EXCITONS AT HIGH DENSITY IN CUPROUS OXIDE AND COUPLED QUANTUM WELLS

There is a 40-year long history in the search for Bose-Einstein condensation (BEC) of excitons in semiconductors. This thesis presents research directed toward this goal in bulk crystal Cu2O in three dimensions and in GaAs-based coupled quantum wells (CQW) in two dimensions.The Auger recombination process in Cu2O plays a major role in limiting the density of the excitons. We find that the rate for this process increases with applied stress and lattice temperature. We create paraexcitons in Cu2O through a resonant two-photon excitation in a harmonic potential trap with the Auger recombination process as small as possible (at low temperature and low stress), and find that the exciton creation efficiency in the resonant two-photon excitation is greater for one-beam excitation than for two colliding pulses, but the colliding pulse method may be useful for direct creation of a condensate in the ground state. The paraexciton density in this work is about thirty times less than the required density for BEC of paraexcitons. One promising direction for BEC of excitons in Cu2O is that with higher laser power from stronger IR laser sources, or at lower temperature, the critical density can be approached under one-beam two-photon excitation resonant with the paraexciton state.In two dimensions, the CQW structure has been modified with four design strategies: highest possible barriers, introducing into the barriers a superlattice of 60 angstrom GaAs wells, p-i-n doping, and wider quantum wells, which provides indirect excitons low disorder and high mobility. With a cold near-resonant excitation, we conclude that the excitons act as a free gas, travelling distances of hundreds of microns. We also present observations of a narrow beam of emitted light, when the indirect excitons are confined in a two-dimensional harmonic potential trap, in a way quite similar to the first observations of BEC in alkali atoms. A beam-like emission has been suggested as a telltale for BEC of excitons. This opens the door to a whole range of investigations, including attempts to observe coherence of the emitted light, proof of superfludity of the excitons, and other fascinating effects.

在寻找半导体中激子的玻色 - 爱因斯坦凝聚（BEC）方面，已有长达40年的研究历史。本论文阐述了朝着这一目标开展的研究，分别涉及三维体晶体Cu₂O以及二维基于砷化镓（GaAs）的耦合量子阱（CQW）。在Cu₂O中，俄歇复合过程在限制激子密度方面起着关键作用。我们发现，该过程的速率会随着施加的应力和晶格温度的升高而增大。我们在尽可能降低俄歇复合过程（在低温和低应力条件下）的谐波势阱中，通过共振双光子激发在Cu₂O中产生仲激子，并且发现单光束激发下共振双光子激发的激子产生效率高于两束对撞脉冲激发，但对撞脉冲方法可能有助于直接在基态产生凝聚体。在本研究中，仲激子密度比仲激子实现玻色 - 爱因斯坦凝聚所需的密度低约三十倍。在Cu₂O中实现激子玻色 - 爱因斯坦凝聚的一个有前景的方向是，利用更强的红外激光源提供更高的激光功率，或者在更低的温度下，通过与仲激子态共振的单光束双光子激发，有可能达到临界密度。 在二维体系中，通过四种设计策略对耦合量子阱结构进行了改进：采用尽可能高的势垒，在势垒中引入60埃砷化镓阱的超晶格，进行p - i - n掺杂，以及使用更宽的量子阱，这些措施为间接激子提供了低无序度和高迁移率。通过冷近共振激发，我们得出激子表现得如同自由气体，能够传播数百微米的距离。我们还观察到，当间接激子被限制在二维谐波势阱中时，会发射出窄光束，这与首次在碱金属原子中观察到玻色 - 爱因斯坦凝聚的方式非常相似。束状发射被认为是激子玻色 - 爱因斯坦凝聚的一个特征。这为一系列研究打开了大门，包括尝试观察发射光的相干性、证明激子的超流性以及其他引人入胜的效应。