Abstract-Enhanced Terahertz Wave Generation from Cytop Antireflection–Coated DAST Crystal

Hirohisa Uchida, Chisa Koyama, Takeshi Takagi, Pei-bin Wang, Takayuki Kamei, Saroj R. Tripathi, Kodo Kawase, Kei Takeya

https://link.springer.com/article/10.1007/s10762-020-00674-6

There was no appropriate antireflection coating material for organic nonlinear optical (NLO) crystal used for terahertz wave generation; therefore, it was difficult to reduce Fresnel’s reflection loss of laser power used to excite the emitter crystal until we found that an amorphous polymer “Cytop” was applicable. Here, we demonstrate enhanced terahertz (THz) wave generation from an organic NLO crystal using a coating made from Cytop. With the Cytop coating, surface reflection losses of the excitation light were minimized, resulting in a significant improvement in THz wave generation over a wide frequency range.

Abstract-Terahertz tag identifiable through shielding materials using machine learning

Ryoya Mitsuhashi, Kosuke Murate, Seiji Niijima, Toshinari Horiuchi, Kodo Kawase,

(a) Tags used for real-time identification. Natural leather was used as the shielding material. Tags are shown for Chemicals A, B, and C (from the right) and were attached to the shielding material. Tags were moved in the direction of the arrow. (b) Images of the detection beams of the multi-wavelength is-TPG without tag insertion. For this measurement, we selected three wavelengths to match the absorption peaks of each tag. (c) The intensity spectrum of each tag measured with a conventional is-TPG. The symbols show the intensities of the three frequencies selected in this study; the red and blue symbols represent the transmission and absorption frequencies, respectively. (d) We took screenshots showing the timing of the measurement of each tag’s position from the video of tag sweeping. The names of tags identified by convolutional neural network (CNN), and the identification probability, are shown in the upper right corner.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-28-3-3517

In recent years, there has been great interest in chipless radio-frequency identification (RFID) devices that work in the terahertz (THz) frequency range. Despite advances in RFID technology, its practical use in the THz range has yet to be realized, due to cost and detection accuracy issues associated with shielding materials. In this study, we propose two types of low-cost THz-tags; one is based on the thickness variation of coated polyethylene and the other on the fingerprint spectra of reagents. In the proposed approach, machine learning, specifically a deep-learning method, is used for high-precision tag identification even with weak signals, or when the spectrum is disturbed by passing through shielding materials. We achieved almost 100% identification accuracy despite using an inexpensive tag placed under thick shielding materials with an attenuation rate of about −50 dB. Furthermore, real-time tag identification was demonstrated by combining a multiwavelength injection-seeded THz parametric generator and a convolutional neural network.

© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Verification of Non-thermal Effects of 0.3–0.6 THz-Waves on Human Cultured Cells

Noriko Yaekashiwa, Hisa Yoshida, Sato Otsuki, Shin’ichiro Hayashi,  Kodo Kawase

https://www.mdpi.com/2304-6732/6/1/33


Recent progress has been made in the development of terahertz (THz) waves for practical applications. Few studies that have assessed the biological effects of THz waves have been reported, and the data currently available regarding the safety of THz waves is inadequate. In this study, the effect of THz wave exposure on two cultured cells was assessed using a widely tunable THz source with a 0.3–0.6 THz frequency range, which can be used and increased in one GHz increments. The THz waves applied to the cultured cells were weak enough such that any thermal effects could be disregarded. The influence of THz wave exposure on both the proliferative and metabolic activities of these cells was investigated, as well as the extent of the thermal stress placed on the cells. In this work, no measurable effect on the proliferative or metabolic activities of either cell type was observed following the exposure to THz waves. No differences in the quantity of cDNA related to heat shock protein 70 was detected in either the sham or exposure group. As such, no differences in cellular activity between cells exposed to THz waves and those not exposed were observed

Abstract-Pump wavelength-independent broadband terahertz generation from a nonlinear optical crystal

