Title: "Luminescence Darkening of Strain-Trapped Excitons in Coupled Quantum Wells" Advisor: Dr. David Snoke Abstract: Interwell excitons in coupled quantum wells are a promising systems for achieving an excitonic Bose-Einstein condensate, as the extremely long lifetime of these excitations allow large populations to accumulate and thermal equilibration of the gas. Such a condensate would be notable for the constituent particles having strong, long-range, repulsive interactions, a property that also makes the condensate formation more di fficult by suppressing wavefunction overlap. It is believed, however, that with suitable traps the bosonic nature of this excitation will prevail and a condensate will form. Within GaAs/AlGaAs coupled quantum wells, large-scale strain traps have been employed to con fine excitons, and a transition is observed in the spatial structure of the exciton photoluminescence under conditions of high strain, low temperature, and high density. Under these conditions, the luminescence at the center of the trap, where the density remains largest, becomes dim compared to the rest of the trap region. Several possible mechanisms for this behavior will be discussed, in particular that of Bose-Einstein condensation, formation of an electron-hole liquid, and strain-induced valence-band mixing. However, none of these mechanisms adequately account for all of the behavior associated with the darkening. The valence-band mixing argument receives particular attention, as it presents the most testable predictions, in addition to offering an explanation for the darkening relying solely on the single-particle spectrum.