氧化鋅微共振腔結構下之激子-極激子雷射之光學特性

Abstract

在本篇論文中，我們利用氧化鋅的微共振腔結構作為研究對象。在特性分析中，主要是光學特性上的相關研究。在發光特性方面，利用光激發螢光光譜(PL)以及反射頻譜，量測不同角度的發光特性，藉由變角度的譜譜可以定義出極激子的能量對波向量的關係圖，藉此了解光與激子之間的耦合情形，並且確認兩者之間耦合現象確實存在。 接著我們利用調變共振腔長度來調整光存在微共振腔內的能量，使激子與光之間的耦合情形發生改變。觀察此時的極激子發光情形。 在波向量等於零，當光子能量較激子能量來的低時， 能量對波向量的曲線可以觀察到明顯的轉折處， 使得當極激子由較大的能量向低能處掉落時， 會容易阻塞再轉折處， 這種瓶頸對於我們的目標-激子極激子雷射-是一項需要克服的問題，要克服這種瓶頸現象，便進行了不同的實驗來探討此一現象，我們藉由改變共振腔長度的方法，了解在光與激子對瓶頸現象的關係，接著我們利用變溫的變角度光激發螢光光譜(ARPL)， 確認了極激子與聲子的散射可以幫助克服瓶頸現象， 最後我們利用變功率的變角度光激發螢光光譜(ARPL)，使瓶頸現象可以藉由極激子對極激子本身的散射現象來克服。 為了在室溫的條件下觀測到玻色-愛因斯坦凝聚的現象，我們使用摻釹釩酸釔晶體脈衝雷射來進行激發並且順利在室溫下觀測到同調性的發光現象，與現有類似規格的面射型雷射相比所需的功率只有十分之一， 因此我們認為這個是因為玻色-愛因斯坦凝聚的現象所形成的激子-極激子雷射。我們初步的證明，一樣是利用變角度的光激發螢光光譜(ARPL)， 可以觀察到極激子克服平井現象後， 集中於底部形成玻色-愛因斯坦凝聚的情形。這是在氧化鋅材料下觀測到玻色-愛因斯坦凝聚的例子。

In this paper, we use zinc oxide micro-cavity structure to study the characteristics of exciton-polaritons. By performing angle-resolved photoluminescence and angle-resolved reflection measurements, we can probe the energy-wavevector dispersion curves of cavity polaritons. This dispersion relationship can be used to understand the coupling strength between photons and excitons, and confirm the existence of cavity polaritons. Furthermore, we observed the coupling between different exciton-photon detunings by varying the length of microcavity. When the detuning is negative, photon energy lower than the exciton energy, the anticrossing can be observed in the dispersion curves, which causes a significant change in density of states. Under the condition, the bottleneck behavior should be observed during the process of polariton relaxation. This consequence may originate from the polariton states with very high photon fractions in the low angle region. The bottleneck effect is an important obstacle to the realization of Bose-Einstein condensation in microcavity. Several systematical experiments are performed to understand the possible physical mechanisms inducing the polariton bottleneck effect. First, we change the cavity length in order to get different exciton-photon detunings, which gives rise to different photon and exciton fractions, and the corresponding density of states. Second, it is found that the polariton relaxation bottleneck can be significantly suppressed by the mechanism of polariton-phonon scattering at high temperature. Consequently, we use the temperature-dependent and angle-resolved photoluminescence to confirm the effect of polariton-phonon scattering. Finally, the polariton-polariton interaction is an important factor under high pumping power condition, and the power-dependent angle-resolved photoluminescence can help us to understand the factor. In order to observe Bose-Einstein condensates at room temperature, we use Nd:YVO4 pulsed laser as excitation source and observe a coherence light at room temperature with a low-threshold pumping power, 1 order of magnitude smaller than in previously reported nitride-based vertical-cavity surface-emitting lasers. This is an important evidence of an exciton-polariton laser induced by Bose-Einstein condensate. In addition, based on the experimental results of angle-resolved photoluminescence, we can observe the phenomenon that the polaritons could overcome the relazation bottleneck, and then approach to the bottom of low polariton branch. This result demonstrates the experimental observation of Bose-Einstein condensation in ZnO-based microcavity at room temperature.