Kei Takeya, Koutarou Okimura, Kengo Oota, Kodo Kawase, and Hirohisa Uchida

https://www.osapublishing.org/ol/abstract.cfm?uri=ol-43-17-4100

In nonlinear optical (NLO) crystals, the selection of pump light wavelengths for the generation of terahertz (THz) waves is limited due to problems associated with coherence length, refractive index, and absorption by the crystal. Relaxation of this limitation would open up potential light sources for THz generation. One such solution, Cherenkov phase matching, removes the coherence length constraint. In this study, we attempted to generate THz waves from an NLO crystal using femtosecond pulses of various wavelengths. Specifically, 805-nm and 1560-nm femtosecond pulses were used to pump a prism-coupled LiNbO3${\mathrm{LiNbO}}_3$ crystal. Broadband THz-wave generation and a THz-wave output proportional to the square of the pump light intensity were observed at both wavelengths. The generation of THz waves by prism-coupled Cherenkov phase matching was not limited by the wavelength of the pump light. Moreover, THz-wave generation at even greater intensities may be possible by optimizing the pump source and coupling to an NLO crystal.

© 2018 Optical Society of America

Abstract-Frequency of the resonance of the human sweat duct in a normal mode of operation

Saroj R. Tripathi, Paul Ben Ishai, and Kodo Kawase

https://www.osapublishing.org/boe/abstract.cfm?uri=boe-9-3-1301&origin=search

The applications of terahertz (THz) waves have been increasing rapidly in different fields such as information and communication technology, homeland security and biomedical engineering. However, study on the possible health implications due to various biological effects induced by THz waves is relatively scarce. Previously, it has been reported that the human sweat ducts play a significant role in the interaction of the THz wave with human skin due to its coiled structure. This structure imposes on them the electromagnetic character of a helical antenna. To further understand these phenomena, we investigated the morphological features of human sweat ducts and the dielectric properties of their surrounding medium. Based upon these parameters, we estimated the frequency of the resonance of the human sweat duct in a normal mode of operation and our estimation showed that there is a broad resonance around 228 GHz. This result indicates that careful consideration should be given while designing electronic and photonic devices operating in the sub-terahertz frequency region in order to avoid various effects on human health due to these waves.

© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Injection-seeded THz parametric generator/amplifier

Kodo Kawase,  Kosuke Murate,

http://ieeexplore.ieee.org/document/8083975/

We report a THz-wave amplifier using nonlinear optical effects in MgO:LiNbO3 crystals. The amplifier operates at room temperature and has a gain of 55 dB based on the fundamental principle of injection-seeded THz parametric generator/detector.

Abstract-Effective Terahertz Wave Parametric Generation Depending on the Pump Pulse Width Using a LiNbO 3 Crystal

 Kouji Nawata ;  Shin'ichiro Hayashi ;  Hideki Ishizuki ;  Kousuke Murate , Kazuki Imayama,  Yuma Takida,   Vincent Yahia,  Takunori Taira,  Kodo Kawase,  Hiroaki Minamide

http://ieeexplore.ieee.org/document/7984855/

Injection-seeded terahertz (THz)-wave parametric generation pumped by subnanosecond pulses was demonstrated as an exploration of the effective pump pulse width. In experiments, pump sources with different pump widths of 35–850 ps were used. The relationship between the undesirable effect of stimulated Brillouin scattering (SBS) and the THz-wave output in LiNbO 3 was analyzed. The transient effect on SBS was not actualized around the acoustic phonon lifetime of LiNbO 3 . Pump widths below 420 ps were found to provide THz-wave generation with an energy conversion efficiency of 10−4. With a pump width of 700 ps, which is around five phonon lifetimes, the conversion efficiency was decreased about one order of magnitude from that of the pump width of 420 ps. Additionally, the peak power of the THz-wave output was maintained at about 15 kW with an actual range of 10–20 kW below 200 ps. A continuous change in the THz-wave pulse energy was obtained for six-fold energy scaling from 100 to 600 nJ.

