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.
Advisor:David Snoke; Gilbert Walker; Xiao-lun Wu; Daniel Boyanovsky; Robert P. Devaty
School:University of Pittsburgh
School Location:USA - Pennsylvania
Source Type:Master's Thesis
Date of Publication:09/24/2004