Spontaneous Coherence of Excitons and Polaritons in GaAs Structures

****非技术摘要****该项目支持一种迷人的新物质状态的实验工作，它在某些方面类似于激光，在某些方面类似于超导体。像激光一样，这个系统发射相干光，但它不是标准意义上的激光，因为不仅发射的光，而且固体中的电子也是相干的。换句话说，就像在超导体中一样，电子是相关的，具有相干的波状性质。然而，超导体不发光，因此它们的相干性质必须间接推导出来。这项研究中使用的半导体结构由几层组成，每层的厚度只有几纳米，由阿尔卡特朗讯的贝尔实验室制造。这些结构被冷却到非常低的（低温）温度，并用最先进的光学光谱和成像技术进行研究。我们的目标是了解物质的这种新状态。一种独特的技术，利用应力将相干电子限制在一个小点上，由匹兹堡大学在美国国家科学基金会支持的先前工作中开发出来，为这些研究提供了相对于类似系统的其他工作的优势。参与该项目的学生将接受最前沿的科学主题和最先进的技术培训，为他们未来在学术界、工业界或政府的职业生涯做好准备。****技术摘要****本项目支持两个相关系统的自发相干性（玻色-爱因斯坦凝聚及相关跃迁）实验工作：耦合量子阱中的空间间接激子和微腔中的激子-极化子。这两种系统的半导体异质结构将在阿尔卡特-朗讯的贝尔实验室制造。在这两种情况下，使用非均匀应力将准粒子限制在一个类似于原子陷阱的谐波势陷阱中，这是匹兹堡大学在NSF支持的先前工作中开发的一种方法。空间间接激子的研究主要集中在由激子-激子相互作用引起的多体重整化效应上，这种效应使冷凝临界温度发生偏移。微腔中的激子-极化子具有极轻的有效质量，这意味着在实验可达到的极化子密度下，自发相干的临界温度为几十到几百开尔文。最近在匹兹堡和其他实验室对微腔中的激子-极化子的研究表明，在一种可以被视为非平衡玻色凝聚态的状态下，有多种形式的自发相干性的证据。参与该项目的学生将接受最前沿的科学主题和最先进的技术培训，为他们未来在学术界、工业界或政府的职业生涯做好准备。

****NON-TECHNICAL ABSTRACT****This project supports experimental work on a fascinating new state of matter, which has some aspects similar to a laser and some aspects similar to a superconductor. Like a laser, this system emits coherent light, but it is not a laser in the standard sense, because not only the emitted light but also the electrons in the solid are coherent. In other words, as in a superconductor, the electrons are correlated and have coherent wave-like properties. Superconductors do not emit light, however, and therefore their coherent properties must be deduced indirectly. The semiconductor structures used for this study consist of layers, each with a thickness of a few nanometers, and are fabricated at Bell Labs of Alcatel-Lucent. The structures are cooled down to very low (cryogenic) temperatures and studied with state-of-the-art optical spectroscopy and imaging. The goal is to understand this new state of matter. A unique technique, which uses stress to confine the coherent electrons in a small spot, developed at the University of Pittsburgh in previous work supported by NSF, provides an advantage for these studies relative to other work on similar systems. The students working on this project will receive training cutting edge scientific topics and in state-of-the-art techniques that will prepare them for future careers in academe, industry, or government.**** TECHNICAL ABSTRACT****This project supports experimental work on spontaneous coherence (Bose-Einstein condensation and related transitions) in two related systems: spatially indirect excitons in coupled quantum wells and exciton-polaritons in microcavities. Semiconductor heterostructures for both systems will be fabricated at Bell Labs of Alcatel-Lucent. In both cases inhomogeneous stress is used to confine the quasiparticles in a harmonic-potential trap analogous to atom traps, a method developed at the University of Pittsburgh in previous work supported by NSF. The studies on spatially indirect excitons will focus on the many-body renormalization effects which arise due to the exciton-exciton interaction, and which shift the critical temperature for condensation. Exciton-polaritons in microcavities have the advantage that they have extremely light effective mass, which implies a critical temperature for spontaneous coherence from tens to hundreds of Kelvin at experimentally achievable polariton densities. Recent work on exciton-polaritons in microcavities at Pittsburgh and other laboratories has shown multiple forms of evidence for spontaneous coherence in a state that can be viewed as a non-equilibrium Bose condensate. The students working on this project will receive training cutting edge scientific topics and in state-of-the-art techniques that will prepare them for future careers in academe, industry, or government.