單石波導耦合式砷化銦量子點微碟調制器

Abstract

本篇論文中我們演示了單石波導耦合式砷化銦量子點微碟調制器，藉由數值模擬以及製程技術來設計元件結構，我們達成了在單一基板上微碟共振腔與波導之間光能量的相互耦合。元件製程方面因結構尺寸要求較為精準，製程主要是由電子束微影及乾式電漿輔助蝕刻來達成。 我們利用光纖為主體之穿透實驗架構來探討元件之光特性。藉由觀察穿透頻譜之規則性凹陷並與模擬數據對照，可證實微共振腔與波導之間的耦合現象。本研究觀察到共振腔品質因子約為300，共振模態所造成之凹陷深度可達15dB。我們可藉由微調微碟大小及共振腔-波導間距來調整光耦合的強度，以最佳化光調製時之消光比(extinction ratio)。我們對元件施加外部電場，觀察到Kerr效應造成的共振模態波長紅移現象，並可藉由調控微碟共振腔之跨壓來控制模態平移量，量測到之Kerr係數達8.88×〖10〗^(-18) m^2/V^2，此係數比砷化鎵塊材與量子井結構都來得高。我們由基本物理原理出發，以量子侷限Stark效應(quantum confined Stark effect)及克拉莫-克若尼關係式(Kramers-Kronig relations)來探討與解釋此增強之電光效應。 最後，我們亦利用時間解析量測實際演示了元件之光調製行為，光訊號之消光比可達1.47，證實了量子點微碟共振腔於光調制器之應用。

In this thesis, we demonstrate the monolithic integration of an InAs quantum-dot microdisk modulator and its waveguide coupling structure. By designing the coupling structure with numerical simulation and fabrication technique, we verify the monolithic mode coupling between the microdisk and the waveguide. The device was fabricated by process mainly based on electron beam lithography and dry plasma etching in order to define the structure precisely. Optical characterization was performed by a fiber-based transmission measurement setup. Mode coupling between the cavity and the waveguide was confirmed by measuring the transmission spectrum through the waveguide. Quality factors of the microdisk cavity were found to be about 300. Depth of dips up to 15dB is observed at the resonance wavelength of whispering-gallery modes of the microdisk cavity. The strength of mode coupling can be controlled by varying the disk size and the cavity-waveguide gap. Mode shift owing to the electric-field-induced refractive index change can be observed when a negative bias is applied to the microdisk cavity. By varying the voltage over the cavity, transmitted light with specific wavelength could be modulated through Kerr effect. The quadratic electro-optic coefficient was found to be 8.88×〖10〗^(-18) m^2/V^2, which is larger than that in bulk GaAs materials and in GaAs/AlGaAs quantum well structures. From the aspect of fundamental physical theory, the measured electro-optic effect is attributed to the quantum confined Stark effect and Kramers-Kronig relations. The optical modulation is also demonstrated through time-resolved measurement, and the extinction ratio of 1.47 can be achieved. The quantum-dot microdisk cavities show the potential for optical modulation applications.