ratio 2:1 1.0 0.5 0.0 0.5 1.0 1.0 0.5 0.0 0.5 1.0 Radius (μm) Radius (μm) 110–112 Figure 5 Schemes of ablation crater shape controlled by shaped femtosecond laser pulse .(a) Ablation crater shapes created by femtosecond pulse − 2 trains consisting of double pulses with different pulse delays at a total ﬂuence of 5 J cm and central wavelength of 780 nm. (b) Ablation crater shapes − 2 created by femtosecond laser pulse trains with different wavelength composition at the total ﬂuence of 5.0 J cm and the pulse delay of 50 fs. (c)Ablation crater shapes created by 800 nm femtosecond pulse trains consisting of double pulses with three different energy ratios at the pulse delay of 50 fs. Reproduced from Ref. 110 (with the permission of IOP), 111 (with the permission of AIP publishing) and 112 (with the permission of Springer). 3) We proposed the plasma model with quantum treatment to investigate plasma generation and changes (ionization and recom- 110,113–115 (μs~ms) bination) . 114,115 In the plasma model , the free electron density distribution in dielectrics under a femtosecond laser pulse is obtained by solving the Fokker − Planck Equation: ∂nðÞ t; r; z (ps~fs) e ¼ a ItðÞ ; r; z nðÞ t; r; z þ dðÞ ItðÞ ; r; z t e N ∂t nðÞ t; r; z ð7Þ where t is time, r is the distance to the Gaussian beam axis, z is the (ps~fs) depth from the surface of the bulk material, n (t,r,z) is the free electron density, τ is the free electron recombination time, α is the impact ionization constant, and δ is the cross-section of N- photon absorption. (ps~fs) The optical properties of ionized dielectrics are calculated using the Drude model. The spatial and temporal dependence of the complex dielectric function for the plasma is expressed as: 2 2 2 2 o ðÞ n t o ðÞ n t e e p e p e εðÞ t; r; z ¼ 1 þ i ð8Þ 2 2 2 2 1 þ o t o 1 þ o t e e pﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ where o ðÞ n ¼ nðÞ t; r; z e =m ε is the plasma frequency and p e e e 0 τ is the free electron relaxation time, calculated as, follows by applying formulas derived from the Boltzmann transport Equation: Figure 6 Temporally shaping femtosecond pulses: (a) a conventional pﬃﬃﬃﬃﬃﬃ 3=2 femtosecond pulse is temporally shaped into a pulse train; (b) the number of 3 mðÞ k TðÞ t; r; z e B e tðÞ t; r; z ¼ pﬃﬃﬃ sub-pulses within a train can be controlled; (c) the delay between sub-pulses e 2 2pðÞ Z nðÞ t; r; z e lnL ð9Þ can be controlled; (d) the energy ratio of sub-pulses can be controlled. ðÞ 1 þ expðÞ mðÞ n ; T =k TðÞ t; r; z F e e B e 1=2 where Z* is the ionization state, m is the electron mass, k is the e B Light: Science & Applications doi:10.1038/lsa.2017.134 Depth (nm) Depth (nm) Depth (nm) Depth (nm) Depth (nm) Electrons dynamics control LJiang et al CCD Reflector lens Grating Lens SLM Lens Grating Reflector Reflector Fs laser Polarizer Pulse shaper Shutter Dichroic mirror Half wave Attenuator plate Objective lens Sample Computer control Hexapod stage Halogen light Figure 7 Schematic of the experimental setup for temporal shaping of femtosecond pulses. Reproduced from Ref. 141(with the permission of Springer). a b c d D1: 185.6441 um 1 μm 1 μm 1 μm D1: 12.28527 um Figure 8 Holes machined using (a) ﬁve 355-nm nanosecond pulses (duration of 30 ns, energy of 0.16 μJ) ,(b) ﬁve 800-nm femtosecond pulses (duration 119 119 of 120 fs, energy of 0.12 μJ) ,(c) ﬁve femtosecond-nanosecond pulse pairs ,(d) an 800 nm femtosecond laser double-pulse train (duration of 50 fs, energy of 20 μJ) . Reproduced from Ref. 119 (with the permission of OSA) and Ref. 120 (with the permission of OSA). Boltzmann constant, F denotes the Fermi-Dirac integrals, μ is transport equation for dense plasmas; and (3) the free-electron heating 1/2 the chemical potential and ln Λ is the Coulomb logarithm.In and interband transition were taken into account using a modiﬁed addition, the wave-particle duality of photon is also considered to Drude model as Equations (8) and (9) to calculate the reﬂectivity and predict the formation mechanism of laser-induced periodic surface absorption coefﬁcient. Table 1 compares the estimates of optical and structures . thermal properties between classical and improved two-temperature 4) We proposed the improved two-temperature model with full-run models. Figure 1 demonstrates the differences in (a, b) electron and quantum treatment to predict the electron and lattice temperature phonon temperatures between the classical approach and the 116,117 distributions . In the improved two-temperature model, improved model. The improved model signiﬁcantly increases the electron and lattice temperatures are given by prediction precisions of the damage thresholds compared with the classical model, as shown in Figure 1c . ∂T C ðÞ T ¼ ∇ ½ kðÞ T ∇T GTðÞ T þ SzðÞ ; t ð10Þ e e e e e e l The four theoretical models are suitable for systems with different ∂t temporal and spatial scales and must be employed in combination to compensate for each other’s limitations. In our improved two- ∂T CðÞ T ¼ GTðÞ T ð11Þ l l e l temperature model, the electron density is assumed to be constant. ∂t Consequently, the model is not suitable when the bound electrons in where T is the electron temperature, T is the lattice temperature, e l non-metals are substantially ionized by a femtosecond laser pulse. The C is the electron heat capacity, C is the lattice heat capacity, k is e l e plasma model is proposed to consider the ionization process in the electron heat conductivity, G is the electron-lattice coupling nonmetals, but material ablation is assumed to commence when the factor, and S is the laser source term. free electron density reaches the critical density without the con- sideration of phase changes, for example, melting, vaporization, The two-temperature model was improved by us as follows :(1) Coulomb explosion and electrostatic repulsion. Moreover, the mole- using the Fermi distribution, the heat capacity of the free electrons was cular dynamics can be used to identify the relative roles of different calculated; (2) free-electron relaxation time and electron conductivity were determined using a quantum model derived from the Boltzmann phase change mechanisms. However, the interatomic potential doi:10.1038/lsa.2017.134 Light: Science & Applications Electrons dynamics control LJiang et al 0 0 (ns~ms) 0 fs 50 fs a b cd e k l –3 –100 –100 n (cm ) 2.000E+21 –200 –200 2 μm 2 μm 5 μm 10 μm Single pulse 20 μm t = 0 t = 20 t = 45 t = 90 t = 150 e e e e e –300 –300 1.500E+21 (fs~ps) f gh ij –400 –400 –1.5 –1.0 –0.5 0.0 0.5 1.0 1.5 –1.5 –1.0 –0.5 0.0 0.5 1.0 1.5 1.000E+21 0 0 100 fs 350 fs Double pulse m n 2 μm 2 μm 5 μm 10 μm 20 μm t = 0 t = 20 t = 45 t = 90 t = 150 –100 –100 =350 fs e e e e e 5.000E+20 –200 –200 0.000 –300 –300 –400 –400 –1.5 –1.0 –0.5 0.0 0.5 1.0 1.5 –1.5 –1.0 –0.5 0.0 0.5 1.0 1.5 Radius (μm) op 20 20 Original beam Original beam Transmitted beam Transmitted beam O 16 16 Si 12 12 8 8 t u 4 4 D D 1 2 3.0 0 0 0 100 200 300 400 500 600 0 100 200 300 400 500 600 Time (fs) Time (fs) 2.6 qr 20 20 Original beam Original beam 2.2 Transmitted beam Double pulse 16 Transmitted beam 16 12 12 1.8 8 8 Single pulse 1.4 200 400 600 800 1000 1200 0 500 1000 1500 2000 0 0 –1 0 100 200 300 400 500 600 0 100 200 300 400 500 600 Wave number (cm ) Pulse delay (fs) Time (fs) Time (fs) Figure 9 Morphology evolution of the sample surface exposed by (a–e) a femtosecond laser single pulse; (f–j) femtosecond laser double pulses at different stages of the etching process, the pulse delay is 350 fs. The SEM images have varying scale bars. (k–n) Simulation of the free electron density distributions and (o–r) center laser intensity distributions in fused silica irradiated by femtosecond double pulses at different pulse delays. (s) Schematic diagram of the manufacturing and Si-O bond structure. (t) Normalized Raman spectra of modiﬁed regions irradiated using femtosecond laser single and double pulses (the pulse delay is 350 fs) in fused silica. Dashed lines below the D2 peaks are baselines used in the peak area measurement in u. Inset is the schematic diagram of 4- and 3-membered ring structures. (u) Percent area of the total reduced Raman spectrum under the D2 line versus different pulse delays. The − 2 femtosecond laser with wavelength of 800 nm, duaration of 50 fs and repetition rate up to 1 KHz. The laser ﬂuence is ﬁxed at 9.46 J cm in all experiments and the energy distribution ratio is 1:1. Reproduced from Ref. 121 (with the permission of NPG). remains constant in the molecular dynamics model, even though it in validated the concept of EDC and predicted its potentials in ultrafast 88–107 laser micro/nanofabrication . fact changes dramatically during strong ionizations. The ab initio method is therefore used to describe a system with no parameteriza- Simulation results tion, which facilitates the investigation of processes whose mechanisms In this section, we report several representative simulation results. As are not fully understood. In a typical ab initio method, the potential shown in Figure 2, the time-dependent electrons dynamics in energy for a particular electronic state is deﬁned by the electronic materials irradiated by temporally shaped femtosecond laser pulse Schrödinger equation. However, the computational demand of ab train have been calculated based on TDDFT and the improved two- initio calculation is substantial, and impact ionization is not considered temperature model . The simulation results indicated that increasing because the theory is based on orbitals that interact only through the pulse delay from 10 to 30 fs markedly increased the number of excited mean ﬁeld. The exact time-dependent xc potential and the functional electrons and absorbed energy (Figure 2a). However, the excitation for physical observables are still being studied. energy decreased as the sub-pulse number per train increased from 1 On the basis of the aforementioned factors, during femtosecond to 4 (Figure 2b). Furthermore, the shaped pulse energy and laser-material interactions, the target material can be conceptually wavelength distribution were also determined to be key parameters divided into three systems: the electron system, atom or molecule for changing the electron density and temperature (Figure 2c and 2d). system and plasma system outside the bulk material. The ab initio The results indicated that localized transient electrons dynamics, model can be applied to sub-nanometers sampling areas to