臺灣博碩士論文加值系統

我們研製了在兆赫輻射(1012 赫茲)波段，以電壓控制其雙折射係數的向列型液晶盒所製作而成的可調式π/2相位調制器及四分之一波片，我們使用了E7這種向列型液晶，在0.2~1.2兆赫這個波段，E7的雙折射係數約為0.17，此外其適用的溫度範圍很大(攝氏-10°~61°)，能夠操作於室溫。我們使用垂直配向的液晶厚樣品，並以銅片作為樣品電極和墊片，以及利用電場控制液晶的排列，讓液晶盒能夠調制相位超過90度，因此可將它當作一個兆赫波段的四分之一波片。在頻率為一個兆赫的時候，室溫下524μm的液晶盒操作電壓為125伏特(均方根值)，最大的調制角度可達到92.2�a，調制的反應時間分別為7.5秒(上升時間)及373.5秒(下降時間)，另外此篇論文中，將會描述和比較電控與磁控的相位調制器的特色與優點。

The electrically controlled refractive index in a nematic liquid crystal cell for switchable π/2 phase shifting and quarter wave plate of terahertz wave was demonstrated. We used the nematic LC, E7, which exhibits a birefringence of ~0.17 (0.2~1.2THz), useable at the room temperature range because of the wide nematic range (-10�aC~61�aC). The thick, homeotropically aligned LC cell with copper sheets used for mylar and electrodes, and the use of electrical field to let the tuning of phase up to 90�a, which equivalent a THz quarter wave plate. The maximum phase shift of 92.2�a was achieved at 1.00 THz when the interaction length was 524 �慆 at room temperature. The driving voltage was 125 V (rms), the rise time and fall time of the cell are 7.5 seconds and 373.5 seconds, respectively. Additionally, characteristics and advantages of electrically and magnetically controlled THz phase shifter are described and compared in this thesis.

Contents
Chinese Abstract (摘要) ------------------------------------------------------------------i
English Abstract-----------------------------------------------------------------------------ii
Acknowledgement(誌謝)----------------------------------------------------------------iii
Contents----------------------------------------------------------------------------------------iv
Figure and table List---------------------------------------------------------------------vii
1 Introduction-----------------------------------------------------------------------------1
2 Terahertz Field, Liquid crystal, and phase shifters-------------------6
2.1 Basics of Generation and Detection of THz beam----------------------6
2.1.1 Generation of THz Radiation Using PC Antennas-------------------------------6
2.1.2 Detection of THz Radiation Using PC Antennas--------------------------------8
2.1.3 Applications---------------------------------------------------------------------------10
2.2 Basics of Liquid Crystal--------------------------------------------------------11
2.2.1 The Liquid Crystal phase-------------------------------------------------------------11
2.2.2 Nematic Liquid Crystal---------------------------------------------------------------12
2.2.3 Birefringence---------------------------------------------------------------------------13
2.2.4 External Influences on Liquid Crystals---------------------------------------------15
2.2.4.1 Electric and Magnetic Field Effects--------------------------------------------16
2.2.4.2 Surface Preparations--------------------------------------------------------------17
2.2.4.3 Freedericksz Transition----------------------------------------------------------19
2.2.5 Threshold Voltage---------------------------------------------------------------------20
2.2.6 Application of liquid crystal ---------------------------------------------------------22
2.3 Phase Shifters-----------------------------------------------------------------------23
2.3.1 Phase Retardation----------------------------------------------------------------------23
2.3.2 Microwave and Millimeter wave Phase Shifter -----------------------------------24
2.3.2.1 Mechanical Phase Shifters------------------------------------------------------25
2.3.2.2 Ferrite Phase Shifters------------------------------------------------------------25
2.3.2.3 Semiconductor Device Phase Shifters----------------------------------------25
2.3.2.4 Active FET Phase Shifters------------------------------------------------------26
2.3.2.5 Bulk Semiconductor Phase Shifters-------------------------------------------26
2.3.2.6 Traveling-Wave Phase Shifters------------------------------------------------26
2.3.3 THz Phase Shifters--------------------------------------------------------------------27
2.3.3.1 Magnetically Tunable THz Phase Shifter------------------------------------27
2.3.3.2 Voltage Dependent Birefringence---------------------------------------------30
2.3.3.2.1 Electrically Controlled 4.07° Terahertz Phase Shifter-----------------30
2.3.3.2.2 Electrically Controlled π/2 Terahertz Phase Shifter--------------------31
2.3.3.2.3 In-Plain Switch Terahertz Phase Shifter---------------------------------36
2.3.4 Wave Plate------------------------------------------------------------------------------38
3 Experimental Methods------------------------------------------------------------44
3.1 THz-TDS system-------------------------------------------------------------------44
3.2 Sample Fabrication---------------------------------------------------------------48
3.3 Data Processing---------------------------------------------------------------------58
4 Results and Discussion------------------------------------------------------------61
4.1 THz-TDS Waveforms and Spectra-------------------------------------------61
4.2 Electrically Controlled Phase Shifter----------------------------------------65
4.2.1 In-Plain Switch Phase Shifter-------------------------------------------------------65
4.2.2 Copper Electrode Phase Shifter------------------------------------------------------69
4.2.3 Compare with the Former Work-----------------------------------------------------84
4.2.3.1 Compare with the Electrically Controlled Sample with Gold Strip Line-----------------------------------------------------------------------------84
4.2.3.2 Compare with the Magnetically Controlled Sample---------------------85
5 Conclusion and Future works--------------------------------------------------89
Appendices---------------------------------------------------------------------------------91
Appendix Composition and Property of Cooper Used In our Experiment-----------------------------------------------------------------------------------91
References-----------------------------------------------------------------------------------93

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