Abstract-Terahertz wave three-dimensional computed tomography based on injection-seeded terahertz wave parametric emitter and detector

Saroj R. Tripathi, Yuya Sugiyama, Kosuke Murate, Kazuki Imayama, and Kodo Kawase

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-24-6-6433

We demonstrate a high dynamic range, three-dimensional (3-D) terahertz (THz) wave computed tomography system in which frequency tunable, Fourier transform-limited, high-power THz waves are emitted by an injection-seeded parametric source and ultrasensitive detection of THz waves is accomplished by heterodyne detection. This system covers the frequency range of 0.95 to 2.7 THz and has a maximum dynamic range in excess of nine orders of magnitude, enabling the acquisition of high-resolution 3-D tomographic images of samples with strong THz absorption. As an illustration, we obtained 3-D computed tomographic images of a pencil and a plastic product with an internal defect that demonstrates the potential applications of our imaging system in non-destructive testing and evaluation of industrial products.

© 2016 Optical Society of America

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Abstract-Kilowatt-peak Terahertz-wave Generation and Sub-femtojoule Terahertz-wave Pulse Detection Based on Nonlinear Optical Wavelength-conversion at Room Temperature

• Hiroaki Minamide, 
• Shin’ichiro Hayashi, 
• Koji Nawata, 
• Takunori Taira, 
• Jun-ichi Shikata, 
• Kodo Kawase

http://link.springer.com/article/10.1007%2Fs10762-013-0041-0

Intense Terahertz (THz)-wave generation and highly sensitive THz-wave detection were obtained by wavelength conversion with nonlinear optical susceptibility χ(2) of LiNbO3 crystals. Maximum peak output of about 50 kW (5 μJ/pulse) was demonstrated in an injection-seeded THz-wave parametric generator pumped by post-amplified emission from a microchip Nd:YAG laser. Using the sub-nanosecond pulse duration of the laser proposed herein provides effective mitigation of stimulated Brillouin scattering in LiNbO3, producing higher gain for wavelength conversion between near-infrared (near-IR) pump light and THz waves. Monochromatic THz radiation was obtained in the continuous tuning range of 0.7–2.9 THz. Additionally, highly sensitive THz-wave detection was demonstrated based on up-conversion from THz waves to near-IR light as well as efficient THz-wave generation. The signal generated with non-collinear phase-matching condition showed spectroscopic detection on the screen apart from the LiNbO3 crystal. Highly sensitive detection with minimum energy of about 80 aJ/pulse (0.8 μW at peak) and a large dynamic range of more than 100 dB were achieved in this experiment.

Abstract-Terahertz Imaging System for Medical Applications and Related High Efficiency Terahertz Devices

Journal of Infrared, Millimeter, and Terahertz Waves

July 2013

• Toshihiko Ouchi, 
• Kousuke Kajiki, 
• Takayuki Koizumi, 
• Takeaki Itsuji, 
• Yasushi Koyama, 
• Ryota Sekiguchi,
• Oichi Kubota, 
• Kodo Kawase

http://link.springer.com/article/10.1007/s10762-013-0004-5

A terahertz (THz) imaging system and high efficient terahertz sources and detectors for medical applications were developed. A fiber laser based compact time domain terahertz tomography system was developed with a high depth resolution of less than 20 μm. Three-dimensional images of porcine skin were obtained including some physical properties such as applied skin creams. The discrimination between healthy human tissue and tumor tissue has been achieved using reflection spectra.

To improve the THz imaging system, a ridge waveguide LiNbO₃ based nonlinear terahertz generator was studied to achieve high output power. A ridge waveguide with 5-7 μm width was designed for high efficiency emission from the LiNbO₃ crystal by the electro-optic Cherenkov effect.

Terahertz electronic sources and detectors were also realized for future imaging systems. As electronic source devices, resonant tunneling diode (RTD) oscillators with a patch antenna were fabricated using an InGaAs/InAlAs/AlAs triple barrier structure. On the other side, Schottky barrier diode (SBD) detectors with a log-periodic antenna were fabricated by thin-film technology on a Si substrate. Both devices operate above 1 THz at room temperature. This electronic THz device set could provide a future high performance imaging system.