在過去的兆赫波時間解析光譜(THz-TDS)的工作裡，我們利用它時間解析的優點去研究半導體當中受光激發產生的暫態超快現象，對於聚焦在半導體上的激發光而言，極高的能量密度足以在半導體表面產生表面電漿態 (surface plasma)。
在這一篇論文裡面，我們將研究兆赫波和表面電漿的交互作用，這部分我們選擇半絕緣的砷化鎵(SI-GaAs)以及InP作為實驗樣品，比較有無光激發情況底下的兆赫波穿透頻譜，光激發的條件是以800nm平均功率0.3W以下的脈衝光作激發，兆赫波探測的時間點為光激發後的240ps，主要量測半導體激發後接近穩態的行為。同時我們也研究旋塗金的奈米顆粒對半導體表面狀態的影響。我們發現存在光激發的時候，半導體表面的電漿態會吸收兆赫波，但是吸收的比例和激發光光強不成比例，研究發現這應該是半導體表面存在很多缺陷所造成。半導體表面旋塗金的奈米顆粒之後，在沒有光激發的情況下，穿透頻譜並沒有特別的改變，給予半導體800nm的脈衝光激發之後，發現兆赫波的吸收和光強呈現近似線性的關係，這是由於金的奈米顆粒改變了半導體的表面狀態。另外我們利用傅利業轉換光譜的相位跟振幅資訊，解析出折射係數跟吸收係數，並佐以古典的Drude model分析近似，得到對應的電漿頻率還有碰撞頻率，最後我們針對反鐵磁性材料碲化錳作變溫的兆赫波穿透頻譜，嘗試解析出鐵磁性隨溫度改變的情形。

For the previous work of Terahertz Time Domain Spectroscopy (THz-TDS) in our group, we have taken the advantages of time resolved property to investigate ultrafast phenomenon in semiconductor which is pumped by optical excitation. For the focused optical power on semiconductor, the extreme high power density would form the surface plasma state. In this thesis, it would be introduced the interactions between plasma and THz wave. For this discussion, we choose SI-GaAs and InP as the experimental samples, comparing the difference of THz spectrum with and without optical pumping. The optical pumping is pulsed laser with 800nm wavelength and average power below 0.3W. It is 240ps that THz probed arrived after optical excitation, we measure the quasi-static optical pumped phenomenon. Also, we investigate the influence of Au nano-particles thin film spin on semiconductor surface. We find that THz wave will be absorbed by the optical pumped surface plasma, but absorption does not proportional to optical pumping power. It has been investigated that this phenomenon may be caused by semiconductor surface traps and defects. For Au nano-particles spin on semiconductor surface, without optical pump, there is no particular influence. With 800nm pulsed optical pump, we find THz absorption is linearly proportional to optical pumping, that is because Au nano-particles change the semiconductor surface state. In addition to that, we used the phase and amplitude from Fourier transform spectrum to extract the index refraction and absorption coefficient. Classical Drude is used to do theoretical calculation and fit our experimental result, then we can get the corresponding plasma frequency and collision rate.
Finally, we check the THz spectrum for anti-ferromagnetic material MnTe under different temperature, for this work, we try to extract the relation of temperature and ferromagnetic properties.

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