Trapping and Transport of Indirect Excitons in Coupled Quantum Wells

Spatially indirect excitons are optically generated composite bosons with a radiative lifetime sufficient to reach thermal equilibrium. This work explores the physics of indirect excitons in coupled quantum wells in the GaAs/AlGaAs system, specifically in the low-temperature, high-density regime. Particular attention is paid to a technique whereby a spatially inhomogeneous strain field is used as a trapping potential. In the process of modeling the trapping profile in wide quantum wells, dramatic effects due to intersubband coupling were observed at high strain. Experimentally, this regime coincides with the abrupt appearance of a dark population of indirect excitons at trap center, an effect originally suspected to be related to Bose-Einstein condensation. Here, the role of band mixing due to the strain-induced distortion of the crystal symmetry will be explored in detail in the context of this effect. Experimental studies presented here and in the literature suggest that Bose-Einstein condensation in indirect exciton systems may be difficult to detect with optical means (e.g., coherence measurements, momentum-space narrowing), possibly due to the strong dipole interaction between indirect excitons. Due to similarities between this system and liquid helium, it may be more fruitful to look for transport-related signatures of condensation, such as superfluidity. Here, a method for performing transport measurements on optically generated indirect excitons is also outlined and preliminary results are presented.