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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.
Characterization of Subwavelength Plastic Fiber Utilizing Terahertz Time-Domain Spectroscopy
(2009-01-01)
You, Borwen
;
Liu, Tze-An
;
Peng, Jin-Long
;
Pan, Ci-Ling
;
Lu, Ja-Yu
We present the first characterization of a simple subwavelength-diameter plastic wire by using wideband terahertz time-domain spectroscopy. The propagation characteristics including waveguide dispersion, group velocity, and attenuation constant of various plastic wires with different diameters and refractive indices are studied. The experimental results show the subwavelength plastic wire has extremely low waveguide dispersion and low attenuation constant (<0.01cm(-1)) at its THz transmission band due to much reduced fractional power delivered inside the lossy core, which is consistent with the theoretical calculations. With the large evanescent-fields, the subwavelength plastic wire has capability to integrate with micro-fluid channel for sensitive bio-sensing applications.
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.
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.
A 340 GHz Triple-Push Oscillator With Differential Output in 40 nm CMOS
(2014-12-01)
Li, Chun-Hsing
;
Ko, Chun-Lin
;
Kuo, Chien-Nan
;
Kuo, Ming-Ching
;
Chang, Da-Chiang
A low-power triple-push oscillator with differential output is proposed in this letter. By extracting signals from the same current loop, the oscillator can naturally provide differential output without any additional active circuit or passive balun required. Therefore, the output power can be increased and the chip area and power consumption can be reduced. Realized in 40 nm CMOS technology, the proposed oscillator can oscillate at 340.6 GHz while providing equivalent isotropically radiated power (EIRP) as -21.8 dBm. The power consumption is only 34.1 mW from a 0.9 V supply. The oscillator core only occupies area of 0.028 mm(2).
利用摻雜之硒化鎵晶體產生兆赫波之研究
(2009)
羅志偉
;
Luo Chih-Wei
;
國立交通大學電子物理學系(所)
本計畫將尋找適當的元素摻雜於非線性半導體硒化鎵晶體中，以提高其產生兆赫波之效率。近來已有研究群利用硒化鎵晶體產生一可調波長之兆赫波幅射。也因為此種晶體在兆赫波波段吸收率較低，所以在眾多非線性光學晶體中最被看好的材料之一。不過其本身的均勻度及光學透明度，卻成為產生高功率兆赫波幅射之限制。一般來說，利用差頻產生兆赫波的方法中，增加晶體厚度應該可增加兆赫波的輻射功率。但到目前為止，實驗上並未觀察到此現象。因此，透過離子半徑較大元素的摻雜來增加其極化率或均勻度以提高兆赫波的輻射功率。另外，利用偶極天線的結構來產生兆赫波幅射，同樣透過不同元素的摻雜，也可增加其電阻率及載子之遷移率以提高兆赫波的輻射效率，同時更可研究其載子之動力學性質。所以，不管利用何種方式產生兆赫波輻射，本計劃的目標在透過不同元素之摻雜，同時配合尋找穩定的長晶條件，以得到一具有高均勻度、高機械強度、高透光率、高電阻率等特性之高品質非線性半導體硒化鎵晶體，進而實現高功率及波長可調之兆赫波幅射光源。最後，將利用此兆赫波幅射進行高溫超導體、多鐵錳氧化物、磁性半導體或半導體量子點之超快動力學研究。
Optical constants of two typical liquid crystals 5CB and PCH5 in the THz frequency range
(2003)
Pan, RP
;
Tsai, TR
;
Chen, CY
;
Pan, CL
;
電子物理學系
The complex refractive indices of two benchmark nematic liquid crystal, 4'-n-pentyl-4-cyanobiphenyl (5CB) and 4-(trans-4'pentylcyclohexyl)-benzonitrile (PCH5) have been determined in the frequency range from 0.2 to 0.8 THz. The technique of coherent THz time-domain spectroscopy (THz-TDS) was used. We show that the birefringence of 5CB is in the range of 0.15 to 0.21, while that of PCH is from 0.01 to 0.08. Both liquid crystals exhibit relatively small absorption in this frequency range. The large birefringence of 5CB indicates possible applications of liquid-crystal-based devices for modulation and polarization control of electromagnetic radiation in the THz frequency range.
The refractive indices of nematic liquid crystal 4'-n-pentyl-4-cyanobiphenyl in the THz frequency range
(2004)
Pan, RP
;
Tsai, TR
;
Chen, CY
;
Wang, CH
;
Pan, CL
The terahertz (THz) refractive indices of a liquid crystal 4'-n-pentyl-4-cycazobiphenyl (5CB) in both of the nematic and isotropic phases were deters mined by terahertz (THz) time-domain spectroscopy for the first time, to our knowledge. We find that the extinction coefficient of 5CB is negligibly small in this frequency range. The birefringence of 5CB, on the other hand, is as large as 0.2. This is comparable to that of 5CB in. the visible and near M. The experimental results indicate that liquid crystals are potentially useful for device applications in the THz frequency range.
An overview of THz research activities in Taiwan
(2005)
Pan, CL
;
光電工程學系
;
Department of Photonics
Reflecting the flourishing THz science and technology in the past decade, a number of groups in Taiwan have initiated research activities in this area. All major research universities as well as the semi-public Industrial Technology Research Institute are represented. Current research topics include THz devices (emitters, detectors, phase shifters) and the applications of THz time-domain spectroscopy to the study the far-infrared optical properties of various materials. Highlights of recent research activities are outlined.

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