| Summary: | 碩士 === 國立交通大學 === 顯示科技研究所 === 98 === 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.
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