Browsing by Author "Wada, Osamu"

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Liquid Crystal Based Terahertz Phase Shifter with Bi-Layer Structure
(2018-01-01)
Sahoo, Anup Kumar
;
Yang, Chan-Shan
;
Yen, Chun-Ling
;
Lin, Hung Chun
;
Wang, Yu-Jen
We propose and demonstrate a liquid crystal (LC) based terahertz phase shifter with a thick (550 mu m in total) bi-layer LC cell structure incorporating a thin transparent polymeric intermediate layer. The present phase shifter exhibited remarkable performance improvement. The phase shift increased from 94 degrees to 111 degrees; threshold and drive voltages decreased from 2.2 V-RMS to 1 V-RMS and from 10 V-RMS to 8.4 V-RMS, respectively, in comparison with those for single-layer cell structure.
Liquid Crystal Based Terahertz Spatial Light Modulator for Imaging Application
(2018-01-01)
Sahoo, Anup Kumar
;
Yang, Chan-Shan
;
Yen, Chun-Ling
;
Lu, Yuan Chun
;
Lin, Hung Chun
We demonstrate a liquid crystal based THz spatial light modulator (SLM). Indium tin oxide or gold finger type patterns were used for transparent (>70 % at 0.08 to 1.2 THz) conductor and self-cross polarizer. The phase shift and amplitude modulation achieved are greater than 90 degrees and 30 % in broadband in THz frequency. This SLM could become a promising candidate for high speed THz imaging application via compress sensing algorithm
Non-Drude Behavior in Indium-Tin-Oxide Nanowhiskers and Thin Films Investigated by Transmission and Reflection THz Time-Domain Spectroscopy
(2013-08-01)
Yang, Chan-Shan
;
Lin, Mao-Hsiang
;
Chang, Chia-Hua
;
Yu, Peichen
;
Shieh, Jia-Min
A comparative study of indium-tin-oxide (ITO) nanowhiskers (NWhs) and thin films as transparent conductors in the terahertz frequency range are conducted. We employ both transmission-type and reflection-type terahertz time-domain spectroscopies (THz-TDTS and THz-TDRS) to explore the far-infrared optical properties of these samples. Their electrical properties, such as plasma frequencies and carrier scattering times, are analyzed and found to be fitted well by the Drude-Smith model over 0.1-1.4 THz. Further, structural and crystalline properties of samples are examined by scanning electron microscopy and X-ray diffraction, respectively. Non-Drude behavior of complex conductivities in ITO NWhs is attributed to carrier scattering from grain boundaries and impurity ions. In ITO thin films, however, the observed non-Drude behavior is ascribed to scattering by impurity ions only. Considering NWhs and thin films with the same height, mobility of the former is similar to 125 cm(2) V-1 s(-1), much larger than those of the ITO thin films, similar to 27 cm(2) V-1 s(-1). This is attributed to the longer carrier scattering time of the NWhs. The dc conductivities (similar to 250 Omega(-1) cm(-1)) or real conductivities in the THz frequency region of ITO NWhs is, however, lower than those of the ITO thin films (similar to 800 Omega(-1) cm(-1)) but adequate for use as electrodes. Partly, this is a reflection of the much higher plasma frequencies of thin films. Significantly, the transmittance of ITO NWhs (congruent to 60%-70%) is much higher (congruent to 13 times) than those of ITO thin films in the THz frequency range. The underneath basic physics is that the THz radiation can easily propagate through the air-space among NWhs. The superb transmittance and adequate electrical properties of ITO NWhs suggest their potential applications as transparent conducting electrodes in THz devices.
THz conductivities of indium-tin-oxide nanowhiskers as a graded-refractive-index structure
(1970-01-01)
Yang, Chan-Shan
;
Chang, Chia-Hua
;
Lin, Mao-Hsiang
;
Yu, Peichen
;
Wada, Osamu
"Indium-tin-oxide (ITO) nanowhiskers with attractive electrical and anti-reflection properties were prepared by the glancing-angle electron-beam evaporation technique. Structural and crystalline properties of such nanostructures were examined by scanning transmission electron microscopy and X-ray diffraction. Their frequency-dependent complex conductivities, refractive indices and absorption coefficients have been characterized with terahertz time-domain spectroscopy (THz-TDS), in which the nanowhiskers were considered as a graded-refractive-index (GRIN) structure instead of the usual thin film model. The electrical properties of ITO GRIN structures are analyzed and fitted well with Drude-Smith model in the 0.2 similar to 2.0 THz band. Our results indicate that the ITO nanowhiskers and its bottom layer atop the substrate exhibit longer carrier scattering times than ITO thin films. This signifies that ITO nanowhiskers have an excellent crystallinity with large grain size, consistent with X-ray data. Besides, we show a strong backscattering effect and fully carrier localization in the ITO nanowhiskers. (c) 2012 Optical Society of America"
Twisted Nematic Liquid-Crystal-Based Terahertz Phase Shifter using Pristine PEDOT: PSS Transparent Conducting Electrodes
(2019-02-02)
Sahoo, Anup Kumar
;
Yang, Chan-Shan
;
Yen, Chun-Ling
;
Lin, Hung-Chun
;
Wang, Yu-Jen
For this study, we demonstrated three different types of twisted nematic (TN) liquid crystal (LC) terahertz (THz) phase shifters using pristine poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT: PSS) thin films as transparent conducting electrodes (TCEs). The transmittance of spin-coated pristine PEDOT: PSS thin film was as high as 92% in the frequency range of 0.2-1.2 THz. This is among the highest reported. Several TN-LC cells were constructed in a comparative study, which confirmed the reliability of pristine PEDOT: PSS as a TCE layer for THz phase shifter applications. The highest phase shift, required root-mean-square (RMS) driving voltage, and threshold voltage achieved by devices tested were 95.2 degrees at 1 THz, 7.2 V-RMS, and 0.5 V-RMS, respectively. The thickness of the LC layer for the phase shifter was 250 mu m, approximately half as thick as previous designs. In addition, the pristine PEDOT: PSS-based TN-LC phase shifter exhibited a figure-of-merit (FOM) value of approximately 6.65 degree.dB(-1).V-1. This compared favorably with previously reported homogeneously aligned phase shifters with an FOM of 2.19 degree.dB(-1).V-1. Our results indicated that a twisted nematic LC cell with pristine PEDOT: PSS thin films as electrodes is a good combination for a THz phase shifter and wave plates as well as other LC-based THz devices.