量子點雷射之光學增益與載子動態特性研究

Abstract

量子點雷射因具有較低的臨界電流密度、高光學增益以及高特徵溫度等特 性，目前已引起廣泛的研究，而半導體光放大器因光纖網路的蓬勃發展，頻寬需 求增加，極高增益與極大調變頻寬的元件更是目前非常熱門的研究主題。本論文 針對量子點的光學增益與載子的動態行為分成兩大主題探討。 在第一部分中，我們利用改良式多段元件量測半導體光放大器的模態增益與 吸收頻譜。文獻上有眾多方法可以得到增益頻譜，例如Hakki-Paoli method、Henry method、長度變化法(Variable stripe length method)、多段元件法(Multisection device) 等等，每個方法都有其優缺點，例如Hakki-Paoli method 需要高解析度的頻譜分析 儀，Henry method 基於一些假設，屬於較為間接的方法，長度變化法在光路的對 準以及耦合上易有誤差，而多段元件法會有漏電流的問題。我們利用接地的方式 消除了漏電流對兩段式元件造成電流密度不均的影響。接著利用三段式結構，也 就是改良式多段元件法(Modified segmented contact method)來消除未被波導侷限的 漏光項。最後利用改良式多段元件法的精確解檢驗第二段與第三段的漏光是否可 忽略，也就是近似解是否合理。 在第二部份中，我們用時間解析之激發-探測(pump-probe)實驗架構來研究量子 點內載子的動態行為(Carrier dynamics)與載子的分佈(Carrier distribution)。首先我 們會探討外部光源由樣品正上方入射時產生的載子在半導體光放大器以及雷射結 構上的分佈有何差異。至於時間解析的pump-probe 實驗，我們用76MHz 重複率 的鎖模藍寶石飛秒雷射(Mode-Locked Ti: Sapphire Femto-second Laser)當作光源，以 半導體光放大器結構來量測載子因為自發放光的時間常數，以及用雷射結構操作 在接近臨界電流時的實驗條件來量測激發放光的動態行為。由兩種不同結構的 pump-probe 實驗比較可以發現自發放光與激發放光的時間尺度上至少差了一個數 量級。

Quantum-dot lasers have attracted much interest in recent years due to their superior properties, such as low threshold current density, high optical gain and high characteristic temperature. Several approaches have been reported in the literature to measure the gain spectra of semiconductor lasers. Henry’s method deduces the gain indirectly from the spontaneous emission spectra under some assumptions. Using Hakki-Paoli method, the gain is calculated from the contrast of modal spectrum oscillations due to the Fabry-Perot cavity below the threshold. A major drawback of this method is the requirement of a high-resolution spectrograph to obtain the true contrasts. In this thesis, net modal gain and absorption spectra were measured by a variable stripe length (VSL) method for an electrically pumped multisection device. This measurement is not limited by the lasing threshold and can be applied even at high excitation densities. Meanwhile, a modified segmented contact (MSC) method is implemented by manipulating the data from single, double, and triple biased sections, respectively. The new approach subtracts background signals from the unguided spontaneous emission, resulting in clean, accurate gain spectra. Besides, the inaccurate estimation of injection current due to current leakage through the finite gap resistance between two adjacent sections is minimized by connecting the unused sections to the ground. The exact solutions of MSC method were also calculated and applied to estimate the magnitude of unguided spontaneous emission of each section. On the other hand, high frequency response and large modulation bandwidth are the other superiorities of quantum dot lasers and amplifiers. Ultrahigh bit rates in the amplifier require ultrafast gain recovery, which is mainly limited by carrier capture and relaxation process in quantum dots1. Time resolved pump-probe experiments were carried out under room temperature in order to understand the carrier dynamics, such as carrier life time, capture time, and relaxation time of our devices. Using optical excitation into GaAs barrier through the window on the top of the device by two degenerate pump and probe laser pulses, we can obtain the carrier life time under spontaneous emission or stimulated emission from semiconductor optical amplifiers or laser devices, respectively. The gain saturation and state filling phenomena were also been observed in our experiments.