determine including the number of excited electrons, excitation energy, electron the ionization mechanisms and multiphoton absorption cross density distribution, and electron temperature, can be controlled by section . The revised molecular dynamics can be employed to shaping femtosecond pulses (pulse delay, sub-pulse number, pulse determine the phase change mechanisms in areas of tens to hundreds energy ratio and dual-wavelength distribution). of nanometers within an electric ﬁeld formed by the plasma system We theoretically demonstrated that localized transient materials outside the bulk material, which is calculated using our plasma properties can be effectively modulated through localized transient model . The plasma model can be used to describe the electron EDC by temporally shaping femtosecond laser pulses (Figure 3). The system for photon absorption and plasma generation (ionization) and reﬂectivity and corresponding peak laser intensity distribution were 110,114 recombination before the lattice actually changes .Our improved completely changed by varying the pulse delays. Moreover, the two-temperature model can be employed to calculate energy transport electron density was adjusted by varying the pulse delay, and 116,117 through electron-phonon interactions .Using themodels,we reﬂectivity decreased as the pulse delay increased because of the free Light: Science & Applications doi:10.1038/lsa.2017.134 Intensity (arbitrary units) Percent area of the reduced spectrum –1 under D =605 cm line Depth (nm) 13 –2 13 –2 Laser intensity (10 W cm ) Laser intensity (10 W cm ) 13 –2 13 –2 Laser intensity (10 W cm ) Laser intensity (10 W cm ) Electrons dynamics control LJiang et al 1.05 F 1.16 F th th Δt = 0 fs Δt = 50 fs Δt = 0 fs Δt = 100 fs Δt = 300 fs a b c eg 2 μm2 μm 1 μm 4 μm 1 μm 1 μm Δt = 50 fs Δt = 200 fs Δt = 400 fs df h 1 μm 1 μm 1 μm 1 μm 4 μm 1 μm 1.8F 2F 0.6F 1.2F ij k th l th th th 2 μm 2 μm 2 μm 2 μm AFM profile 2F m 2F n 2F o th th th –0.3 –0.4 LSFL(//E ) HSFL(⊥E ) –0.5 –0.6 –0.7 0.0 0.5 1.0 1.5 2.0 2 μm 2 μm 2 μm Distance (μm) Figure 10 Control of period, orientation and topology of the LIPSS on the surface of fused silica via symmetrically shaped femtosecond pulses. (a–h)LSFL changes to HSFL with different orientation under certain pulse ﬂuences and pulse delays on fused silica .(i–o) SEM images of representative HSFL (i–k), LSFL (m–o) and double-grating structure (l) on fused silica for single-pulse trains (Ns= 1), double-pulse trains (Ns= 2), triple-pulse trains (Ns= 3) and quadruple-pulse trains (Ns= 4), respectively . Reproduced from Ref. 122 (with the permission of OSA) and Ref. 123 (with the permission of OSA). electron density was lower. Thus, the original laser intensity distribu- slightly above the volumetric phase change threshold ﬂuence of single tion was considerably reshaped, providing substantial control over the pulse ablation. Therefore, the results demonstrated that the phase energy absorption process. The results demonstrated that by control- change mechanism can be controlled by carefully designing the pulse ling localized transient electron dynamics through shaping femtose- train parameters. cond pulses, the localized transient material properties, such as the The ablation shape modulated by designed femtosecond pulse trains 110–112 on fused silica was validated using our plasma model . The crater reﬂectivity and absorption coefﬁcient, can be modiﬁed. Meanwhile, we studied the phase change mechanisms in materials shapes of the fused silica and the spacing of the subwavelength ripples such as nickel thin ﬁlms under irradiation by femtosecond laser pulse differed greatly among sub-pulses with different delay times, wave- trains by using molecular dynamics simulations and the two- lengths and energy distributions. The ablation depths were approxi- temperature model . The theoretical simulation revealed that more mately 170, 145, 102 and 91 nm for pulse delays of 0, 25, 50 and 75 fs, and smaller high-quality uniform nanoparticles can be obtained on the respectively (Figure 5a). The ablation depths were approximately 186, nickel thin ﬁlm by femtosecond laser pulse trains compared with 43 and 139 nm, and the ablation radii were approximately 860, 630 conventional pulses (Figure 4a–4h). The use of pulse train reduced the and 410 nm for pulse trains of 400+800, 800+400 and 800+800 nm, − 2 electron temperature and electron thermal conductivity dramatically respectively (Figure 5b). For 4.0 J cm , the depths of the microholes because of the lower intensity of the sub-pulses or higher transient were 77, 84 and 71 nm (depth from the surface to the position of line surface temperatures, which left the absorbed energies deposited AB), and the corresponding radii of the ablation craters were 250, 160 mainly within the nanoscale layers of the dynamic ﬁlm surface and 270 nm for the energy ratios of 1:2, 1:1 and 2:1, respectively; For − 2 (Figure 4i and 4j). By designing the pulse train, smaller ﬁlm 5.0 J cm , the depths of the microholes were 80, 95 and 70 nm compressive stresses and tensile stresses can be obtained, which (depth from the surface to the position of line CD), and the reduced microcracks (Figure 4k and 4l). Furthermore, a transition corresponding radii of the ablation craters were 430, 410 and from phase explosion to critical point phase separation (Figure 4m– 440 nm for the energy ratios of 1:2, 1:1 and 2:1, respectively 4p) enabled small uniform nanoparticle generation. The modulated (Figure 5c). Calculations based on the plasma model revealed that phase change mechanism was considered the main factor for the the time-dependent free electron density differed substantially among morphology control. Both the compressive and tensile stresses can be laser pulses with different parameters. Changing the ionized electron reduced by the pulse trains, leading to the critical point phase density distributions signiﬁcantly modiﬁed the optical and thermal separation within the uppermost ﬁlms and no liquid-vapor phase properties of the material. This interaction process greatly altered separation within the subsurface ﬁlms when total laser ﬂuence was ablation shape and subwavelength ripple. The results indicated that by doi:10.1038/lsa.2017.134 Light: Science & Applications Ns=1 Ns=1 Ns=2 Ns=2 Ns=3 Ns=3 Ns=4 Depth (μm) Electrons dynamics control LJiang et al 1.0 μm 1.0 μm a b f Δt Δt ~ ms Y 2nd 1st 2nd 1st hv + + – Ag +H O Ag+H +OH Ripple Fs laser Fs laser pulse train Nanopillar array AgNO Z Water 3 Y Si Si Y STEP TWO STEP ONE Reference 0 fs Δt =0 fs 400 fs Δt =800 fs 600 fs 800 fs 500 nm 500 nm 18 1000 fs ik 60 8 0 fs 600 fs 800 fs 1000 fs Reference 0 40 80 120 160 180 240 600 1000 1400 1800 –1 Partical diameter (nm) Raman shift (cm ) 8 500 nm 500 nm h 6 –1 Raman shift (cm ) 0 fs 400 fs 600 fs 800 fs 1000 fs 610 9.2E+06 6.2E+07 4.5E+07 2.2E+07 1.9E+07 1310 7.1E+06 2.7E+07 1.9E+07 8.4E+07 1.2E+07 1360 1.1E+07 4.8E+07 3.3E+07 1.6E+08 2.0E+07 600 800 1000 1200 1400 1600 1800 1510 1.1E+07 5.2E+07 3.5E+07 1.6E+08 2.3E+07 –1 Raman shift (cm ) 1650 8.0E+06 4.7E+07 3.0E+07 1.5E+08 1.8E+07 Figure 11 (a–e) One-step method fabrication of controllable SERS substrates .(a and b) SEM images of the silicon irradiated at pulse delays of 0 fs, and 800 fs, respectively. (c) Size distribution of silver nanoparticles in a (black), and b (red). (d) SERS signals of R6G molecules on the as-prepared substrates at various pulse delay. (e) Enhancement factors with different pulse delays. (f–k) Two-step method fabrication of controllable SERS substrate . f Schematic diagram of the SERRS substrate fabrication process. (g–j) SEM images of silicon substrates irradiated at pulse delays of g 0 fs, and i 1000 fs in deionized water, (h) 0 fs and (j) 1000 fs in 10-mM silver nitrate solution. (k) SERS spectrum of substrates fabricated at different pulse delays. Reproduced from Ref. 125 (with the permission of OSA), and Ref. 126 (with the permission of OSA). controlling femtosecond pulses trains, the ﬁnal material modiﬁcation us to further modify the chemical and physical properties of the can be improved. materials . By spatially shaping a femtosecond laser, a metal The model simulations validate theoretical feasibility of EDC, nanowire with a super sub-diffraction-limit precision (1/14th of the providing a theoretical prediction and guide for practical experiments wavelength) can be achieved . and applications. Furthermore, our models and subsequent multiscale measurements can form a mutually-supporting system. According to Femtosecond laser temporal pulse shaping the theoretical prediction, a novel fabrication method based on During femtosecond laser fabrication, most of the photon energy is temporally and spatially shaped pulse trains was proposed. Localized initially absorbed by electrons. However, conventional femtose- cond laser pulses are separated by a time scale that ranges from transient electron dynamics and the corresponding material properties microseconds to milliseconds (Figure 6), which is much longer can be actively controlled based on the proposed method. We discuss than the time scale of electron-lattice coupling (typically a few several pieces of experimental evidences in detail in the following sections. picoseconds to tens of picoseconds). Temporal pulse shaping enables sub-pulse generation with a pulse delay shorter than the EXPERIMENTS: EDC-BASED NOVEL FABRICATION METHOD characteristic time scale of electron-lattice coupling so that we can To validate the proposed method of EDC-based fabrication method control femtosecond laser photon–electron–phonon interactions. through temporal and spatial shaping of femtosecond laser pulses, As Figure 6a shows, a conventional femtosecond laser pulse can be 119–128 many experiments have been conducted . By designing tempo- split into several sub-pulses with a delay, which is called the pulse rally shaped pulse trains, we can control the localized transient train. The separation between the sub-pulses occurs in the time electron density to induce resonance absorption; thus, high efﬁciency scale—tens of femtoseconds to several picoseconds—which is 119,120 fabrication can be achieved . We can adjust the phase change generally similar to the characteristic time scale of electron- mechanism from thermal phase change to nonthermal phase change; lattice coupling. Before the photon energy that is absorbed by subsequently, high-quality fabrication can be