Browsing by Author "Yang, Chan-Shan"

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Broadband terahertz conductivity and optical transmission of indium-tin-oxide (ITO) nanomaterials
(2013-07-15)
Yang, Chan-Shan
;
Chang, Chan-Ming
;
Chen, Po-Han
;
Yu, Peichen
;
Pan, Ci-Ling
Indium-tin-oxide (ITO) nanorods (NRs) and nanowhiskers (NWhs) were fabricated by an electron-beam glancing-angle deposition (GLAD) system. These nanomaterials are of interests as transparent conducting electrodes in various devices. Two terahertz (THz) time-domain spectrometers (TDS) with combined spectral coverage from 0.15 to 9.00 THz were used. These allow accurate determination of the optical and electrical properties of such ITO nanomaterials in the frequency range from 0.20 to 4.00 THz. Together with Fourier transform infrared spectroscopic (FTIR) measurements, we found that the THz and far-infrared transmittance of these nanomaterials can be as high as 70% up to 15 THz, as opposed to about 9% for sputtered ITO thin films. The complex conductivities of ITO NRs, NWhs as well films are well fitted by the Drude-Smith model. Taking into account that the volume filling factors of both type of nanomaterials are nearly same, mobilities, and DC conductivities of ITO NWhs are higher than those of NRs due to less severe carrier localization effects in the former. On the other hand, mobilities of sputtered ITO thin films are poorer than ITO nanomaterials because of larger concentration of dopant ions in films, which causes stronger carrier scattering. We note further that consideration of the extreme values of Re{sigma} and Im{sigma} as well the inflection points, which are functions of the carrier scattering time (tau) and the expectation value of cosine of the scattering angle (gamma), provide additional criteria for accessing the accuracy of the extraction of electrical parameters of non-Drude-like materials using THz-TDS. Our studies so far indicate ITO NWhs with heights of similar to 1000 nm show outstanding transmittance and good electrical characteristics for applications such as transparent conducting electrodes of THz Devices. (C) 2013 Optical Society of America
Characterization and Comparison of GaAs/AlGaAs Uni-Traveling Carrier and Separated-Transport-Recombination Photodiode Based High-Power Sub-THz Photonic Transmitters
(2010-01-01)
Li, Yu-Tai
;
Shi, Jin-Wei
;
Huang, C. -Y.
;
Chen, Nan-Wei
;
Chen, Shu-Han
We describe in detail the characterization of two high-power photonic transmitters based on two different kinds of high-power photodiodes, one a GaAs/AlGaAs based uni-traveling-carrier photodiode (UTC-PD) and the other a separated-transport-recombination photodiode (STR-PD). The diodes operate under optical pulse excitation at the 800 nm wavelength. Both PDs have the same total depletion layer thickness (same theoretical RC-limited bandwidth) and are m integrated with the same broadband micro-machined circular disk monopole antennas to radiate strong sub-THz pulses. However the STR-PD based transmitter exhibits very different dynamic and static performance from that of the UTC-PD based transmitter due to the existence of a low-temperature-grown GaAs (LTG-GaAs) based recombination center inside the active region, and the much thinner thickness of effective depletion layer. Under optical pulse excitation (similar to 480 pJ/pulse), the STR-PD based transmitter exhibits a much lower maximum averaged output photocurrent (1.2 mA versus 0.3 mA) than that of the UTC-PD transmitter, although the radiated electrical pulse-width and maximum peak-power, which are measured by the same THz time-domain spectroscopic (TDS) system, of both devices are comparable. These results indicate that although the recombination center in the STR-PD degrades its DC responsivity, it effectively improves the high-speed and output power performance of the device and eliminates the DC component of the photocurrent, which should minimize device-heating problem during high-power operation. The radiated waveforms of both devices under intense optical pulse illumination also exhibit excellent linearity and strong bias dependent magnitude. This suggests their suitability for application as photonic emitters and possibly as a data modulator in sub-THz impulse-radio communication systems.
Distinct Dynamic Behaviors of High-Power Photonic-Transmitters Based on Uni-Traveling Carrier and Separated-Transport-Recombination Photodiodes
(2009)
Li, Yu-Tai
;
Yang, Chan-Shan
;
Pan, Ci-Ling
;
Shi, J. -W.
;
Huang, C. -Y.
We demonstrate two high-performance photonic-transmitters based on uni-traveling-carrier-photodiode (UTC-PD) and separated-transport-recombination-photodiode (STR-PD). Thanks to recombination center, STR-PD exhibits comparable maximum-peak-power and electrical pulse-width with much smaller photocurrents and less serious device-heating to those of UTC-PD.
Dynamic Analysis of Cascaded Laser Power Converters for Simultaneous High-Speed Data Detection and Optical-to-Electrical DC Power Generation
(2011-07-01)
Shi, Jin-Wei
;
Kuo, F. -M.
;
Yang, Chan-Shan
;
Lo, S. -S.
