Abstract-Liquid-crystal-based magnetically tunable terahertz achromatic quarter-wave plate

Cho-Fan Hsieh, Chan-Shan Yang, Fang-Cih Shih, Ru-Pin Pan, and Ci-Ling Pan

Fig. 1 The LC-based achromatic wave plate has three elements, A, B and C. Each tunable retarder (TR) consists of a pair of rotatable permanent magnets and a homeotropically aligned LC cell. The FP and RA represent polarizer with fixed angle at −75° and rotating analyzer, respectively. Thicknesses of LC layers in TRA, TRB and TRCwere dA = 2.56 mm, dB = 3.86 mm, and dC = 2.56 mm, respectively.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-7-9933

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° ± 9° 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.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Quantum-critical conductivity of the Dirac fluid in graphene

Patrick Gallagher, Chan-Shan Yang, Tairu Lyu, Fanglin Tian, Rai Kou1, Hai Zhang, Kenji Watanabe, Takashi Taniguchi

Probing the electrodynamics of graphene using on-chip terahertz spectroscopy. (A) Current carrying modes of a graphene sheet. The zero-momentum mode corresponds to a plasma of counterpropagating electrons and holes and can be relaxed by electron-hole interactions. The finite-momentum mode corresponds to a fluid of co-propagating electrons or holes with nonzero net charge and cannot be relaxed by charge-carrier interactions. The vector J denotes the net current flow. (B) Cartoon of the sample. Photoconductive switches (“emitter” and “detector”) triggered by a pulsed laser emit and detect terahertz pulses within the waveguide. The transmitted pulse is reconstructed by measuring the current collected by the preamplifier (“A”) as a function of delay between laser pulse trains illuminating the emitter and detector. The graphene is optionally excited by a separate pulsed beam (“pump”) to heat the electron system. (C) Photograph of the heterostructure embedded in the waveguide. Few-layer graphene (FLG) electrodes make contact to the monolayer graphene sheet under study and the WS2 gate electrode. Scale bar: 15 micron. Credit: Science, doi:10.1126/science.aat8687

http://science.sciencemag.org/content/early/2019/02/27/science.aat8687?rss=1
Graphene near charge neutrality is expected to behave like a quantum-critical, relativistic plasma—the “Dirac fluid”—in which massless electrons and holes rapidly collide at a rapid rate. We measure the frequency-dependent optical conductivity of clean, micron-scale graphene at electron temperatures between 77 and 300 K using on-chip terahertz spectroscopy. At charge neutrality, we observe the quantum-critical scattering rate characteristic of the Dirac fluid. At higher doping, we uncover two distinct current-carrying modes with zero and nonzero total momenta, a manifestation of relativistic hydrodynamics. Our work reveals the quantum criticality and unusual dynamic excitations near charge neutrality in graphene.

Abstract-Twisted Nematic Liquid-Crystal-Based Terahertz Phase Shifter using Pristine PEDOT: PSSTransparent Conducting Electrodes

Anup Kumar Sahoo, Chan-Shan Yang,  Chun-Ling Yen, Hung-Chun Lin, Yu-Jen Wang,  Yi-Hsin Lin, Osamu Wada ,Ci-Ling Pan,

https://www.mdpi.com/2076-3417/9/4/761

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° 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 µm, 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. 

Abstract-Voltage-controlled liquid-crystal terahertz phase shifter with indium–tin–oxide nanowhiskers as transparent electrodes

Chan-Shan Yang, Tsung-Ta Tang, Po-Han Chen, Ru-Pin Pan, Peichen Yu, and Ci-Ling Pan  »View Author Affiliations

Optics Letters, Vol. 39, Issue 8, pp. 2511-2513 (2014)
http://dx.doi.org/10.1364/OL.39.002511

http://www.opticsinfobase.org/ol/abstract.cfm?URI=ol-39-8-2511

Indium–tin–oxide nanowhiskers were employed as transparent electrodes in a liquid-crystal terahertz phase shifter. Transmittance of the device was as high as ∼75%. Phase shift exceeding π/2 at 1.0 THz is achieved in a ∼500  μm-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.

© 2014 Optical Society of America

Abstract-Liquid crystal terahertz phase shifters with functional indium-tin-oxide nanostructures for biasing and alignment

Chan-Shan Yang^1,2, Tsung-Ta Tang³, Ru-Pin Pan⁴, Peichen Yu⁵ and Ci-Ling Pan^1,6,a)

1 Department of Physics, National Tsing Hua University, Hsinchu 30013, Taiwan
2 Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
3 Taiwan Semiconductor Manufacturing Company, Hsinchu, Taiwan
4 Department of Electrophysics, National Chiao Tung University, Hsinchu 30078, Taiwan
5 Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan
6 Frontier Research Center on Fundamental and Applied Science of Matters, Hsinchu 30013, Taiwan
a) Author to whom correspondence should be addressed. Electronic mail: clpan@phys.nthu.edu.tw
Appl. Phys. Lett. 104, 141106 (2014); http://dx.doi.org/10.1063/1.4871255

Indium Tin Oxide (ITO) nanowhiskers (NWhs) obliquely evaporated by electron-beam glancing-angle deposition can serve simultaneously as transparent electrodes and alignment layer forliquid crystal (LC) devices in the terahertz (THz) frequency range. To demonstrate, we constructed a THz LC phase shifter with ITO NWhs. Phase shift exceeding π/2 at 1.0 THz was achieved in a ∼517 μm-thick cell. The phase shifter exhibits high transmittance (∼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.