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Characterization and Applications of Liquid Crystals in the THz Frequency Range
(2012-01-01)
Pan, Ci-Ling
;
Pan, Ru-Pin
;
電子物理學系
;
Department of Electrophysics
The applications of liquid-crystal-based devices the sub-millimeter wave or THz (1 THz = 10(12) Hz) frequency range has blossomed recently. In this paper, we review the methodology for determination of the THz optical constants of nematic liquid crystals, using E7 as an example. To demonstrate potential applications, we report an electrically tuned THz Solc filter.
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.
Recent progress in liquid crystal THz optics
(2006-01-01)
Pan, Ci-Ling
;
Pan, Ru-Pin
;
電子物理學系
;
光電工程學系
;
光電工程研究所
Recently, there have been increasing interests in the study of liquid-crystal-based devices for application in the submillimeter wave or THz frequency range. In this paper, we present recent progress in liquid crystal THz optics from our group. Using time-domain THz spectroscopy, we have determined the complex indices of refraction of nematic liquid crystals, 5CB, PCH5 and E7 from 0.2 to beyond 1 THz. Significantly, the birefrincrence of 5CB and E7 are found to be as large as 0.2 at THz frequencies, while the absorption is negligible. Electrical-field and magnetic-field-controlled birefringence in LC were also investigated. A tunable room-temperature THz phase shifter using magnetic-field-controlled birefringence in nematic 5CB gives a phase shift as large as 108 degrees at 1.0 THz. Phase shift exceeding 360 degrees at 1 THz, an important milestone, was realized by using a sandwiched LC (E7, Merck) cell as thick as 3 nun. The magnetically tuned LC phase shifter served as key components in a two-element tunable LC Lyot filter. The tuning range of the filter is from 0.388 to 0.564 THz or a fractional tuning range of similar to 40%. Our work clearly demonstrates the potential of liquid crystal devices for THz applications. Finally, we will present initial works on control of enhanced THz transmission through a metallic hole array with nematic liquid crystals. Our work clearly demonstrates the potential of liquid crystal devices for THz applications.
Liquid-crystal-based electrically tunable THz optical devices
(2007-01-01)
Pan, Ci-Ling
;
Pan, Ru-Pin
;
電子物理學系
;
光電工程學系
;
光電工程研究所
In this work, we report recent progress in liquid-crystal-based electrically tunable THz optical devices. Tunable phase shift up to 360 degrees at I THz is demonstrated using electrically controlled birefringence in a vertically aligned nematic liquid crystal (E7) cell, 1.83 nun in thickness. The driving voltage and corresponding field required for a phase shift of 360 degrees at 1 THz are 100 V and 90.5 V/cm, respectively. A sandwiched NLC cell about 2 mm in total thickness is used to increase the interaction length while minimizing Fresnel losses at the interfaces. A phase shift of 367 degrees is demonstrated at 1.05 THz, significantly improving the dynamic response of the device.
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.
Sub-single-cycle optical pulse train with constant carrier envelope phase
(2008-04-25)
Chen, Wei-Jan
;
Hsieh, Zhi-Ming
;
Huang, ShuWei
;
Su, Hao-Yu
;
Lai, Chien-Jen
We describe the synthesis of periodic waveforms consisting of a train of pulses that are 0.83 cycles long and have an electric field pulse width of 0.44 fs using 7 Raman sidebands generated by molecular modulation in H(2). We verify by optical correlation that the carrier-envelope phase is constant in these waveforms when they are synthesized from commensurate sidebands. The estimated overall shift of the carrier-envelope phase is less than 0.18 cycles from the first to the last pulse of nearly 10(6) pulses in the pulse train.
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.
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
Liquid crystal surface alignments by using films composed of magnetic nanoparticles - art. no. 691104
(2008)
Pan, Ru-Pin
;
Wu, Hsin-Ying
;
Hsieh, Cho-Fan
;
電子物理學系
;
Department of Electrophysics
A method for liquid crystal surface alignment by using a one-step, ion beam sputtering on glass substrates is demonstrated. Pre-coating by polyimide is not necessary. We use a diode-type sputter to treat the glass substrates with Ar ion-beam. The homeotropic alignments for nematic liquid crystals are achieved. The alignments are characterized by using the polarizing optical microscope and the conoscope. To find out the alignment mechanism, the studies by using super conducting quantum interference device and scanning probe microscopy are carried out. The surveyed surface morphology reveals that the films are amorphous and composed of nanoparticles with dimensions around 30 nm. The magnetization anisotropy of the sputtered magnetic films is analyzed. The polar anchoring strengths of the coated films with different thicknesses are measured and compared with their saturation magnetization. We deduce that the homeotropic alignment is achieved due to the orientation of the diamagnetic nematogenic molecules in the magnetic field caused by the gamma-Fe2O3 ferrimagnetic thin films. A simple model of alternatively distributed magnetic moments with opposite direction is proposed. The profile of magnetic field strength near the surface is then calculated to compare with the measured alignment strength.
The Anchoring Strength of Liquid Crystals on Polyimide Surfaces Treated by a Direct-Current Ion Beam Sputter
(2011)
Tai, Chao-Yu
;
Wu, Hsin-Ying
;
Huang, Meng-Chiou
;
Pan, Ru-Pin
;
電子物理學系
In this work we report the alignment property of nematic liquid crystal with variable pretilt angle on surfaces treated by argon plasma. The scanning electron microscope (SEM) image indicates that nanoparticles are deposited on treated PI film. Groove-like aggregating structures are observed and the direction of aggregate tends to parallel to the ion beam direction. X-ray photoelectron spectroscopic (XPS) analysis confirms that -Fe2O3 nanoparticles were deposited on the surface of polyimide (PI) films. We also deduce that the maghemite is the main factor determining the pretilt angle while a competition exists between the homogeneous and homeotropic alignments.

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