耦合量子井結構的光學吸收特性及共振穿透行為之研究

Abstract

本文分二部份，第一部份計算比較對稱及非對稱性的耦合量子井結構的光學吸收係數，第二部份研究電子在耦合量子結構中的共振穿透行為。耦合量子井由兩個單一量子井及一層很薄的位能障所組成，與單一量子井比較他們有較大的史塔克效應，因為有二個位能井的關係，我們可以調整結構上的參數以達到我們想獲得的光電吸收效能，本文計算出具有二個導帶能階且不同結構的耦合量子井結構的光學吸收係數，經由有限差分束傳輸法（FDBPM）解薛丁格方程式及帕桑方程式獲得波函數及能階，並利用狀態密度的公式去算出非線性光學的極化率進而可以算出吸收係數，我們可藉由調整結構的各項參數去得到可調性好吸收係數高的耦合量子井結構。另外我們模擬將一定大小電場加在非對稱的耦合量子井可以獲得電子在位能井之間的穿透振盪，這些振盪的行為可以讓我們分析電子的運動情形，我們改變各項條件如電場大小，位能障厚薄、及電子的濃度大小去了解電子的共振行為，在對稱性耦合量子井結構除了加電場外再加上平行成長面的磁場去了解電子的共振穿透，在共振時侯穿透時間隨磁場增加而減少及迴旋共振中心有明顯的變化趨勢，我們可以很清楚的了解電子在這些量子結構中的的運動情況。

There are two parts in the thesis. In the first part, we investigate the optical absorption in symmetric and asymmetric coupled quantum wells that consist of two wells and a thinner barrier. Comparing with single quantum well, the coupled quantum well has a larger Stark effect. It also has more freedom to adjust the widths of wells and barrier to acquire a desirable optical property. In the study, we calculate the absorption coefficient of various coupled quantum wells with two eigenstates. By using beam propagation method (BPM), we simultaneously solved time dependent Schrödinger equation and Poisson equation to acquire of wave functions and eigenenergies. By invoking of density matrixes, we calculated the linear susceptibility and the linear absorption coefficient of coupled quantum-well systems. In the second part, we study the resonant tunneling effect of coupled quantum wells in the external electric. We explored electron dynamical evolution relating to the variation of the external electric field strength, barrier thickness, and electrons sheet density. Finally, we apply longitudinal magnetic field to the coupled quantum wells to study electron resonant effects. In a resonant condition, the tunneling time decreases with the increasing of magnetic field.