performed .In addi- the electrons transfers to lattice, the subsequent sub-pulses con- tion, we can modify the free electron density, the corresponding tinuously interact with the materials so that controlling ultrafast photon-absorption efﬁciency and the material properties, enabling us photon-electron-phonon interactions is possible. Moreover, a to further control the chemical reaction . We can adjust the electron conventional femtosecond laser single pulse can be shaped into density and its distribution so that the periods, orientations and almost any arbitrary pulse shapes; for example, (1) a pulse can be 122,123,140 structures of the surface ripples can be effectively modulated split into a pulse train with different numbers of sub-pulses and a high sensitivity of SERS substrate (controllable micro/nano (Figure 6b); (2) the delay between the sub-pulses can be controlled 124–125 hierarchical structures on materials’ surfaces) can be achieved . (Figure 6c); and (3) the energy ratio of the sub-pulses can be Furthermore, we can control the electron density distribution controlled (Figure 6d). By shaping a femtosecond laser pulse in to induce surface chemical reduction activity in materials, enabling temporal domains to obtain a speciﬁc pulse shape, we can control Light: Science & Applications doi:10.1038/lsa.2017.134 Number Intensity (×10 a.u.) Intensity (×10 a.u.) Electrons dynamics control LJiang et al ab c d (ns~ms) 500 nm Single pulse 500 nm 500 nm 500 nm t = 0 t = 20 s t = 1 min t = 30 min r r r r ef g h (fs~ps) Double pulses ( = 5ps) 500 nm 500 nm 500 nm 500 nm t = 0 t = 20 s t = 1 min t = 30 min r r r r ij k MoS modified by temporally MoS modified by conventional 2 S 2p 2 1/2 S 2p 1/2 shaped femtosecond pulses femtosecond pulses Mo/S=1:4.28 Mo/S=1:2.85 Unbound Unbound sulfur sulfur (27.78%) (27.74%) S 2p 3/2 S 2p 3/2 4+ 4+ MoS film 172 171 170 169 168 167 166 165 164 163 162 161 160 159 158 172 171 170 169 168 167 166 165 164 163 162 161 160 159 158 Binding energy (eV) Binding energy (eV) l m n 3+ Au Shape-controlled Dangling bond Au NPs Unsaturated Saturated S atoms 500 nm Fs laser treated MoS S atoms Mo S 2 Figure 12 Morphology evolution of gold NPs reduced on MoS surfaces irradiated by femtosecond (a–d) single and (e–h) double pulses, at different stages of the reduction process, where tr represents the chemical reduction time, the pulse delay is 5 ps. (i) Schematic diagram of the manufacturing and Mo-S bond structure. XPS S 2p spectra of modiﬁed regions irradiated by femtosecond laser (j) double and (k) single pulses on MoS , where the percentage value represents the content of unbound sulfur and the atomic ratio represents the relative atomic concentration ratio of Mo and S atoms. (l) AFM image and (m) atomic scale schematic of the laser-broken micro/nano MoS debris. (n) Mechanism of chemical reduction of gold cations on laser-treated MoS 2 2 (Ref. 127). Reproduced from Ref. 127 (with the permission of ACS). the photon-electron interactions, which allow us to control the Temporal shaping of the femtosecond laser pulses can be achieved localized transient electron dynamics, including electron density, by using a commercial 4f-conﬁguration-based pulse shaper (BSI temperature, excited state distribution and further modify loca- MIIPS BOX 640, Biophotonic Solutions, Inc., East Lansing, MI, lized transient material properties, adjust material phase changes USA), which allows us to split each conventional single pulse into a and ultimately implement the novel fabrication methods. pulse train and control the number of sub-pulses, delay between sub- pulses and energy ratio of the sub-pulses within a pulse train. The Experimental setup for temporal pulse shaping.The schematic of irradiation time (that is, number of pulse bursts) is precisely controlled the experimental setup for temporal shaping of femtosecond laser by using an electromechanical shutter. The sample is mounted on a pulses is shown in Figure 7 (Ref. 141). An ampliﬁed Ti: sapphire laser computer-controlled, six-axis translation stage (M-840.5DG, PI, Inc., system (Spectra Physics Inc., Santa Clara, CA, USA) is used to generate Karlsruhe, Germany) with a positioning resolution of 1 μm. The entire 35 fs (full width at half maximum) linearly polarized laser pulses on a fabrication process can be observed by using a charge-coupled device central wavelength of 800 nm with a repetition rate of 1 kHz. In (CCD) camera along with a white-light source irradiate on the sample addition, the pulse energy can be up to 3.5 W and can be continuously surface. adjusted by combining a half-wave plate with a polarizer. Pulse energy can also be reduced to the desired values according to speciﬁc High-efﬁciency fabrication by temporal pulse shaping based on experimental conditions by using a neutral density (ND) ﬁlters. EDC. By designing femtosecond laser pulse trains on the basis of doi:10.1038/lsa.2017.134 Light: Science & Applications Intensity (a.u.) Intensity (a.u.) Electrons dynamics control LJiang et al 0.40 WS CCD 0.36 BS 0.32 SLM 0.28 DM 0.24 0.20 Phase pattern 0 200 400 600 800 Lens Pulse delay (fs) bc 0 3 μm μm –200 –200 Translation stage Figure 15 Schematic diagram of the experimental setup. WS: white light nm nm source; BS: beam splitter; DM: dichroic mirror; L1, L2: two convex lenses consisting of a 4f relay system; L3: convex lens; Inset: section of the de samples and the focusing laser in the yz plane . Reproduced from Ref. 129 (with the permission of Wiley). Figure 8a, when ﬁve 355 nm nanosecond laser pulses irradiated on fused silica, no apparent damage was observed because the photon energy (B3.5 eV) of the 355 nm wavelength is lower than the bandgap (B8.9–9.3 eV) of fused silica, resulting in little absorption of laser energy .For ﬁve femtosecond laser pulses, a shallow hole was machined (Figure 8b), because of the low-efﬁcient 800 nm (B1.55 eV) photon absorption through multiphoton ionization in Figure 13 (a) Dependence between the recast ratio (recast area/ablation wide bandgap dielectrics. In Figure 8c, femtosecond-nanosecond dual- area) and pulse delay on fused silica fabrication using femtosecond laser beam laser manufacturing revealed that a much higher fabrication pulse trains consisting of two identical sub-pulses with an identical total ﬂuence. AFM proﬁles of the structures of the fused silica fabrication using efﬁciency (that is, a 50.7-fold enhancement in material removal (b) a conventional single pulse and (c) femtosecond laser pulse train with a volume) was obtained. This enhancement was attributed to the high pulse delay of 300 fs. SEM images of the structures on fused silica free electron density generated by the femtosecond laser pulses, which fabrication using a femtosecond laser pulse train with different energy ratios leading to the signiﬁcantly-increased absorption of the nanosecond between the two sub-pulses: (d)1:1 and(e) 2:1. The femtosecond laser laser pulses energy. However, the femtosecond-nanosecond dual-beam with wavelength of 800 nm, duration of 35 fs and repetition rate up to − 2 system was complex, and the quality of the as-fabricated holes could 1 KHz. The total ﬂuence of the pulse trains in all experiments is 5 J cm ; not be guaranteed. To date, temporally shaped femtosecond laser the scale bar in d and e is 500 nm. Reproduced from Ref. 128 (with the permission of OSA). double-pulse train was used to manufacture high-quality microholes with high-efﬁciency . Through temporal pulse shaping, free electron density can be adjusted to be around the critical point, at which the laser frequency equal to the plasma frequency, nearly optimizing to the resonance absorption so that the fabrication efﬁciency is enhanced 56 folds and the aspect-ratio is enhanced 3 fold (Figure 8d). Chemical etching controlled by temporal pulse shaping based on EDC. By designing femtosecond pulse trains on the basis of EDC, Flat-top beam Vortex beam the free electron density and corresponding photon-absorption efﬁciency can be modiﬁed. Thus, the material properties can be modiﬁed to improve the etching rate of fused silica (Figure 9). Compared with conventional femtosecond laser pulses, femtosecond Gaussian beam laser double-pulse trains achieved a 37-fold enhancement in the laser- assisted chemical etching rate. Simulations indicated that by optimiz- Bessel beam Airy beam ing the pulse delay between the two sub-pulses, the free-electron density can be modiﬁed, leading to the change of localized transient Figure 14 Shaping conventional Gaussian beam into different beam types. material properties, such as the physical properties (that is, reﬂectiv- ity), so that the laser ﬁeld was reshaped, then the free electron density EDC, we can control the localized transient electron density to induce distributions (Figure 9k–9n) and absorbed laser intensity distributions resonance absorption between laser and its generated plasma; thus, (Figure 9o–9r) can be controlled, contributing to the enhancement of 119,120 high efﬁciency fabrication can be achieved .As illustrated in the photon absorption efﬁciency and result in the modiﬁcation Light: Science & Applications doi:10.1038/lsa.2017.134 Recast ratio Electrons dynamics control LJiang et al ae Ablation area Metal nanowire 112 nm 190 nm 56 nm 1 μm 40 nm 20 nm 10 nm 0.0 0.5 1.0 1.5 2.0 2.5 Energy (keV) 5 μm cd f –200 –100 0 100 200 X (nm) (nm) Figure 16 Fabrication of nanowire by spatial pulse shaping. (a) Single spot fabricated by the shaped beam. (b) Scanning electron microscope (SEM) images of nanowire. (c and d) AFM images of the nanowire and its cross section. (e) EDXS measurements of the metal nanowire and the ablation area. (f) Five-ring patterns fabricated by the proposed methods . Reproduced from Ref. 129 (with the permission of Wiley). a b Electrode pads –2 220 J m –2 260 J m –2 –4 305 J m Resistor –2 345 J m –2 –8 390 J m –1000 –500 0 500 1000 Current (μA) Figure 17 Testing of the resistivity of the nanowire. (a) The SEM images of the nanowire and the electrode pads. (b) The volt-ampere characteristics of the nanowires. Reproduced from Ref. 129 (with the permission of Wiley). improvement in the irradiated zone. We also conducted micro-Raman which in turn improve the manufacturing efﬁciency. Overall, femto- spectroscopy to characterize the internal structure of the sample. As second laser temporal pulse shaping fabrication that is based on EDC Figure 9s–9u shows, by optimizing the