;
Pan, Ci-Ling
We analyze the dynamic behaviors of a novel device, i.e., cascaded high-speed laser power converters (LPCs), which can detect the direct-current (dc) component of an incoming highspeed optical data stream and efficiently convert its dc component to dc electrical power. By utilizing a p-type photoabsorption layer in our LPC, the problem of slow-motion holes can be eliminated, and only the electrons act as the active carriers. We can thus achieve high-speed performance with the LPC under forward-bias operation with a small electric field inside. Furthermore, according to our modeling and measurement results, there are a significant alternating-current capacitance reduction and an electron-trapping effect at the interface between the absorption and collector layers with a significant degradation in the carrier drift velocity. These become more serious with the increase in optical pumping power and forward-bias voltage and truly limit the net optical-to-electrical (O-E) bandwidth of the device. In order to overcome such a transient-time-limited bandwidth and further increase the maximum dc output voltage of the LPC, we connect two single LPCs in series (cascade). Error-free data detection of 10-Gb/s and an O-E dc power-generation efficiency of 21.1% can be achieved simultaneously at a wavelength of 850 nm by the use of such two cascaded LPCs.
High-Transmittance 2 Electrically Tunable Terahertz Phase Shifter with CMOS-Compatible Driving Voltage Enabled by Liquid Crystals
(2019-01-02)
Yang, Chan-Shan
;
Kuo, Chun
;
Chen, Po-Han
;
Wu, Wei-Ta
;
Pan, Ru-Pin
We have investigated tunable terahertz (THz) phase shifters that are based on a sandwiched liquid crystal (LC) cell with indium-tin-oxide (ITO) nanowhiskers (NWhs) as transparent electrodes. More than 360 degrees of phase shift at 1.0 THz was achieved at a driving voltage as low as similar to 2.6 V (rms). This is approximately 40 times smaller than that reported in previous works using an electrically tuned LC device. Significance of the NWhs in reducing the required voltage is demonstrated. Overall transmittance of the device is as high as 30%, which is accountable by absorption losses of ITO NWhs, quartz substrate and LC. Experimental results are in good agreement with a theoretical formulism while taking into account super-thick LC cells (similar to 1 mm) and pretilt angles. We also propose and demonstrate a novel THz technique for measuring pretilt angles of liquid crystals.
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
Liquid crystal terahertz phase shifters with functional indium-tin-oxide nanostructures for biasing and alignment
(2014-04-07)
Yang, Chan-Shan
;
Tang, Tsung-Ta
;
Pan, Ru-Pin
;
Yu, Peichen
;
Pan, Ci-Ling
Indium Tin Oxide (ITO) nanowhiskers (NWhs) obliquely evaporated by electron-beam glancing-angle deposition can serve simultaneously as transparent electrodes and alignment layer for liquid crystal (LC) devices in the terahertz (THz) frequency range. To demonstrate, we constructed a THz LC phase shifter with ITO NWhs. Phase shift exceeding pi/2 at 1.0 THz was achieved in a similar to 517 mu m-thick cell. The phase shifter exhibits high transmittance (similar to 78%). The driving voltage required for quarter-wave operation is as low as 5.66 V (rms), compatible with complementary metal-oxide-semiconductor (CMOS) and thin-film transistor (TFT) technologies. (C) 2014 AIP Publishing LLC.
Liquid crystal terahertz photonics with indium tin oxide nanowhiskers and graphene as functional electrodes
(2015-01-01)
Yang, Chan-Shan
;
Pan, Ru-Pin
;
Pan, Ci-Ling
;
電子物理學系
;
Department of Electrophysics
We have constructed and characterized THz phase shifters based on liquid crystals (LCs) with graphene grown by chemical vapor deposition (CVD) and indium-tin-oxide nanowhiskers (ITO NWhs) as transparent conducting electrodes. A graphene-based phase shifter can achieve a phase shift of pi/2 at 1.0 THz with the operating voltage of similar to 2.2 V (rms) as opposed to similar to 5.6 V (rms) for ITO-NWhs-based phase shifter in previous work. On the other hand, 2 pi phase shift at 1.0 THz was achieved in an ITO-NWhs-based phase shifter with a multi-sandwiched structure by applying similar to 2.6 V (rms). The low operation voltage of both two kinds of phase shifters imply compatibility of both type of devices with thin-film transistor (TFT) and complementary metal-oxide-semiconductor (CMOS) technologies. The experimental results of phase shifters are in good agreement with the theoretical predictions.
Liquid-Crystal Terahertz Quarter-Wave Plate Using Chemical-Vapor-Deposited Graphene Electrodes
(2015-12-01)
Yang, Chan-Shan
;
Kuo, Chun
;
Tang, Chiu-Chun
;
Chen, J. C.
