纳米结构ZnO晶体薄膜室温紫外激光发射

文章综述了纳米结构的氧化锌半导体薄膜在室温下自由激子的自发辐射以及由自由激子引起的受激发射的特性，阐述了在不同激发密度下室温紫外受激发射的机理．纳米结构氧化锌半导体薄膜是用激光分子束外延(L-MBE)技术生长在蓝宝石衬底上的．薄膜由密集而规则排列的纳米尺度的六角柱组成．这些纳米六角柱起着限制激子运动的作用，激子的量子尺寸效应，使激子的跃迁振子强度大幅度增强．同时六角柱之间的晶面组成了一个天然的激光谐振腔．室温下用三倍频的YAG脉冲激光激发，可从这些纳米结构的氧化锌薄膜中观测到很强的紫外激光发射．研究发现，在中等激发密度下，紫外受激发射是由于激子与激子间碰撞而引起的辐射复合．在高密度激发条件下，由于激子趋于离化，紫外受激发射主要由电子-空穴等离子体的辐射复合引起．由于纳米结构中激子的跃迁振子增强效应，在室温下测量到的光学增益高达320cm^-1，这比在同样条件下测量到的块状氧化锌晶体的光学增益要高一个量级以上．与传统的电子-空穴等离子体激光辐射相比，激子引起的受激发射可在较低的激发密度条件下实现．这在实际应用上很有价值．

Self-assembled ZnO nano-crystals and ultraviolet exciton lasing at room temperature

We review our recent studies on ultraviolet (UV) stimulated emission and lasing observed at room temperature from nano-structured ZnO thin films, which were grown on sapphire substrates using laser-molecular beam epitaxy. The thin film consists of regularly arrayed hexagonal nano-crystallite columns, whose facets form natural micro-cavities. These nano-crystallites confine the centre-of-mass motion of excitons. As a result of the quantum size effect, the oscillation strength of the excitons is greatly enhanced, which is favorable to the radiative recombination of excitons at room temperature. On excitation with the frequency-tripled output of a YAG laser, the nano-structured ZnO thin film shows strong UV lasing at room temperature with a threshold as low as 24kW/cm 2. At a moderate pump intensity, the room temperature stimulated emission is associated with an exciton-exciton collision process. At higher pump densities the excitons dissociate, and the UV stimulated emission is dominated by an electron-hole plasma recombination process. Because of the large enhancement of oscillator strength of the excitons, the optical gain of the stimulated emission measured at room temperature can reach 320 cm -1, which is an order higher than that observed in bulk ZnO crystals. In comparison with the electron-hole plasma stimulated emission in most commercial semiconductor lasers, the excitonic stimulated emission can be realized at relatively low external pumping density. The observation of excitonic lasing effects at room temperature might be valuable in the realization of practical UV semiconductor laser devices.