pulse delays between the two represents a preliminary attempt to control the chemical reaction. sub-pulses, we can adjust the chemical properties (such as the Si-O bond structure) in irradiated material, which results in a higher Modulation of femtosecond laser-induced periodic surface structures based on EDC. On the basis of the aforementioned theory on EDC, number of 3- and 4-membered ring structures in double pulses we demonstrated that femtosecond laser-induced periodic surface modiﬁed regions than that in single pulse modiﬁed regions. These structures (LIPSS, also referred as ripples) can be deliberately results lead to the increases in the reactivity of the oxygen atoms, modulated by controlling the electron density and its distribution which contributes to a higher etching rate induced by femtosecond via designed femtosecond laser pulse trains. LIPSS have been studied laser double-pulse train. In short, by varying the pulse delay of pulse 142–144 extensively in various materials, including semiconductors , trains, we can control the localized transient free electron dynamics, 145 110,146,147 including bound electron ionization, free-electron density, tempera- metals and dielectrics , because of their promising 124,148–154 ture and excited state distribution, and further modify localized applications . The periodicity, orientation and structure are transient material properties, such as physical and chemical properties, the typical parameters in the study of ripples. According to its doi:10.1038/lsa.2017.134 Light: Science & Applications Height (mm) Voltage/mV Intensity/(Arb. Unt.) Fs laser system Electrons dynamics control LJiang et al Coulomb explosion Electrostatic ablation Plasma expansion Impact ionization Shockwave Radiation Electron heating Structure evolution Photoionization Vaporization/Melting Time scale (s) –15 –14 –13 –12 –11 –10 –9 –8 –7 –6 –5 –4 –3 –2 –1 0 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 Time-resolved plasma ICCD/CCD Femtosecond laser pump-probe photography and LIBS imagery Multipulse induced Plasma expansion Laser propagation Early plasma and plasma evolution shockwave expansion Ejected material composition Laser-induced free Multipulse induced elctrons evolution Energy deposition Plasma density and temperature structure evolution a b Beam splitter Half wave plate Fs laser CCD Mirror Shutter Polarizer Illuminator Shutter Half wave plate Mirror Delay Pump stage Polarizer Lens Lens Lens Objective filter BBO Filter Probe Z 20× X Y PMMA CCD PMMA c d Lens Lens ICCD CCD Illuminator Zoom lens Zoom lens Objective Objective PMMA PMMA Figure 18 Schematic of multiscale measurement of femtosecond laser drilling, including laser propagation and laser-induced material excitation, plasma and shockwave evolution and hole formation and so on. (a) Pump-probe shadowgraph imaging technique. (b) Laser-induced breakdown spectroscopy (LIBS). (c) Time-resolved plasma photography with gated intensiﬁed charge-coupled device (ICCD). (d) Industrial continuous imagery. 159,170–172 periodicity, LIPSS can be divided into low spatial frequency LIPSS state into a metallic state . Subsequently, at the interface (LSFL) and high spatial frequency LIPSS (HSFL). It is now widely between the metallic state surface and air, SPs can be excited by the accepted that the excitation and propagation of surface plasmon coupling between the surface electrons of the irradiated sample and 155–157 polaritons (SPPs) plays a crucial role in LSFL formation .The the incident ﬁeld when the real part of the dielectric function is less formation of LSFL is affected by the initial laser-SPPs interference and than − 1 (Ref. 173). The SPs are characterized by surface electro- 158,159 the subsequent grating-assisted SPPs-laser coupling effect .Up to magnetic waves so that the coupling ﬁeld is a superposition of the now, the formation mechanism of HSFL is still under investigation. incident ﬁeld and the SP ﬁeld. When the free electron density reaches Recently, Wang et al demonstrated that structure evolution of LSFL 21 − 3 the critical density (B1.74 × 10 cm for the wavelength of and HSFL is highly dependent on the localized effective laser ﬂuence, 800 nm), the SPs can be resonantly excited . The SPPs excitation which determines the instantaneous optical permittivity by the laser- and resonance can reshape the laser intensity distribution in the excited electrons creating an active plasma layer. In general, the material and affect the subsequent linear/nonlinear ionization process. 147,161 formation mechanisms include self-organization , second har- Thus, the transient free electron density and its distribution is the key 162–164 165 monic generation (SHG) third harmonic generation (THG) , factor that affects SP excitation and properties, and ultimately, the 166 167 168 excitation of SPPs ,split , Coulomb explosion and cavitation 110,147,173 corresponding ripple formation . Meanwhile, the increased instability and so on. When a femtosecond laser irradiates the localized electron density can be further affected by trapping, diffusion surface of dielectric/semiconductor materials, free electrons can be and recombination with a time scale of several hundred femtose- generated, leading to the formation of electron-hole plasma (surface conds. Therefore, the time delay within the picosecond timescale is plasma) with time scales shorter than the electron-phonon relaxation proposed to control the electron dynamics to modulate the electron time. The localized transient free electron density is rapidly increased density and distribution, thus to modulate the resulted ripple through linear and nonlinear (multiphoton and avalanche) ionization, structures. leading to the material transforming from a dielectric/semiconducting Light: Science & Applications doi:10.1038/lsa.2017.134 Electrons dynamics control LJiang et al Femtosecond-picosecond Picosecond-nanosecond Air probe 600 ps 4 ns 16 ns ab c d e fg h 1.0 SF 0.9 100 fs 300 fs 500 fs 700 fs 900 fs 0.8 Air 0.7 PMMA 60 μm 0.6 60 μm PMMA 0.5 Laser propagation and maetrial excitation Plasma and shockwave expansion Nanosecond-submillisecond Millisecond-second Air Air Air 200 μm 500 μm PMMA PMMA PMMA Air Air Air 0 ms 0.5 s 5 s 20 s 33 s 45 s 60 s PMMA PMMA PMMA Plasma images above the surface Plasma images in the hole Multiple pulse induced structure in second scale 200 μm 0 s 10 s 20 s 30 s 40 s 50 s 60 s 70 s 80 s 90 s 100 s 110 s 120 s Figure 19 Multiscale measurement results of deep-hole drilling process in PMMA. Multiscale measurement results of deep-hole drilling process in PMMA with 100 μJ pulse energy focused by plano-convex lens (f= 100 mm). The dynamics include femtosecond-picosecond electron excitation, picosecond– nanosecond plasma and shockwave evolution and multiple pulse-induced structure in second scale. Optimal EDC using suitably shaped temporal pulse trains thus orientation parallel to the laser polarization, were obtained by gives the possibility to modulate the LIPSS artiﬁcially, offering changing the pulse delay (Δt) and pulse ﬂuence (Figure 10 upper extended ﬂexibility in material processing. Studies show that for panel). Thus, three types of LIPSS under speciﬁc conditions can be femtosecond (fs) laser pulse train processing of materials, the pulse obtained. (1) LSFL with orientation parallel to the laser polarization delay between sub-pulses strongly impacts the formation of direction; (2) HSFL with orientation parallel to the laser polarization 7,176–179 176–178 nanostructures , especially the morphology of LIPSS . direction with low pulse ﬂuence; and (3) HSFL with orientation Here, we performed relevant experiments as examples on the surface perpendicular to the laser polarization direction at higher pulse 122,123 of dielectrics to illustrate the aforementioned mechanisms . ﬂuence. The experimental results indicate that: (1) at lower pulse Fused silica was used as a dielectric material in a case study on the ﬂuences, a transition from LSFL to HSFL occurred at a pulse delay of control of the LIPSS period, area and orientation. For conventional 50 fs with a decrease in area (Figure 10a and 10b,); (2) whereas, at femtosecond laser irradiation, only LSFL with an orientation parallel higher ﬂuences, LSFL were replaced by another type of HSFL with an to the laser polarization and HSFL with an orientation perpendicular orientation perpendicular to the laser polarization at Δt4100 fs to the laser polarization were obtained on fused silica depending on (Figure 10c–10h). The average periods of LSFL and HSFL were the laser ﬂuences (F) or pulse number (N). Nevertheless, compared 560± 8 nm and 255± 30 nm, respectively. with the conventional situation, both types of ripples with controllable During processing, the second sub-pulse of the doublepulse train periods, areas and orientations, especially the HSFL with an signiﬁcantly affects the free electron density and distribution generated doi:10.1038/lsa.2017.134 Light: Science & Applications Commercialized CCD imaging Plasma photography Pump and probe Electrons dynamics control LJiang et al 3 3 2 2 C swan (d Π -a Π ) CN (B Σ–X Σ) 2 g u SP DP @ 80 ps Δv =0 Δv =0 Na I Ca I Δv =+1 Ca II 2 2 CH (A Δ–X Π,Δv =0) Δv =–1 Δv =–1 380 400 420 440 460 480 500 520 540 560 580 Wavelength (nm) bc 75 8500 70 8000 65 7500 60 7000 55 6500 50 6000 45 5500 0.1 1 10 20 40 60 80 100 120 0.1 1 10 20 40 60 80 100 120 DP delay (ps) DP delay (ps) − 2 Figure 20 Typical spectra of PMMA plasma irradiated by a single pulse and a double pulse with the same total ﬂuence of 7.8 J cm . Reproduced from Ref. 134 (with the permission of SPIE). by the ﬁrst sub-pulse, thereby inﬂuencing the mechanism of LIPSS recombination, which is determined by the electron density. By formation and the surface morphology. Consequently, by controlling controlling the pulse delay, the electron occupation is adjusted , the pulse delay and pulse ﬂuence, we can control excited electron leading to the manipulation of electron density, thus facilitating production, distribution, motivation and the interaction between SHG and resulting in a 50% cut in LIPSS periods; surface plasmon (SP) and the incident laser, then to control the 3) The strong decrease in the ripple area was due to the electron periodicity, orientation and morphology of the LIPSS. decay. The free electrons in the conduction band excited by the ﬁrst sub-pulse relaxed and returned to the valence band during the 1) LSFL obtained here at a low pulse ﬂuence were oriented parallel to time interval between the two pulses, in terms of diffusion and 184–186 the laser polarization, which cannot be explained by SPs recombination , leading to the reduced electron density; 147,180 excitation . According to calculations based on the Sipe- therefore, the energy coupling by the second sub-pulse to the 180 53 Drude model and pump-probe results , femtosecond energy excited material decreased, leading to the decay of ablation, which deposition by the ﬁrst/previous pulses can occur at speciﬁc (LSFL) resulted in the reduced rippled area; spatial frequencies, reshaping the electron density distribution 4) The transition of HSFL at a higher pulse ﬂuence was mainly along the polarization direction, and then determines the forma- attributed to the periodic plasma enhancement of the incident laser tion of LSFL with an orientation parallel to the polarization ﬁeld, which is related to the excited free electron density. When the direction. Meanwhile, thermal effects also play a critical role in pulse delay was low (o100 fs), the effects of induced SPs on the subsequent material removals at the ﬂuences for LSFL incident laser were insufﬁcient due to the low absorbed intensity. 