Assessing the hydration state of agarose-based hydrogels using terahertz spectroscopyZapanta, Mark Justine; Postelmans, Annelies; Saeys, Wouter
doi: 10.1117/12.2620986pmid: N/A
Agarose is a gel-forming polysaccharide extracted from marine red algae. This biopolymer is an important analytical separation medium, stabilizer and thickener in the food and pharmaceutical industries, and phantom material for biomedical research. The distribution of water to the different hydration zones in the gel can significantly influence the functional properties of agarose-based hydrogels. In this work, the application of terahertz spectroscopy in assessing the hydration state of biomolecules was extended from aqueous solutions and lipid-water emulsions to polysaccharide gels. The agarose gel system, whose structure is constituted by water confined in a complex network of polymer assemblies, was investigated. THz time-domain spectroscopy (THz-TDS) measurements were performed in transmission mode. The behavior of the absorption and dielectric loss spectra of the gels relative to the polymer concentration indicate that the agarose network acts as a kosmotropic agent that favors the formation of the regular hydrogen bond structure. The hydration number equation of Hishida and Tanaka was modified to account for heterogeneity and confinement effects using effective medium theory. The hydration number of the agarose monomers was found to decrease with increasing polymer concentration. This trend in hydration number can be attributed to the increase in fiber density as a result of increasing agarose concentration which causes the thinning of the hydration shells and the sequestration of water molecules to the inside cavity of the agarose double helices.
Comparison of time-domain antenna performances for the terahertz photoconductive sources and detectorsSongur, Ahmet C.; Ünlü, Mehmet
doi: 10.1117/12.2622125pmid: N/A
The aim of this study is to investigate and design broadband, terahertz antennas for the time-domain, pulsed operation of the photoconductive sources and detectors. Different antenna types, i.e., thick dipole, bowtie, and spiral antenna, are designed, and their performances are analyzed both analytically and numerically for radiation in 0.8-2 THz band. A very effective ultra-wideband antenna configuration, Archimedean spiral antenna, which has promising time-domain radiation results according to analytical studies, is proposed for terahertz radiation. To the best of the authors’ knowledge, this is the first study on fully time-domain analysis of terahertz antennas operating in 0.8-2 THz band.
Shear interferometry for terahertz wavefront sensingAgour, Mostafa; Falldorf, Claas; Taleb, Fatima; Castro-Camus, Enrique; Koch, Martin; Bergmann, Ralf B.
doi: 10.1117/12.2621696pmid: N/A
We propose a new solution for sensing a terahertz (THz) wavefront based on a THz reference-less shear interferometer. The key component of the experimental configuration of the proposed interferometer is a THz Ronchi phase grating (RPG). The RPG is custom designed and fabricated for a 0.28 THz source using mechanical milling on a block of high-density polyethylene (HDPE) with a computer numerical control (CNC) machine. It acts as a shearing element that generates two diffraction orders, thereby creating two laterally shifted copies of the investigated wavefront in the sensor plane where a THz camera is placed. The direction of the shear can be varied by rotating the grating. Since the grating is a phase grating, the diffraction efficiency is very high. The approach is verified experimentally by demonstrating interferograms of a spherical wave and wavefront reconstruction from five different shears using a gradient-based iterative process.
Rapid genetic discrimination in plants through terahertz spectroscopyKundu, Urbi; Bandyopadhyay, Aparajita; Bertling, Karl; Rakić, Aleksandar D.; Sengupta, Amartya
doi: 10.1117/12.2621415pmid: N/A
Rapid changes in the agricultural sector in the past two decades have given rise to several new technologies and superior products including genetically modified crops; the identification of which still requires robustness and rapidity. In this work, we report the use of continuous wave terahertz (CW THz) spectroscopy as a means to identify biomechanical changes at the tissue level based on systemic dehydration. We have also identified the factors affecting this progression and propose a biomechanical model towards genetic discrimination in plants. Our results indicate that within the same family, factors such as cell size and age, tissue composition, hydration retention capacity and water percentage to cell volume ratio affect the systemic dehydration in the plant and thereby show unique biomechanical profiles.
