利用同調多級光整流技術於硒化鎵晶體中產生兆赫輻射

Abstract

本論文成功地建構一套理論模型以解釋非線性光學中光整流的機制行為。在考慮相位匹配參數及晶體的吸收係數的情況下，理論模擬利用超快雷射於硒化鎵晶體內光波混頻以產生兆赫輻射。而理論模擬於頻域上的光譜及時域上的波形的結果，可配合實驗數據來證實理論模型的正確性。進一步，將理論模型延伸於多級光整流的範疇，並檢視兆赫輻射於多級光整流情形下的同調機制與行為。 我們於實驗中利用多級光整流技術產生兆赫輻射，並透過調控兩級兆赫輻射之間的時間延遲關係，可以使兆赫輻射同調疊加並且擁有較高能量輸出，進一步證實此技術有應用於兆赫輻射任意波形調控的潛力。接著，利用光譜干涉法的分析方法，可得知兩級產生的兆赫輻射在不同的時間延遲下的同調關係在1 THz以下可達0.8~0.95。另外，經由實驗結果擬合出兆赫輻射於硒化鎵晶體內的非線性吸收截面係數範圍約為(1.3－5.9)×10-17 cm2。 多級光整流技術預期可以克服非線性晶體長度與晶體內群速度不匹配的限制，以同調串聯的方式產生高強度的兆赫輻射。經由理論模型計算的最佳化結果得知，使用超快雷射高脈衝能量產生經由多級光整流方式產生兆赫輻射的輸出可達400.8 nJ。

In this thesis, we successfully construct a theoretical model to interpret the optical rectification process in nonlinear optics. Generation of terahertz radiation via frequencies mixing in GaSe crystal by use of the ultra-fast pulses laser is theoretically simulated by taking into account of the phase-matching parameter and absorption coefficient of nonlinear optical crystal. The frequency spectra and time-domain waveforms of terahertz radiation deduced from the numerical simulation are verified by the experimental results. Furthermore, this theoretical model can be extended to the application of coherent multi-stage optical rectification technique. We experimentally generated terahertz radiation via multi-stage optical rectification process. By accurately adjusting the time delay between the two terahertz radiation from the two stages, the terahertz radiation can be coherently superposed with higher output power. This technique verifies the great potential of spectral synthesis of terahertz radiation. Moreover, it is observed that the coherence between the two terahertz radiation from the two stages is as high as 0.8~0.95 by the analysis of spectral-interfeometry method. Besides, the nonlinear absorption coefficient cross-section of GaSe crystal at terahertz frequencies is experimentally fitted in the range of (1.3－5.9)×10-17 cm2.

Multi-stage optical rectification technique is expected to overcome the interaction length inside the nonlinear crystal, which is usually limited by group velocity mismatching. The high power terahertz radiation can be generated by coherently cascaded optical rectification processes in GaSe crystals. Under the numerical optimization, the single-cycle terahertz radiation can be generated with pulse energy as high as 400.8 nJ by use of the multi-stage optical rectification technique.