181–183 generation ; With pulse delays of 100–500 fs, however, SPs excitation can easily 2) The periods of LSFL and HSFL (with orientation parallel to the be achieved at the initial stage of the second sub-pulse due to the polarization direction) were close to the fundamental (λ/ accumulation of the ﬁrst sub-pulse (see modeling section n= 551 nm) and second-harmonic (λ/2n= 275 nm) wavelengths for details). The interaction between SPPs and the incident laser in fused silica with lower pulse ﬂuence. Thus, SHG plays a key role ﬁeld resulted in the periodic modulated intensity enhancement at in HSFL formation with an orientation parallel to the polarization the surface. The incubation effects with multiple bursts irradiation 155,156 direction at a lower pulse ﬂuence. SHG is a result of electron led to the evolution of the local intensity distribution along the Light: Science & Applications doi:10.1038/lsa.2017.134 Intensity (a.u.) 16 –3 Electron density (×10 cm ) Temperature (K) Electrons dynamics control LJiang et al 0 ps 0.2 ps 0.3 ps 0.4 ps 0.5 ps 0.6 ps 0.7 ps 0.8 ps 2 ps 6 ps 10 ps 40 ps 80 ps 120 ps 1mm –2 3.3 J cm –2 4.4 J cm –2 6.6 J cm –2 8.8 J cm –2 11 J cm bc 2.6 –2 –2 6.6 J cm 6.6 J cm –2 –2 8.8 J cm 8.8 J cm –2 2.4 –2 11 J cm 11 J cm 2.2 2.0 1.8 1.6 1.4 0 0.2 1 10 40 80 120 0 0.2 1 10 40 80 120 Interpulse delay (ps) Interpulse delay (ps) Figure 21 Characterizations of femtosecond laser double pulse induced plasma of fused silica (a) plasma images, (b) electron densities and (c)plasma temperature. electric ﬁeld direction , resulting in HSFL formation at a speciﬁc analysis of chemical and biological species because of its high 188,189 period with an orientation perpendicular to the electric ﬁeld sensitivity and ﬁngerprint-identiﬁcation features . Previous stu- direction. dies have demonstrated that surface morphologies (for example, 190,191 ripples, nanoparticles and nanopillars) play key roles in SERS More complicated morphology control of LIPSS on fused silica can enhancement in which the enhanced electromagnetic ﬁeld on surface be obtained by EDC via shaping the conventional femtosecond laser nanostructures induced by the localized surface plasmons resonance 191–193 pulses into symmetrical triple- and quadruple-pulse sequences, (LSPR) effect dominated . Consequently, tailoring the surface structures into different morphologies and sizes, in this aspect, is especially, double-grating structures and an HSFL period as small as signiﬁcant for tuning LSPR features to further improve the sensitivity 190nmwereobtained(seeFigure10i–10o) . In addition, the and push SERS devices into practical applications. As shown in geometric morphology modulation on Si can also be obtained by Figure 11d, the SERS intensity gradually reached a maximum when EDC via adjusting double pulse delay . Therefore, the aforemen- the pulse delay was increased from 0 to 800 fs, and then decreased tioned experimental studies demonstrate that by designing a femto- when the pulse delay was further increased to 1000 fs . Although the second laser pulse train, the electron dynamics can be controlled, that small changes (that is, the modulation of pulse delays) occurred in the is, electron density, distribution, thus to control the coupling between incident pulses, the obtained surface structures signiﬁcantly improved the SPP and incident laser, and ﬁnally guide the material response the signal intensity (Figure 11e). Compared with the conventional (LIPSS morphology) towards user-designed directions with various femtosecond laser ablation (Δt= 0 fs), the designed pulse train could morphologies (for example, periods, orientations, distributions and reduce more silver nanoparticles (Figure 11a and 11b) and lead to a geometric morphologies). more uniform distribution of the nanoparticles deposited on the Detection sensitivity improvement of SERS based on EDC.By subwavelength ripples (Figure 11c). Consequently, the SERS sensitivity designing femtosecond laser pulse trains on the basis of EDC, we was improved. can control the localized transient electron density and its distribution To further conﬁrm that we controlled the properties of SPs, a two- to modulate the properties of SPs, then promote the energy transfer to step experiment was conducted using a designed pulse train, as materials and control the surface structures and photochemical schematically shown in Figure 11f . The laser polarization and pulse reduction process; thus, high detection sensitivity SERS substrates delay were synergistically controlled. Compared with the ripples formed can be achieved. SERS has been recognized as the most promising by conventional femtosecond laser pulses (Figure 11g and 11h), regular trace analyte detection method for rapid and accurate label-free nanopillar arrays were generated by a double pulse train (i.e., Δt40fs) doi:10.1038/lsa.2017.134 Light: Science & Applications 16 3 Electron density (×10 cm ) Electron temperature ( °C) Electrons dynamics control LJiang et al M M Bessel beams ac 2 3 CCD2 0 1 0 0.5 –5 –5 Initial Bessel beams –10 0 500 1000 1500 –10 –5 05 10 DM y (μm) z (μm) PBS Axicon Gaussian beams 10 1 Micro-Bessel beams MO S2 HWP z 0 0.5 –5 –5 CCD1/ICCD L x 0 –10 0 0 500 1000 1500 –10 –5 05 10 Fs laser y M y (μm) z (μm) S1 b d Bessel beams Gaussian beams 1 μm 5 μm 5 μm 5 μm 3.5 2.5 5 μm 50 μm μm 10 μm 123456 μm Figure 22 (a) Schematic of the spatial shaping femtosecond laser pulses microdrilling setups; (b) Morphology images of microholes drilled with a single pulse Bessel beam and Gaussian beam, respectively; (c) Intensity distribution simulations of the Bessel beam and Gaussian beam; (d) The hollowness characterization of microholes drilled by single-pulse Bessel beam. Reproduced from Ref. 132 (with the permission of Springer). with the polarization direction rotated by 90°, as shown in Figure 11i on the basis of EDC, bound electrons can be ionized, chemical bonds and 11j. In the case of conventional femtosecond laser ablation between atoms can be interrupted, atoms can be selectively removed and material properties can be modiﬁed so that gold cations can be (Δt= 0), SP induced by the previous pulse of the subwavelength ripples would fast damp to its original state before the subsequent pulse spontaneously reduced on MoS surface , as shown in Figure 12. Compared with the conventional femtosecond laser pulses, the arrived because of the long-time interval (millisecond scale), leading to chemical reduction rate of gold cations on MoS surface modiﬁed the formation of large and non-uniformly distributed silver nanopar- by femtosecond double-pulse trains was signiﬁcantly enhanced (Figure ticles on conventional grating-like ripples. By contrast, because the 12a–12h). We conducted X-ray photoelectron spectroscopy (XPS) and pulse delay of a pulse train is shorter than the damping time of SP, the atomic force microscope (AFM) to characterize and analyze the properties of SPs would be signiﬁcantly controlled to change the energy element valence, atomic ratio and morphology of the sample, as transfer efﬁciency, which could contribute to the formation of showninFigure 12i–12m. When material was irradiated by the abundant small and uniformly distributed silver nanoparticles on the femtosecond laser pulses, electrons were excited from bonding to anti- nanopillar arrays. In addition, the larger enhancement of the incident bonding states (bound electrons were ionized), hence abundant laser electric ﬁeld on nanopillar arrays lead to generation of much more chemical bonds between atoms (such as Mo-S bonds) were instanta- silver nanoparticles, which resulted in a lager SERS signals (maximum neously weaken or even broken entirely, resulting in an integral enhancement factor up to 2.2 × 10 ). Overall, by designing a pulse train increase in the binding energy of S atoms and the appearance of on the basis of EDC, the electron density and its distribution induced unbound sulfur (unsaturated S atoms with dangling bonds). Mean- by laser irradiation can be controlled to modulate the properties of SPs, while, abundant Mo atoms were selectively removed, and the surface resulting in a more effective energy transfer and changes in the resulting lattice structure of the material was non-perfect and broken into a structures. These ﬁndings provide new insights regarding tuning LSPR large number of micro/nano debris terminating with unsaturated S features for related applications. atoms (edge active sites). These unsaturated-terminal S atoms induced on MoS can reduce gold cations to gold atoms (Figure 12n). Surface chemical reduction activity of MoS controlled by temporal 2 2 pulse shaping based on EDC. By designing femtosecond pulse trains Compared with conventional femtosecond laser pulses, more Mo Light: Science & Applications doi:10.1038/lsa.2017.134 r (μm) r (μm) x (μm) x (μm) Electrons dynamics control LJiang et al a c 0~5 ns 5~10 ns 10~15 ns 15~20 ns 20~25 ns 25~30 ns 30~35 ns 200 fs c1 400 fs 0.5 ns c2 c11 Air 600 fs c3 PMMA 800 fs 1 ns c12 c4 1 ps c5 100 μm 1st pulse 1.2 ps 3.6 ns c6 c13 1.4 ps c7 0~5 ns 5~10 ns 10~15 ns 15~20 ns 20~25 ns 25~30 ns 30~35 ns 1.6 ps 8.6 ns c8 c14 10 ps Air c9 13.6 ns PMMA 