Study on the Liquid-Crystal Tunable Devices with Grating Structure for Terahertz waves
| Autor: | Lin, Chia-Jen, 林家任 |
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| Rok vydání: | 2010 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 98 In the past two decades, terahertz photonics and applications have progressed remarkably. However, tunable components in terahertz range are relatively underdeveloped. The present tunable terahertz devices, however, have limited range of tunability and have to be operated at cryogenic temperatures far below room temperature. To conquer the drawbacks, various tunable terahertz devices operated at room temperature based on nematic liquid crystal have been demonstrated in our group. In this work, based on the experience of fabricating the nematic liquid-crystal-based tunable devices, we demonstrated several tunable devices with grating structure to manipulate THz waves. We constructed a Kerr-lens mode-locking Ti:sappire laser with maximum power 450 mW, 90 MHz repetition rate, and 64 fs in pulse width. A conventional photoconductive antenna based THz-TDS was constructed that can characterize the broadband terahertz signal in the range, 0.1 - 4.0 THz. To investigate the terahertz beam steering, an improved THz-TDS with movable detecting part was constructed with an optical fiber. The improved THz-TDS can characterize the terahertz signal below 1.0 THz. It can detect the terahertz pulse directly and can be more convenient and lower cost than that using the liquid-helium-cooled Si bolometer. We choose the fused silica and nematic liquid crystal E7 and MDA-00-3461 to fabricate our devices. The materials for tunable devices design should have the properties of large birefringence and small absorption. The complex refractive indices of these materials were characterized by the conventional THz-TDS at room temperature 24.5?aC. In the THz range, 0.2-2.0 THz, the complex refractive indices of fused silica are n=1.95, k < 0.01. The ordinary and extraordinary indices of E7 are no = 1.58, ne = 1.71, ?菣o=0.03, and ?菣e=0.01, and which of MDA-00-3461 are no = 1.54, ne = 1.72, ko=0.03, and ?菣e=0.01, respectively. In this frequency range, these materials do not show any sharp resonant absorption and clear dispersion. The studies of the optical constants of NLC E7 and MDA-00-3461 in terahertz range show the attractive potential of the applications due to the comparable large birefringence and relative small extinction coefficient. In this work, we demonstrated several liquid-crystal-based tunable devices with grating structure. These devices including magnetically and electrically controlled phase gratings, and the electrically phase shifter array. We designed the phase gratings that can be utilized as tunable beam splitters, and the phase shift array can be utilized as a beam steerer. |
| Databáze: | Networked Digital Library of Theses & Dissertations |
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