基于激子极化激元玻色爱因斯坦凝聚的超低阈值激光

Exciton-polaritons are qusi-particles that exist in the so-called strong-coupling systems. Due to their bosonic nature, they can be used to generate Bose-Einstein Condensates (BEC), where the ground state of a system alone can have a particle number of macroscopic order. This means the signal from a single quantum state can be amplified to macroscopic level thus have beautiful application prospect. The best part about Exciton-polariton BEC is its critical temperature. Unlike cold atom system, this qusi-particle allows BEC to happen at a much higher temperature----all the way to room temperature, making BEC devices realistic under more common conditions. One amazing possible application is polariton laser, sometimes referred to as the “zero-threshold laser”. It amplifies light through condensation on the ground state instead of spontaneous emission, meaning it doesn’t require population inversion thus has ultra-low or no threshold. In order to obtain reasonable amount of these exciton-polaritons, the material system need to be chosen carefully. GaN stands up to the demands with its high exciton bonding energy, high carrier mobility and high-quality growing and manufacture techniques. Given our group’s previous work on high-quality GaN growth and our physics background, especially that we have already achieved k-space condensation in pure GaN microrods, we feel confident to apply for this project to push our work further and do research on exciton-polariton BEC as well as polariton laser. In the project we will cover the designing and growth of core-shell GaN microrods with very high Q-factors, the generating and observing of excition-polaritons and the realization of their BEC together with some other macroscopic quantum phenomenon.

激子极化激元是半导体强耦合体系中的一种服从玻色统计的准粒子，可在特定条件下实现玻色爱因斯坦凝聚（BEC）。此时基态上聚集了宏观量级的相同量子态粒子，从而把单个微观量子态信息放大到宏观水平，其相变温度可比冷原子提高8个量级，使室温工作的凝聚器件成为可能。基于此的极化激元激光，利用凝聚对基态放大而非受激辐射放大，不需布居数反转，也叫零阈值激光，近年来备受关注。其实现对材料和工艺都具有严苛的要求，GaN材料以其高激子束缚能、高载流子迁移率和优质工艺积累成为了理想的最佳选项。我们从本课题组在高质量GaN材料生长和物理研究的优势背景出发，特别是在GaN微米柱中实现动量空间凝聚的基础上，提出本项目申请，研究GaN体系中的室温激子极化激元的BEC凝聚，实现极化激元激光。主要工作包括设计生长高Q值的GaN核壳结构纳米柱、在新结构中产生观测激子极化激元、实现室温凝聚并探索相关宏观量子现象等。

激子极化激元是强耦合半导体微腔中的元激发，这些准粒子具有半物质半光的特殊性质。其自发辐射携带着整个体系的能量、动量、乃至角动量信息，使得室温条件下观测宏观量子效应成为可能。当激子极化激元发生玻色-爱因斯坦凝聚（BEC）时，来自色散曲线底部的自发辐射具有高度相干性，这便是极化激元激光。它具有远低于传统激光的阈值，是一种高能效的新型微型光源。课题组进行了基于激子极化激元玻色-爱因斯坦凝聚的超低阈值激光的研究，从理论、实验和工艺等多方面开展了研究工作，制成极化激元激光器，全面高质量完成课题各项研究内容。取得的主要进展包括：通过自组织方法外延生长了高质量的GaN微米柱，成功实现了室温BEC；首次观测、识别和操控了两周准回音壁模式激子极化激元，获得了高达230 meV的巨拉比分裂；进行了微纳工艺研究，包括化学机械抛光（CMP）、键合剥离的设备改造和工艺优化；设计、外延、制备了两面介质膜分布式布拉格反射镜（DBR）的垂直微腔；为实现光和激子的强耦合，进行了多量子阱精细结构和外延生长机制的研究，使用结晶恢复层实现了超厚多量子阱特殊结构的外延，揭示了高温引入氮空位和其在量子阱间隔层外延过程中被掩埋的机理；制备了二维微腔，测得了创纪录的超高自发辐射因子0.16，较之一般的垂直腔激光器有几个数量级的提高，表明成功实现了光子-激子相互作用的显著增强。