Theory of Propagation and Manipulation of Excitons in GaAs Structures
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PublisherThe University of Arizona.
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EmbargoRelease after 08-Jul-2013
AbstractThis dissertation presents research on the propagation and manipulation of excitons in GaAs. There are three main aspects to be addressed. In the first part, we provide a comprehensive understanding of the slow- and fast-light propagation based on excitonic resonances. Propagation (or transit) times of optical pulses through a medium near an absorptive resonance with and without spatial dispersion are studied and contrasted. We show that, when the broadening of the resonance (i.e., dephasing rate) is below a critical value, a frequency range exists near resonance where the transit times are determined by interference between co-propagating exciton-polaritons and deviate strongly from expectations based on the group velocities of the individual exciton-polariton branches. Our theory puts the well-known slow- and fast-light effects in systems without spatial dispersion into a broader context by interpreting them as a limiting case of systems with spatial dispersion. An important ingredient of the exciton theory is the light-matter coupling that can be expressed either in terms of the dipole or the momentum matrix elements. We re-examine the validity of a frequently used relation between the interband momentum and dipole matrix elements ('p-r relation') in semiconductors. An additional correction term was obtained when we applied the 'p-r relation' to finite-volume crystals treated with periodic boundary conditions. The correction term does not vanish in the limit of infinite volume. We illustrate this with numerical examples for bulk GaAs and GaAs superlattices. For bulk GaAs, the correction term is found to be always important; while for a GaAs superlattice, the importance of the correction term for the transition depends strongly on the origin of the unit cell.As an example for manipulation of excitons, we consider mechanical deformations of GaAs nanomembranes. The nanomembranes with lateral sizes much larger than their thicknesses exhibit great flexibility in non-planar deformations and thus promise applications in flexible electronic and photonic devices. These non-planar deformations do not fit in the well-established theory for planar deformations induced by lattice mismatch. Our theory relates the general mechanical deformations (planar or non-planar) of nanomembranes to their excitonic spectra, and is numerically evaluated within the average-strain approximation.
Degree ProgramGraduate College