100 ps c15 c10 The final hole fs laser 18.6 ns c16 100 μm c17 200th pulse 485 μm Air PMMA Figure 23 Time-resolved images of the plasma expansion during the Gauss beam drilling for the 1st pulse (a) and 200th pulse (b), respectively ,pump- probe shadowgraph study (c) of Bessel beam drilling . c1–c16 Time-resolved images of femtosecond-picosecond-nanosceond dynamics of Bessel beam drilling. c17 The ﬁnal hole morphology drilled by Bessel beam. Reproduced from Ref. 133 (with the permission of OSA) and Ref. 131 (with the permission of OSA). atoms were selectively removed and more unbound sulfur was including melting (various phase change mechanisms coexisted during formed, which might result from the signiﬁcantly enhanced ionization the process) dominated the ablation process, which resulted in inferior of bound electrons by femtosecond double-pulse trains (ﬁrst, bound fabrication quality. While in the case of femtosecond laser pulse train electrons were ionized by the ﬁrst sub-pulse and the electron-hole with the identical total ﬂuence, a single pulse was split into two sub- pairs separated; second, the photoexcited electron-hole pairs recom- pulses with the identical ﬂuence lower than the ablation threshold of bined, which began within 500 fs and would last for more than a fused silica. Therefore, through optimizing the pulse delay between the hundred picoseconds; last, the second sub-pulse could further ionize two sub-pulses, the laser-induced electron density can be controlled to the recombining electron-hole pairs, which might further facilitate the be slightly higher than the critical density. Subsequently, the non- 38,194,195 photochemical bond breaking) and inevitably resulted in thermal phase-change mechanisms mainly dominate the fabrication stronger chemical reduction activity and higher chemical reduction process, contributing to less recast and high fabrication quality. rate on laser-treated MoS . In conclusion, by designing pulse trains Furthermore, by adjusting of the energy ratio between the two sub- based on EDC, the bound electrons ionized by laser irradiation can be pulses, the ionization processes were altered to change the pulse energy controlled to modulate chemical bond cleavage and increase chemical absorption. Consequently, the free electron distribution can be reduction ability, which can reduce gold cations to obtain metal-MoS adjusted to make the phase change process a nonthermal one. hybrids for relevant applications. Therefore, we achieved much higher fabrication quality and more controllable structures (Figure 13d and 13e). Overall, through High-quality fabrication by temporal pulse shaping based on EDC. femtosecond laser temporal pulse shaping, the localized electron By designing femtosecond laser pulse trains on the basis of EDC, the dynamics of the materials can be controlled, thus the phase-change phase change mechanism can be controlled to achieve high-quality mechanisms can be adjusted to be dominated through nonthermal fabrication . As Figure 13a shows, the recast ratio (recast area/ phase change process so that higher fabrication quality and more ablation area) of the ablation structures on the fused silica decreased as controllable structures can be achieved. the pulse delay increased within a femtosecond laser double-pulse train. Compared with a conventional single pulse, the recast height Femtosecond laser spatial pulse shaping surrounding the ablation spot decreased by 60% when fabrication was Spatial laser shaping is another key aspect of EDC. The spatial states of performed using a femtosecond laser pulse train (see Figure 13b and electron dynamics are closely related to the spatial distribution of laser 13c, for atomic force microscope (AFM) proﬁles). In this case, the − 2 energy. Conventionally, most commercial laser systems provide a total ﬂuence of the pulse train was 5 J cm (greater than the ablation fundamental Gaussian intensity proﬁle, which has a short Rayleigh threshold of fused silica). The conventional femtosecond laser single length, small beam waist of focusing spot, homogeneous phase/ pulse induced an electron density much higher than the critical density polarization state and limited numbers of laser spots. These char- and a higher Coulomb barrier, therefore leading to signiﬁcant electron acteristics of Gaussian laser cannot satisfy the increasingly higher screening effects. Consequently, the accumulation of positive charge demands for ﬂexibility, precision and efﬁciency in high-end laser during ablation was reduced, thus the electric ﬁeld was weakened so fabrication. Thus, it is essential to spatially shape the laser pulses to that the effectiveness of the nonthermal processes (Coulomb repulsion and/or electrostatic ablation) was reduced. The thermal phase change control the spatial electron density/temperature distribution. By spatial doi:10.1038/lsa.2017.134 Light: Science & Applications Electrons dynamics control LJiang et al a c 50 μm bd 2.23 μm 5 μm 20 μm Figure 24 (a and b) The image of a microhole array throughout a 1 cm × 1 cm large area using the ﬂying punch method for Bessel beam; (c) the local magniﬁed OTM image of a microhole array; (d) the local magniﬁed OTM image of microholes in the central array under 100 × microscope objective. Reproduced from Ref. 132 (with the permission of Springer). pulse shaping, the Gaussian beam can be converted into different By using a spatially modulated femtosecond beam based on spatial beam types, such as ﬂat top beam, vortex beam and Bessel beam, Airy EDC, we achieved high-resolution nanowire patterning that breaks the beam and so on (Figure 14) Some unique characteristics of these beam light diffraction limit . The initial beam had a Gaussian intensity types can lead to special functionality for laser fabrication, which has a proﬁle with an even phase wave front. The phase of the initial beam large advantage over conventional Gaussian beams. was modulated using a liquid crystal on silicon spatial light modulator. After modulation, a relative phase difference was created between two Spatial pulse shaping experimental setup. The experimental setup is equal parts of the incident beam. Subsequently, the modulated beam shown in Figure 15. An ampliﬁed Ti: sapphire laser system (Spitﬁre was focused by an objective lens, forming a dual-peak focusing spot Ace-35F Spectra Physics Inc.) provides a fundamental Gaussian mode with an intensity valley in the center because of a diffraction effect. with a central wavelength of 800 nm and a pulse duration of 35 fs. The spatially modulated beam was used to pattern a gold thin ﬁlm, Phase patterns were generated using the liquid crystal on a silicon which was deposited on a silica substrate by electron beam evaporator. spatial light modulator (SLM, Holoeye Pluto). The size of the liquid The central thin ﬁlm was preserved due to the intensity valley thus a crystal screen was 15.36 × 8.64 mm. The modulated laser beam passes nanowire was formed in the beam center. Arbitrary nanowire can be through a 4f relay system, which consisted of two plano-convex lenses generated on the substrate by dynamically adjusting the orientation of (L1, L2). The distance between the SLM and lens L1 was equal to the the intensity valley. A minimum nanowire width of approximately L1 focal length. The distance between the two lenses was the sum of their focal lengths. The laser beam can then be transmitted to the focal 56 nm (B1/14 of the laser wavelength) can be achieved (Figure 16a– plane of the second lens L2 without any distortion. The focal length 16d). The high resolution is achieved by combining the ultrashort was 100 mm (L1) and 150 mm (L2), respectively. The samples were nature of the femtosecond and the low thermal conductivity of the mounted on a nanometer-precision stage (Newport, NPXYZ100) has a thin ﬁlm. We tested the amount of Au residue using an electron resolution of 0.2 nm in the X, Y and Z planes. The fabrication process diffraction X-ray spectrum (EDXS) experiment (Thermo Scientiﬁc, was monitored using a CCD camera and white light source (WS). MA, USA; Figure 16e). By changing the direction of the phase pattern loaded on the SLM, the center nanowire changed its orientation High precision nanowire fabrication by spatial pulse shaping based accordingly. By dynamically adjusting the direction of the phase on EDC. Diffraction is a universal phenomenon in wave optics, pattern to tangent of the scanning route, arbitrary curves can be which greatly limits the resolution of laser fabrication to half of the fabricated. Figure 16f shows Olympic rings fabricated using this wavelength level. One of the greatest challenges in laser micro/nano method, which evidence its effective curve patterning ability. fabrication is ‘overcoming’ the diffraction limit. Several methods have To investigate the electronic characteristics of the nanowire, the volt- been developed to solve this problem using different mechanisms, such 196,197 198 ampere characteristics curve of the nanowire fabricated by different as near-ﬁeld fabrication , two/multi-photon polymerizations energy pulses was measured. (Figure 17) No inner nanopores and and plasmonics-based fabrications .However,these methods all have 196,197 particle intervals are generated inside the nanowire, thus endowing the different disadvantages, including the low efﬁciency ,complex 196,197 199 procedures ,weak ﬂexibility . and limited types of applicable nanowire with favorable electronic characteristics: the conductivity of 198 7 − 1 the nanowires wasashighas1.2×10 Sm ,and themaximum materials . A novel simple, repeatable, mask-free, high-throughput, 8 − 2 broad-applicability and high-ﬂexibility method is highly desired. current density was up to 1.66 × 10 Am . This approach offers a Light: Science & Applications doi:10.1038/lsa.2017.134 Electrons dynamics control LJiang et al simple, robust alternative for high-quality nanowire fabrication as a strongly depended on the pulse delay; the signal was stronger than that complementary method to conventional lithography methods. of single-pulse irradiation when the pulse delay exceeded 10 ps. The maximum enhancement value was about 7 at the pulse delay of MULTISCALE MEASUREMENT OF ELECTRON DYNAMICS approximately 80 ps. To determine the electron dynamics difference, DURING FEMTOSECOND LASER-MATERIAL INTERACTIONS the plasma temperature and electron density were calculated using the To comprehensively