Spontaneous Coherence of Excitons and Polaritons in GaAs Structures

University Of Pittsburgh, Pittsburgh PA

Investigators

Abstract

****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.