Applied Physics A (2018) 124:265
Pulse‑by‑pulse depth proﬁle measurement of femtosecond laser
ablation on copper
· Yohei Kobayashi
Received: 8 January 2018 / Accepted: 19 February 2018 / Published online: 21 February 2018
© Springer-Verlag GmbH Germany, part of Springer Nature 2018
Pulse-by-pulse depth proﬁle measurements were performed during femtosecond laser pulse irradiation on copper. The evolu-
tion of the ablation depth and the surface roughness during multi-pulse irradiation was investigated using a three-dimensional
microscope. We found that a number of pulse irradiation was required to activate constant-rate ablation and the number of
pulses depended on the ﬂuence. We conclude that laser-induced embrittlement of a target material plays a decisive role to
determine the ablation rate during multiple-pulse irradiation.
Laser processing using femtosecond laser pulses has been
emerging as a powerful tool for micromachining and surface
functionalization owing to its precise processing capability.
The rapid energy deposition into a target material by the
pulses leads to the non-thermal removal and modiﬁcation
of the target, thereby enabling high-precision machining .
Extensive studies have been performed to reveal the under-
lying mechanism, namely laser ablation, in terms of single
shot and multi-shot characteristics.
Studies on multiple shot characteristics have mainly
focused on ablation rate and surface roughening owing to
their key importance in applications. Here, we have used
the term “ablation rate” to refer to the ablation depth per
pulse. The ablation rate increases monotonically as a func-
tion of laser ﬂuence with a couple of threshold behaviors
[2, 3], except with a high repetition rate pulse irradiation,
where heat accumulation can impede or promote the abla-
tion processes [4, 5]. The ﬁrst threshold, namely the ablation
threshold, can vary with the number of irradiated pulses [6,
7]. The second threshold at which the ablation-rate-to-ﬂu-
ence ratio increases is regarded as the border of the “gentle”
and “strong” ablation regimes. As for the surface roughness,
surface functionalization has attracted considerable atten-
tion, and the formation of laser-induced periodic structures,
spikes, and other structures has been extensively studied.
The morphologies of these structures depend upon the ﬂu-
ence and the number of irradiated pulses at a spot [8–12].
In contrast with the single shot experiments, in which the
laser pulses interact with a material having a fresh surface,
pulses after the ﬁrst shot experience changes in material
property, which are induced by the prior pulses in multi-
shot irradiation. Vorobyev et al. reported the increase of
the absorbance of a gold ﬁlm as the number of irradiated
pulses increased, which was attributed to the morphology of
the laser-irradiated surface . As the surface roughness
changes the absorbance of the target, the ablation rate can
be correlated with the surface roughness. Thus, the evolu-
tion of the ablation depth and surface roughness, i.e., the
depth proﬁle, during multiple-pulse irradiation is of great
importance for understanding the laser ablation processes.
However, these two characteristics are typically measured
as a net result of multi-pulse irradiation on a fresh surface,
and their evolution during the irradiation has been rarely
discussed [14, 15]. Speciﬁcally, no study has reported the
evolution of the depth proﬁle during multi-pulse irradiation.
In this letter, we report the ﬁrst measurement of pulse-
by-pulse depth proﬁle changes during multiple-femtosecond
pulse irradiations. Our experimental system can compare the
depth proﬁles before and after the pulse irradiation, which
enables the investigation of changes in the depth proﬁle dur-
ing multiple-pulse irradiation.
* Shuntaro Tani
Institute for Solid State Physics, The University of Tokyo,
Kashiwa, Chiba 277-8581, Japan