Polarization Independent THz Photonics : Phase Shifter and Tunable Bandpass Filter

• Main Authors: Po-sheng Fang, 方柏勝
• Other Authors: Chao-Kuei Lee
• Format: Others
• Language: zh-TW
• Published: 2014
• Online Access: http://ndltd.ncl.edu.tw/handle/rv674c

碩士 === 國立中山大學 === 光電工程學系研究所 === 102 === In recent years, terahertz (THz) photonics have attracted plenty of attraction due to its remarkable progress. As its potential in the field of communication, spectrum analysis and so on, the demand of high performance quasi-optic components such as phase shifters, band pass filters or polarizers is strongly increasing. However, these devices need polarizer or other polarization control system to achieve polarization independency. The first part of this thesis, polarization independent electrical controlled 2p THz phase shifter with sub-second modulation speed was demonstrated by using cholesteric liquid crystal (CLC). The device was constructed by stacking three CLC layers. Each CLC layer with thickness of 1.5mm was sandwiched by two fused silica glasses coated with ITO for electrode. The CLC was made by chiral dopant mixed in a nematic liquid crystal (NLC) host BL006 and pitch length is around 20μm. The structure of CLCs became visible homeotropic texture from invisible focal conic texture when the driving voltage increases from zero till over 360V. The modulation speed increases as the driving voltage increasing. The response time up to 500 ms was successfully achieved using the multi-layer structure. The polarization independence of this component was performed about l/60. Feasibility of 100ms modulation speed was discussed as well. Second, we demonstrated a polarization independent tunable THz band pass filter based on one-dimensional photonic crystal cavity and using CLC. The CLC layer, made by chiral dopant mixed in a nematic liquid crystal (NLC) host BL006 with around 20μm pitch length, was sandwiched by two Bragg mirror with half-wavelength optical thickness of 650μm. The THz Fabry-Perot(FP) is demonstrated using two three-pairs THz high reflective structures. The stop band of 85GHz and 3dB bandwidth of 11GHz are demonstrated. The corresponding Q-factor is about 38. By filling with CLC in the THz FP, the frequency can be tuned from 406GHz to 420GHz as applying voltage on CLC cell. This is the first time to our best knowledge that the electrical controlled THz narrow-band-pass is polarization independent and with tuning rang of 14GHz.