週期性分散式砷化鋁鎵銦多量子井的光學特性: 自發輻射、受激輻射及飽和吸收特性

Abstract

週期性排列的半導體多量子井結構作為半導體雷射的增益介質上相較於傳統的異質結構具有波長可調、低激發放射的閾值及較好的載子侷限特性。而相較於離子摻雜的晶體，其作為被動式Q開關固態雷射的飽和吸收體上也具有非飽和損失較小及作用長度極短的優點。結合以上特性半導體多量子井結構很有潛力作為一個同時進行飽和吸收及波長轉換的光學元件。此外由其近似二維的量子侷限結構加上高強度的激發使得受激載子間的多體效應相當顯著。因此本論文架構基本上可分為三個部份，第一部份首先討論高濃度電子電洞對在近似二維侷限的多量子井結構下展現的多體效應，藉由較高的侷限能障及週期性排列的多量子井結構我們觀察到了常溫下重整化能隙的自發輻射頻譜，相較於過去文獻僅對此輻射頻譜作不同受激載子濃度下的探討，我們對此多體效應下新發生的能態對溫度的頻譜變化及輻射頻譜的集成強度作詳細的探討，並配合其發生的閾值強度隨溫度增加指數提升的結果驗證了週期性的多量子井結構對常溫下重整化能態自發輻射頻譜的觀察的重要性。 在了解了高激發強度下多量子井結構的螢光特性後，本論文的第二部份則是實現以砷化鋁鎵銦多量子井結構作為光激發半導體雷射的增益介質。首先藉由掺釔光纖的功率放大器作為激發光源實現了出光波長在1220 nm的高重覆率及高尖峰功率光激發半導體雷射，在脈衝頻率及寬度可調的激發光源下可優化最佳的雷射輸出條件。接著在相同的激發光源下將同樣砷化鋁鎵銦半導體多量子井結構設計在光通訊及人眼安全波段以得到1520 nm的高重覆率及高尖峰功率光激發半導體雷射輸出，其中經由高透明度及高熱傳導率的單晶鑽石散熱片與半導體增益介質的鍵合技術解決了高重覆率及高激發功率下的熱反轉現象，使得最佳雷射操作重覆率從30 kHz提升到200 kHz且最高平均功率輸出提升了4.7倍，即使將重覆率提升到500 kHz仍有2.32 W的平均功率及170 W的尖峰功率輸出。 本論文最後一部份則是利用設計在1530 nm的砷化鋁鎵銦多量子井結構同時作為1064 nm掺釹釩酸釔固態雷射的飽和吸收體及波長轉換元件，其多量子井結構n=2的能階提供1064 nm的非線性飽和吸收作用而n=1的能階搭配適當耦合共振腔則可進行增益轉換產生1530 nm的脈衝雷射輸出，此一架構結合了自調Q雷射、腔外脈衝式幫浦雷射及半導體多量子井結構的優點，由於被動式Q開關不需額外電子驅動元件且和受掺離子晶體飽和吸收體及非線性晶體相比量子井結構的作用長度相當短可縮減腔長，因此若搭配第二部份所述不同介質間的鍵合技術其相當具潛力作為低成本、簡單、一體成形且波長可設計範圍廣的微片被動式調Q雷射。

Recently, the semiconductor multiple-quantum-wells (MQWs) structure is extensively used in the semiconductor lasers owing to the characteristics like versatile emission wavelength, lower laser threshold and excellent performance under room temperature operation. It is also applied as a promising saturable absorber in the diode-pumped passively Q-switched solid-state laser. Compared to the doped crystal saturable absorber, the MQWs absorber has lower non-saturable loss and allows the shorter cavity length. According to these characteristics, the MQWs structure has the potential to be designed simultaneously as a saturable absorber and an active medium in the intra-cavity pumped solid-state lasers. Besides, the many-body effect of the high density electron-hole plasma (EHP) under this quasi-2D confinement structure is also an interesting issue. Therefore, the contents of this dissertation are organized to be three parts. At the first part we investigate the many-body effect of the high density EHP in the quasi-2D confined MQWs structure. The room temperature spontaneous emission of the renormalized bandgap is observed under the designation of high confinement energy and periodically aligned gain structure. The temperature dependent luminescence features such as photoluminescence (PL) spectrum and the integrated PL intensity are discussed and the threshold excitation intensity of the luminescence of renormalized band-edge is shown to be exponentially increased with increasing temperature. As a result we have confirmed that the periodically aligned MQWs structure is beneficial to the observation of room temperature many-body state emission In the second part, an AlGaInAs MQWs structure is designed to be the gain chip of a high repetition rate and high peak power 1220 nm optically-pumped semiconductor laser (OPSL). By using an Yb-doped master oscillator fiber amplifier as the pump source, the output performance could be optimized with the free controlled pump conditions such as pump repetition rates and pulse durations. Then the same pump laser is used to excite the high repetition rate and high peak power AlGaInAs MQW OPSL at the communication and eye-safe spectral region of 1520 nm. By capillary bonding the highly transparent and thermal conductive single crystal diamond heat spreader to the gain chip, the thermal roll-over effect was eased at high average pump power under high repetition rate operation. The optimized repetition rate was raised from 30 to 200 kHz and the maximum average output power was scaled to be 4.7 times higher. When operated at as high as 500 kHz, the maximum average power of 2.32 W and peak power of 170 W were obtained. At the end of this dissertation an AlGaInAs MQW structure is used simultaneously as the saturable absorber (SA) and wavelength-converted component in the Nd:GdVO4 laser. It is fabricated to accommodate the pump level of 1064 nm and the emission level of 1530 nm in the quantum well region. The upper state is served as the nonlinear saturable absorption device at 1064 nm and the lower state is served as the active medium at 1530 nm with additional couple-cavity in the Nd:GdVO4 laser. This configuration combines the advantages of the self-Q-switched lasers, pulse-pumped solid state lasers and the semiconductor MQW structures. Because there is no need of additional electronic drivers for passively Q-switches and the shorter action lengths of the MQWs compared to the bulk crystal SAs and nonlinear crystals, the device configuration could be simplified and cavity length is shortened, respectively. According to these characteristics and the capillary bonding technique in second part, this laser system has potential to be served as a low cost, simple and monolithic passively Q-switched microchip laser with broad achievable spectral range.