understand the electron dynamics during femto- Saha–Boltzmann plot and the Stark broadening method, respectively. second laser micro/nano fabrications, a multiscale measurement The plasma temperature variation was correlated with the signal system was developed to monitor the spatiotemporal electron enhancements achieved using the double-pulse delay, implying that dynamics of laser-material interactions. This system integrates the the plasma reheating effect of the second pulse was the main 198,199 widely applied ultrafast pump-probe microscopy , time-resolved mechanism of the enhancement effect. By contrast, the maximum plasma photography with a gated intensiﬁed charge-coupled device enhancements were different for molecules, atoms and ions, which 200,201 202–204 205 (ICCD) ,LIBS and industrial continuous imagery were related to the different upper excitation energy of the emission 200,201 (Figure 18). These techniques have different characteristic time transition and the ionization stage of the electrons . resolutions, ranging from femtoseconds to seconds. By virtue of the A comparison of fused silica plasma emission induced by a single multiscale ability, the electron dynamics of femtosecond laser micro/ pulse and that induced by a double pulse is presented in Figure 21a nano fabrications can be revealed at different scales. and 21b. The plasma emissions at the pulse delay below 10 ps were As a case study, the deep-hole drilling process in poly (methyl stronger than they were in the single-pulse case. This phenomenon methacrylate) (PMMA) was investigated using the multiscale measure- was explained by the existence of free electrons and self-trapped ment system, as shown in Figure 19. In the femtosecond to picosecond excitons at this pulse delay. The free electrons and self-trapped time scale, focused laser propagation in the material, as well as free excitons left by the ﬁrst pulse increased the absorption efﬁciency of electron generation, diffusion and recombination, was detected the second pulse. Moreover, extraordinarily high enhancement factors through pump-probe shadowgraphy (Supplementary Movie 2, were observed at a pulse delay above 10 ps. The maximum enhance- Supplementary Information). Nonlinear phenomena were observed ment factor of double-pulse irradiation wasB35 times at a pulse delay − 2 at a high pulse energy as a result of the strong self-focusing effect of of 120 ps for a ﬂuence of 11 J cm . The plasma consisted of a fast the intense femtosecond laser. In the picosecond to nanosecond time component (ionized atoms and ions) and a slow plume component scale, the early plasma expansion and following shockwave evolution (partially ionized nanoparticles), with the slow part contributing little in the atmosphere can also be studied through pump-probe shadow- to plasma emission . The ionization of the slow part by the second graphy (Supplementary Movie 3, Supplementary Information). Shock- pulse greatly increased the plasma quantity and was demonstrated to wave expansion properties were investigated on the basis of the Sedov- be the main cause of the high enhancement factor when the pulse Taylor solution, revealing the change of environment and its effect on delay was larger than 10 ps. The plasma temperature was also laser ablation. For larger time scales, time-resolved plasma photo- calculated to ascertain its changes with respect to the pulse delay graphy with the gated ICCD and LIBS were employed to collect the (Figure 21c). Hence, the electron dynamics (free electrons, self- plasma expansion morphology (Supplementary Movie 4, trapped excitons or nanoparticles) left by the ﬁrst pulse dominated Supplementary Information) and plasma emission spectroscopy, the plasma emission intensity of the femtosecond double pulse LIBS. respectively. These results not only provided the expansion dynamics of plasma but also the plasma intrinsic information, including the APPLICATIONS OF EDC IN HIGH QUALITY AND ASPECT-RATIO material composition, plasma density and temperature. In the milli- MICROHOLES DRILLING second to second time scale, the plasma evolution induced by multiple As discussed before, because of the signiﬁcant electron-lattice none- pulses was studied using time-resolved plasma photography, revealing quilibrium state in the femtosecond laser fabrication process, the laser- the effect of the prior structure on the plasma intensity and material interactions, including phase change and material removal, 26,27 distribution (Supplementary Movie 5, Supplementary Information). are actually determined by the initial photon–electron interactions . Furthermore, microhole formation was studied using industrial Furthermore, the electron density distribution can be manipulated by continuous imagery (Supplementary Movie 6, Supplementary modifying the laser intensity distribution. By implementing EDC Information), providing information concerning the evolution of the through spatial pulse shaping, forming an intense, long and uniform depth, diameter and quality of the high-aspect-ratio microhole. Bessel beam, to adjust the localized transient electron density By using the measurement system, we determined the nanosecond- distribution, and thus control phase change, we fabricated high quality scale electron temperature and density evolution under double pulse and high aspect-ratio microholes. The technique was applied in key irradiation. Large differences in plasma plume ejection of PMMA and structure fabrication in one of the 16 Chinese National S&T Major fused silica were demonstrated using the LIBS technique, following the Projects. different electron dynamics designed by femtosecond laser double- Microholes fabrication in the key structure faced many challenges, pulsetrain. Bychanging the pulsedelay, wecould modify the including the high aspect-ratio (420:1), small diameter (o10 μm), generation and distribution of electrons and thus effectively improve taper-free, high quality, reduced recast/ejected materials and minimized the electron density and temperature. Finally, we could improve the in-cavity residues. Although various novel methods had been proposed plasma enhancement factor and optimize the ablation accuracy for microholes fabrication, there were many challenges that limited through EDC. their application in the key structure fabrication. By using a particle The plasma spectra of PMMA induced by single pulse and double based near-ﬁeld nanostructuring to overcome the optical diffraction pulse irradiation at the same laser ﬂuence were compared to explore limit, Quentin et al achieved nanoholes with diameters below the LIBS enhancement mechanism. The plasma spectra of PMMA 200 nm, but the structure was a shallow crater and the aspect-ratio consisted of emission peaks of molecular species (CN, CH and C )as was below 1:1. He et al reported that high aspect-ratio microholes well as atomic and ionic species (Ca I, Na I and Ga II) .AsFigure 20 were fabricated in fused silica with femtosecond laser transverse direct- shows, double-pulse-induced plasma emission signal enhancement writing followed by wet chemical etching, but the cross-sections of the doi:10.1038/lsa.2017.134 Light: Science & Applications Electrons dynamics control LJiang et al microholes fabricated by this method are usually in poor shape Bessel beam and a focused Gaussian beam in the longitudinal and ascribing to the asymmetric shape of the focal spot. Gottmann transverse plane, as shown in Figure 22c. The micro-Bessel beam et al constructed a selective laser-induced etching system of 3D exhibited a large focal depth (Z = 597 μm) in comparison with the max precision quartz glass components. By the combination of a three-axis focused Gaussian beam (Raleigh range R= 18.5 μm). According to the system to move the glass sample and a fast 3D system to move the laser simulation results, the single-pulse Bessel beam exhibited an intense, focus, the selective laser-induced etching process (LightFab 3D Printer) long, consistent and stable intensity distribution without any nonlinear is suitable to produce more complex structures in a shorter time. beam distortion along the propagation direction. These unique However, the over-etching for some complex structures and the properties of the spatial shaping pulse allow for uniform energy formation of cracks during the etching process limit advanced deposition over extended propagation lengths and then adjust free applications. In addition to the aforementioned methods, percussion electron density distribution to be intense, long and uniform through 204,206,207 drilling has attracted the most interests . Nevertheless, the photo–electron interactions. aspect-ratio of microholes with percussion drilling is usually smaller Using the aforementioned multiscale time-resolved measurement than 10:1 in air because of the saturation effect and the occurrence of system developed by us, we further revealed the forming mechanisms the bending effect. We recently reduced the ambient pressure from of the high aspect-ratio and high quality microholes using a approximately 10 Pa (air) to B1 Pa (rough vacuum) to control the femtosecond laser spatial pulse shaping beam (Bessel beam) by 133,208 expansion dynamic of the ablated plasma/material .The aspect comparing it with the Gaussian beam percussion drilling. Figure 23a ratio of the microholes was signiﬁcantly improved from ~ 40:1 (in air) shows the time-resolved plasma photography of a Gaussian beam to approximately 100:1 (in vacuum), and the bending effect was drilling with a single-pulse, the ablation plume was detected only simultaneously eliminated. Nonetheless, conventional Gaussian beam above the PMMA surface. As shown in Figure 22c, the focused drilling could barely achieve a much higher aspect-ratio of taper-free Gaussian beam was condensed, thus, the laser energy could only be microholes with diameters below 10 μm. localized within several microns below the surface. Such a high-energy To overcome the disadvantages of the traditional Gaussian beam concentration would make free electron density much higher than the drilling, we proposed a new processing method for high aspect-ratio critical density. Therefore, the Coulomb explosion and electrostatic and high-quality microdrilling by optimizing the localized transient ablation are weakened, and the material removal is mainly attributed electron density distribution in plasma in the focal spot through