- 學位論文
以激發-探測法研究微碟共振腔之載子動力學
Study of Carrier Dynamics in Microdisk Resonators by a Pump-Probe Technique
摘要
微碟共振腔由於其迴音廊模態具有良好的光場侷限，因此擁有高品質係數及小巧尺寸的特性，非常適合用在超大型光電積體整合電路。在本論文中，我們利用兩步驟濕式蝕刻法得到平滑的砷化鎵微碟共振腔側面。此外，我們製作出錐形光纖與元件漸逝場耦合去量測與研究被動與主動微碟共振腔的光學特性。 首先，我們使用錐形光纖耦合，利用飛秒激發-探測技術展示了砷化鎵微碟共振腔之超快全光開關。由於砷化鎵微碟共振腔內的光激發載子密度的改變而導致折射率的變化，使得共振模態發生調變。共振模態在砷化鎵微碟共振腔的穿透頻譜中可被調變超過10 dB，在關閉的模式操作下其開關時間窗口只有8皮秒，而且激發能量只要小到17.5皮焦耳即可達到。經由擬合的結果，砷化鎵微碟共振腔的載子生命期為42皮秒，遠小於塊材砷化鎵數個奈秒的載子生命期。上述的發現指出表面覆合在增加開關速度有重要的影響。 最後，砷化銦/砷化鎵量子點微碟共振腔內的載子動力學也利用錐形光纖耦合飛秒激發-探測技術來量測其特性。由於微碟共振腔內主動層的載子密度的改變而導致折射率的變化，使得共振模態瞬間藍移被發現。較慢的共振模態藍移是由於較慢的載子被量子點補捉的過程及載子在量子點內的鬆弛過程。這歸因於量子點的離散能階使得聲子參與散射過程速率被抑制。在載子覆合過程中，微碟共振腔的載子生命期遠快於塊材的載子生命期。微碟共振腔的表面覆合是重要的因素。
並列摘要
Microdisk resonators exhibit features of high quality factors and compact sizes due to strong light confinement of whispering gallery modes, which makes them suitable for very large scale integration in photonic integrated circuits. In this dissertation, GaAs-based microdisk resonators were fabricated by using two-step wet etching method to obtain smooth sidewalls. Furthermore, to measure and study the optical properties of the passive and active microdisk resonators, tapered fibers were made to couple these devices evanescently. First, ultrafast all-optical switching in GaAs microdisk resonators was demonstrated using a femtosecond pump-probe technique through tapered-fiber coupling. The temporal tuning of the resonant modes resulted from the refractive index change due to photoexcited carrier density variation inside the GaAs micoridisk resonator. Transmission through the GaAs microdisk resonator can be modulated by more than 10 dB with a switching time window of 8 ps in the switch-off operation using pumping pulses with energies as low as 17.5 pJ. The carrier lifetime was fitted to be 42 ps, much shorter than that of the bulk GaAs, typically of the order of nanoseconds. The above observation indicates that the surface recombination plays an important role in increasing the switching speed. Finally, carrier dynamics in InAs/GaAs quantum-dot microdisk resonators were also characterized by femtosecond pump-probe measurements through tapered-fiber coupling. A momentary blue shift of the resonant modes caused by carrier induced refractive index change in the active region of the microdisk resonator was observed. The slower blue shift of the resonant modes results from the slower processes of carrier capture into the quantum dots as well as carrier relaxation inside the quantum dots. This is attributed to the discrete energy levels in quantum dots, which suppresses the rate of phonon-mediated scattering processes. In the recombination process, the carrier lifetime in the microdisk resonator is faster than that of the bulk material. The surface recombination in the microdisk resonator is suggested to be the crucial factor.
參考文獻
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