以電激發砷化鎵塊材為兆赫波增益介質之研究

Abstract

本論文是在研究砷化鎵塊材在電激發與抽熱的條件下，是否可以達到聲子的居量反轉，形成兆赫波段的增益介質。我們將元件施加高偏壓以利用高電場將電子加速來激發塊材中聲子，並且控制抽熱的條件，觀察反射率頻譜中的特徵譜線隨偏壓的變化。透過與熱平衡狀態下的反射率頻譜相比較，我們發現施加偏壓能使塊材中聲子的居量能有異於熱平衡狀態下的分布。我們進一步嘗試利用恆溫器提供一個良好的抽熱環境將低能量的聲子抽離，預期能使整體系統有利於產生聲子間的居量反轉。實驗結果並無觀察到反射率頻譜隨偏壓增加而有明顯的變化，且沒有反射率超過1的特徵，說明熱沉(heat sink)沒有選擇性的將元件內部的聲子抽離且在我們的實驗條件下不足以觀察到聲子態間的居量反轉。

In this thesis, we study the possibility of obtaining terahertz gain through population inversion between phonon states in semi-insulating gallium arsenide crystals. Phonons are excited by applying voltage biases to the crystals under controlled and uncontrolled heat sink conditions. Variations of phonon population are probed by measuring bias-dependent reflection spectra. In uncontrolled heat sink experiments, we find the phonon population of an electrically excited crystal differs greatly from that of a crystal under thermal equilibrium in the spectral region where two-phonon sum processes dominate. In controlled heat sink experiments, we do not observe obvious spectral changes with biases. The results indicate that the sink extracts the excited phonons effectively but unselectively. In both heat sink conditions, we do not observe evidence of population inversion between phonon states from reflectance spectra in the spectral region dominated by two-phonon difference processes.