spatial to melting and evaporation, resulting in a large heat-affected zone. pulse shaping . Taper-free microholes with a diameter of approxi- Meanwhile, the shielding-effect induced by the ultrahigh free electron mately 1.6 μm and an aspect ratio of up to 330:1 using a single spatial density suppresses the laser propagation, leading to a short optical shaping pulse (Bessel beam). The aspect ratio of these fabricated penetration depth; consequently, only a shallow crater can be microholes was 52 times larger than those formed by using a Gaussian fabricated with a single-pulse Gaussian beam. Furthermore, during beam in similar focusing conditions. The formation of these high the multipulses percussion drilling process (Figure 23b), the subse- aspect-ratio microholes is attributed to the intense, long and uniform quent pulses energy was not sufﬁciently absorbed by the material transient localized electron density distribution, which is adjusted on because of the strong reﬂection of the generated dense ablated plasma. the basis of the unique intensity distribution and propagation stability The generated plasma would disturb the propagation of the subse- of the Bessel beam through spatial pulse shaping. quent laser pulses so that leads to an unstable ﬁlament bending slightly In our experiments, we normally used an axicon (Edmund Inc., deep inside the hole, then the laser beam energy was deposited along a Barrington, NJ, USA, base angle α= 2°, refractive index= 1.45) to bent direction. Moreover, for the deeper microholes, the plasma transform a Gaussian beam into a Bessel beam for high aspect-ratio, would cool down gradually, then adhered to the microholes sidewalls, high quality and high efﬁciency microdrilling. This is a convenient and which negatively affected the quality of the microholes. effective method for spatial pulse shaping. Figure 22a shows the By contrast, spatial shaping pulse (Bessel beam) drilling exhibits a schematic of the experimental setup used for femtosecond laser completely different fabrication mechanism. Figure 23c displays the microhole drilling . Subsequently, we performed the experiments time-resolved images of Bessel beam drilling in PMMA. As the Bessel to prove that high aspect-ratio and high quality microholes can be beam entered into the PMMA, the rising edge induced electron obtained by using a single spatial shaping pulse. As Figure 22b shows, excitation, which formed a dark strip along the light path that a microhole with a mean diameter of 1.5 μmand adepth of indicated the plasma channel dynamics. The maximum length (optical approximately 523 μm can be drilled using a single-pulse Bessel beam penetration depth) of the Bessel plasma channel was reached at 1.6 ps at a pulse energy (E) of 20 μJ in PMMA. Under identical processing and effectively coincided well with the ﬁnal hole-depth, indicating the conditions, the diameter and depth of a microhole fabricated using the dominant role of initial electron excitation and the free electron Gaussian beam were 7.2 and 41 μm, respectively. Subsequently, to distribution on the ﬁnal structure formation. In the following tens of further demonstrate the hollowness of the microholes through single picoseconds, there existed electron-ion energy transfer occurred that spatial shaping pulse drilling, the two different methods were mainly induced an extremely high pressure and temperature in the focal area, adopted, namely the liquid inﬁltration method and the cross-section resulting in an explosive and supersonic expansion of the material in proﬁle test, as shown in Figure 22d. The microholes drilled using this the nanosecond domain, as shown in Figure 23c11–23c16. In contrast method were small in diameter and exhibited a high aspect-ratio, a to the pressure waves in air and some other cases, the pressure wave taper-free sidewall, a highly circular entrance and fewer surface-ejected induced by the Bessel beam in PMMA was a cylindrical shockwave and materials. gradually expanded outward along the radial direction. This phenom- Moreover, we performed a series of theoretical investigations and enon suggests that the formation of microholes is an extrusion effect, simulations on the spatial intensity distribution of Bessel beams and leading to the formation of high quality microholes with taper-free, Gaussian beams to more thoroughly understand the physical mechan- reduced recast/ejected materials and minimized in-cavity residues. isms of pulse microdrilling with spatial shaping. On the basis of the After the understanding of the processing mechanisms of the Bessel theory of spatial pulse shaping using the axicon, we respectively beam, we proposed the ﬂying punch method for machining large-area simulated the spatial intensity distribution of single-pulse, micro- microhole arrays in PMMA by using a Bessel beam, as shown in Light: Science & Applications doi:10.1038/lsa.2017.134 Electrons dynamics control LJiang et al Figure 24. A 1 cm × 1 cm microhole array (with 251 001 ultrahigh- system limited the spatial/temporal resolution and the sensitivity aspect-ratio holes in total, at a processing speed of 100 holes of the measurement, which shall be improved to monitor the per second) was fabricated within 42 min, indicating the high- electron dynamics in more detail. In addition, more time-resolved efﬁciency and high repeatability of the fabrication process using the measurement techniques should be integrated into the multiscale ﬂying punch method . system, so that it can provide more information of electrons from different aspects. Furthermore, the measurement results should CONCLUSIONS AND OUTLOOKS correspond with the theoretical models, such as providing the In this paper, we comprehensively reviewed our decade-long efforts on characteristic values of electrons for theoretical calculations. four parts of EDC in femtosecond laser micro/nano fabrications: the (4) Broader applications: High-aspect-ratio and high-quality micro- theoretical fundamentals, experiments, multiscale measurements and holes by using spatial shaping pulses have been applied in applications. Theoretically, based on the four models with different fabricating some key structure fabrications. However, the current − 3 − 15 − 3 − 10 time scales (10 –10 s) and space scales (10 –10 m), we method is limited to a few transparent materials. We will extend demonstrated that the localized transient electron dynamics (including the range of materials and then explore novel applications of electron density, temperature and excited state distribution), and microholes, such as for microﬂuidic devices and three- subsequent phase change can be controlled by temporally/spatially dimensional integrated chip packaging. In addition, we will shaping femtosecond laser pulses. Experimentally, seven experiments substantially expand the applications of the novel method, such were reported as examples to validate the feasibility of EDC by as for the adjustment of chemical reaction pathways by ultrafast temporally/spatially shaping femtosecond pulses in micro/nanofabri- laser EDC, transient or permanent adjustment of material proper- cation. The experiments revealed that the precisions, efﬁciencies and ties through ultrafast laser EDC. qualities can be signiﬁcantly improved and that various surface micro/ nano-structures can be effectively modulated by the proposed EDC- based methods. Additionally, multiscale measurements further directly CONFLICT OF INTEREST demonstrated the fundamentals of EDC from femtosecond scale to The authors declare no conﬂict of interest. nanosecond scale and to millisecond scale. Finally, EDC was applied in high aspect-ratio (330:1) and high-quality microholes drilling at the ACKNOWLEDGEMENTS speed of 100 holes per second (251 001 holes fabricated in 1 cm × 1 cm This research was supported by the National Natural Science Foundation of area within 42 min), in which multiscale measurements were used to China (NSFC) (Grant Nos. 90923039, 91323301, 50705009, 51105037, analyze and optimize the electron dynamics. The high aspect-ratio 51322511 and 51025521), National Basic Research Program of China (973 microholes drilling was applied to key structure fabrication in one of Program) (Grant No. 2011CB013000), the 863 Project of China under Grant the 16 Chinese National S&T Major Projects. No. 2008AA03Z301, the Cultivation Fund of the Key Scientiﬁc and Technical Innovation Project, Ministry of Education of China (No. 708018), the 111 We have devoted the past ten years to studying the mechanisms, Project of China (Grant No. B08043), Multidisciplinary University Research methodologies and applications of femtosecond laser micro/nano Initiative (MURI) program of USA under Grant No. N00014-05-1-0432 and fabrications. However, many challenges still remain, especially on National Science Foundation of USA under Grant No. 0423233. We thank Prof the following topics: Xin Li, Prof Jie Hu, Prof Sumei Wang, Prof Xiaowei Li, Prof Jingya Sun, Prof Jianfeng Yan, Dr Xiaoxing Su, Dr Weina Han, Dr Qingsong Wang, Dr Zhitao (1) Comprehensive models: Femtosecond laser-material interactions Cao, Mr Zhi Wang, Dr Changji Pan, Dr Peng Ran and Dr Pei Zuo for dis- are a comprehensive nonlinear, nonequilibrium process ranging cussing, revising and proofreading this manuscript. from a nanometer scale to a millimeter scale and from a femtosecond scale to a millisecond scale. However, our present models, including the plasma and improved two-temperature models, are not applicable to some materials. Furthermore, a 1 Malinauskas M, Žukauskas A, Hasegawa S, Hayasaki Y, Mizeikis V et al. Ultrafast laser comprehensive, integrated multiscale physical-chemical modeling, processing of materials: from science to industry. 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Electrons dynamics control by shaping femtosecond laser pulses in micro/nanofabrication: modeling, method, measurement and application

Electrons dynamics control by shaping femtosecond laser pulses in micro/nanofabrication: modeling, method, measurement and application

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Electrons dynamics control by shaping femtosecond laser pulses in micro/nanofabrication: modeling, method, measurement and application

Electrons dynamics control by shaping femtosecond laser pulses in micro/nanofabrication: modeling, method, measurement and application

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