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  4. Study of Optical Gain and Carrier Dynamics in Quantum-Dot Laser Structures
 
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Study of Optical Gain and Carrier Dynamics in Quantum-Dot Laser Structures

Date Issued
2007
Date
2007
Author(s)
Huang, Yu-Sheng
DOI
zh-TW
URI
http://ntur.lib.ntu.edu.tw//handle/246246/57458
Abstract
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.
Subjects
量子點雷射
光學增益
載子動態
Quantum dot laser
optical gain
carrier dynamics
Type
thesis
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