以光整流方式研究兆赫波輻射產生於摻碲硒化鎵及碲化鋅晶體

Abstract

在本論文中，我們利用高功率超快雷射振盪器研究兩個兆赫波產生的議題，一個為研究摻碲硒化鎵晶體應用於兆赫波產生的各種特性，另一個為探討碲化鋅晶體的載子吸收對兆赫波產生所造成的影響。

在第一部分，我們藉由非相位匹配光整流方式使摻碲硒化鎵晶體產生兆赫波輻射，硒化鎵晶體摻雜之碲原子可提高兆赫波產生之效率，且產生之中心頻率可以利用改變晶體厚度來調變，且該晶體具有大範圍的光致兆赫波輻射的線性轉換特性，這些特性證明了摻碲硒化鎵晶體具有兆赫波波段應用之潛能。另外，我們利用兆赫波時間解析光譜進一步了解摻碲硒化鎵晶體的光學特性與摻雜濃度的關係，並將此結果與樣品產生兆赫波輻射之效率做比較，我們發現吸收峰強度(對應E(2) 聲子振動模態，~0.584 THz)與兆赫波產生強度呈現高度正相關，因此，提出以吸收峰強度作為不同碲摻雜濃度之硒化鎵晶體品質判定的依據。

在第二部分中，我們利用不同波長及能量密度的激發光，系統性的研究碲化鋅晶體中自由載子對於兆赫波產生之影響。在激發能量密度小於6.36 mJ/cm2 (中心波長為800 nm)，碲化鋅晶體中自由載子數量隨激發光能量密度增加而增加，此現象進一步造成兆赫波時域波形之時間位移、兆赫波頻譜權重的變化、兆赫波輻射強度的飽和以及光致螢光頻譜的紅移。在高能量密度的激發下(> 6.36 mJ/cm2)，由於受激自由載子數量的飽和，使得兆赫波輸出強度呈現二次方成長。另一方面，我們也利用光子能量高於碲化鋅晶體能隙的激發光源(中心波長400 nm)研究兆赫波產生與自由載子的關係。在方位角相依實驗中，證實以波長400 nm作為激發光源的兆赫波產生機制仍為光整流效應，這與使用800 nm作為激發光源產生機制相同。此外，由光致螢光頻譜的持續紅移，顯示碲化鋅晶體表面上的自由載子數量隨著激發能量密度增加而增加，且在高激發能量密度下(> 2.31 mJ/cm2)，我們觀察到過多自由載子所造成的游離效應，因而導致兆赫波與螢光產生之效率急遽下降。以上之結果將在本論文中詳細討論。

In this dissertation, we investigate two main topics on the terahertz (THz) generation. One is Te-doped GaSe for THz generation. The other one is the competition between the free carrier absorption and the optical rectification on ZnTe crystals.

In the first part, we demonstrated the wide-range linearity between the optical pumping intensity and the electric field of radiated THz on GaSe:Te crystals, that can be promising and potential nonlinear crystals for THz applications. It was found that the dopant, Te atoms, in GaSe crystals significantly enhances the efficiency of THz generation, and its central frequency can be tuned by varying the crystal thickness through non-phase-matched optical rectification. Moreover, the evolution of the absorption peaks of the phonon modes E(2) (~0.584 THz) and E(2) (1.77 THz) on Te-doped GaSe crystals was studied by THz time-domain spectroscopy. This study proposes that the evolution of both E(2) and E(2) absorption peaks correlates well with the optical quality of Te-doped GaSe crystals, which was confirmed by experimental results on the efficiency of THz generation by optical rectification. Maximal intensity of the absorption peak of the rigid layer mode E(2) is proposed as a criterion for identification of optimal Te-doping in GaSe crystals.

In the second part, we systematically investigated the influence of free carriers on the generation of THz in ZnTe crystals with 800 nm and 400 nm excitation over a wide range of pumping fluences. On the 800 nm excited ZnTe crystals, as the pumping fluences increase (< 6.36 mJ/cm2), the concentration of free carriers gradually increases and the THz output power saturates, which were clearly demonstrated by the time delay in the THz temporal waveforms, the changes in the THz spectral weight and the redshift in the PL spectra. For high pumping fluences (> 6.36 mJ/cm2), spectacularly, there is a quadratic increase in the THz output power when the pumping fluences increase, as well as at low pumping fluences of < 0.58 mJ/cm2, because of the saturation of free carriers. On the other hand, from the azimuth angle-dependent experiments, the mechanism of THz generation on the 400 nm excited ZnTe crystals is optical rectification, which is same as that in the 800 nm excited ZnTe crystals. Additionally, we also demonstrated the pumping fluence-dependent experiments on the 400 nm excited ZnTe crystals. The concentration of free carriers on the 400 nm excited ZnTe continuously increases as pumping fluences increase, which was disclosed by the continuously shift in the peak of the PL spectra. For the high pumping fluences (> 2.31 mJ/cm2), the ionization effect on the 400 nm excited ZnTe crystals was observed. These results will be discussed in the dissertation.