Investigation of the loss performance of the spoof surface plasmon polariton waveguides at 1 THz bandDemircioglu, Mesut; Unutmaz, Muhammed A.; Ünlü, Mehmet
doi: 10.1117/12.2622091pmid: N/A
The terahertz imaging systems bring the advantage of both optical and microwave frequency spectrums, thanks to the invasion capability of the terahertz waves through different media providing high-resolution imaging at real terahertz frequencies such as 1 THz. Nevertheless, the state-of-the-art terahertz technologies employ bulky optical system design approach. In consequence, the state-of-the-art terahertz systems are not suitable for high mobility terahertz imaging applications. On the other hand, the state-of-the-art terahertz integrated circuits (TICs) suffer from high attenuation due to conventional terahertz waveguides, and hence, a novel high-performance terahertz waveguide is needed. In this paper, we present the investigation of loss performance of spoof surface plasmon polariton (SSPP) waveguides (WGs) that operate at 1 THz, which will enable the demonstration of compact and high-performance TICs. We present a relationship between the corrugation dimensions, radiation, and metallic losses and guided wavenumber for the first time. The proposed SSPP WGs are able to transmit the terahertz wave in expense of an insertion loss of -4.93 dB through 250 µm at 1 THz.
Optimizing the performance of the CPW-to-spoof surface plasmon polariton waveguides for 0.3 THz bandUnutmaz, Muhammed A.; Ünlü, Mehmet
doi: 10.1117/12.2620946pmid: N/A
The competition to suggest high performance solutions for terahertz communication targets 0.22-0.32 THz band because of its bandwidth and attenuation advantages over other terahertz frequencies. However, the state-of-the-art suffers from conventional terahertz waveguide performance. Alternatively, the spoof surface plasmon polariton waveguides (SSPP WGs) measurements achieve the record-low insertion loss per unit length at 0.3 THz. On the other hand, the SSPP WGs require high performance transitions to interface with terahertz active devices such as transistors and diodes. In this paper, we present design, optimization, and experimental verification of high-performance coplanar waveguide-to-SSPP WG (CPW-to-SSPP WG) transitions at 0.25-0.3 THz band. The measurements show that the insertion loss of a CPW to SSPP WG transition can be suppressed up to -0.5 dB at the proposed frequency band.
Injection-seeded backward terahertz-wave parametric oscillators at 0.3 and 0.5 THz bands for nondestructive imaging applicationsTakida, Yuma; Nawata, Kouji; Notake, Takashi; Minamide, Hiroaki
doi: 10.1117/12.2626827pmid: N/A
We have demonstrated injection-seeded backward terahertz (THz)-wave parametric oscillators (BW-TPOs) based on a slant-strip-type periodically poled lithium niobate (PPLN) crystal with two different poling periods. The BW-TPOs were pumped by sub-nanosecond pump pulses at 1064 nm and designed for generating backward-propagating THz waves around 0.30 and 0.46 THz with PPLN poling periods of 53 and 35 μm, respectively. As a result of an optical injection seeding for the forward-propagating idler wavelength in the BW-TPO process, we achieved over a 1000-fold enhancement in backward-propagating THz-wave output energy, a 63% reduction of the oscillation threshold, and long-term stable operation compared to the unseeded case. Furthermore, we demonstrated that the oscillation frequency of backward-propagating THz waves is continuously tunable in the range of 0.27–0.35 and 0.41–0.52 THz for the poling periods of 53 and 35 μm, respectively, by angle tuning of the PPLN crystal in parallel with seed wavelength tuning. Using the developed injection-seeded BW-TPOs, we also performed the THz-wave imaging test in transmission geometry for various materials, including glass, wood, and liquids.
THz resonant-tunneling diodes, oscillators, detectors, and applicationsFeiginov, Michael
doi: 10.1117/12.2625809pmid: N/A
The paper gives a short overview on the present status of the resonant-tunneling diodes (RTDs) in the area of THz electronics. In particular, the RTD oscillators are discussed: the achieved level of their output power and operating frequencies; an overview of the different types of RTD oscillators, their advantages and disadvantages; tunability and frequency stability of RTD oscillators. The use of RTDs as THz detectors is also shortly discussed, in particular: operating principles, limitations, application examples. Further on, an overview on the application examples of RTDs is given: RTDs oscillators in high-data-rate wireless transmission systems, imaging applications, spectroscopy, etc.