;
Pan, Ru-Pin
Quarter-wave operation or a phase shift of more than pi/2, which is approximately ten times greater than that reported in previous works using liquid crystals (LCs) and graphene electrodes, was demonstrated. The device is transparent to the terahertz (THz) wave, and the driving voltage required was as low as approximately 2.2 V (rms), which is also unprecedented. Experimental results supported a theoretical formalism adapted for LC cells with THz wavelength-scale thickness. The scattering rate, DC mobility, and carrier mean free path of bilayer graphene were also determined using THz spectroscopic techniques; the parameters were inferior to those of monolayer graphene. This observation can be attributed to the higher density of charged impurities in the bilayer graphene. The device performances of LC phase shifters using monolayer and bilayer graphene as electrodes were essentially identical.
Liquid-crystal-based magnetically tunable terahertz achromatic quarter-wave plate
(2019-04-01)
Hsieh, Cho-Fan
;
Yang, Chan-Shan
;
Shih, Fang-Cih
;
Pan, Ru-Pin
;
Pan, Ci-Ling
Development of the wideband and tunable quasi-optic terahertz (THz) components is in high demand. In this work, we demonstrate a tunable achromatic quarter-wave plate (AQWP) for the THz frequency range. The phase retardation of this device can be set at 90 degrees +/- 9 degrees from 0.20 to 0.50 THz. The operation range from 0.20 to 0.50 THz can be tuned to from 0.30 to 0.70 THz by introducing three nematic liquid crystals phase retarders, of which the birefringence can be magnetically tuned. The frequency-dependent phase retardation is in good agreement with theoretical predictions. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Liquid-crystal-enabled Electrically Tunable Terahertz Achromatic-wave Plate
(2016)
Yang, Chan-Shan
;
Shih, Fang-Cih
;
Pan, Ru-Pin
;
Yu, Peichen
;
Pan, Ci-Ling
A THz achromatic quarter-wave plate (AQWP) based on liquid-crystal (LC) phase shifters which have different thickness and orientations, respectively, has been designed and demonstrated. Electrical properties for LC cells is taken into account as well to be used to compensate the thickness of the cell and to accomplish the aim of achromatic tunability. Indium-tinoxide nanowhiskers which show outstanding THz transmittance are employed as the transparent electrodes in AQWP. The performance of the AQWP analyzed by Stokes parameters have shown the achromatic ability in the frequency range of 0.2 similar to 0.8 THz, and can be electrically tuned to 0.3 similar to 0.9 THz.
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.
The complex refractive indices of the liquid crystal mixture E7 in the terahertz frequency range
(2010-09-01)
Yang, Chan-Shan
;
Lin, Chia-Jen
;
Pan, Ru-Pin
;
Que, Christopher T.
;
Yamamoto, Kohji
We have used terahertz time-domain spectroscopy to investigate the complex optical constants and birefringence of a widely used liquid crystal mixture E7 in both nematic and isotropic phases (26 degrees C-70 degrees C). The extinction coefficient of E7 at room temperature is less than 0.035 and without sharp absorption features in the frequency range of 0.2-2.0 THz. The extraordinary (n(e)) and ordinary (n(o)) indices of refraction at 26 C are 1.690-1.704 and 1.557-1.581, respectively, giving rise to a birefringence of 0.130-0.148 in this frequency range. The temperature-dependent (26 degrees C-70 degrees C) order parameter extracted from the birefringence data agrees with that in the visible region quite well. Further, the temperature gradients of the terahertz optical constants of E7 are also determined. The optical constants of E7 in the terahertz or sub-millimeter wave range are found to deviate significantly from values predicated by the usual extended Cauchy equations used in the visible and near-infrared. (C) 2010 Optical Society of America
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.
Voltage-controlled 2 pi Liquid-crystal Terahertz Phase Shifter with Indium-tin-oxide (ITO) Nanowhiskers as Transparent Electrodes
(2014-01-01)
Yang, Chan-Shan
;
Pan, Ru-Pin
;
Yu, Peichen
;
Pan, Ci-Ling
;
電子物理學系
Phase shift exceeding 2 pi at 1.0 THz with high transmittance was achieved in liquid-crystal THz phase shifters of three different designs. Indium-tin-oxide (ITO) nanowhiskers (NWhs) were employed as transparent electrodes. The driving voltage required for a 2 pi wave plate is as low as 5 Vrms.
Voltage-controlled liquid-crystal terahertz phase shifter with indium-tin-oxide nanowhiskers as transparent electrodes
(2014-04-15)
Yang, Chan-Shan
;
Tang, Tsung-Ta
;
Chen, Po-Han
;
Pan, Ru-Pin
;
Yu, Peichen
Indium-tin-oxide nanowhiskers were employed as transparent electrodes in a liquid-crystal terahertz phase shifter. Transmittance of the device was as high as similar to 75%. Phase shift exceeding pi/2 at 1.0 THz is achieved in a similar to 500 mu m-thick cell. The driving voltage required for the device operating as a quarter-wave plate was as low as 17.68 V (rms), an improvement of nearly an order of magnitude over previous work. (C) 2014 Optical Society of America