An analysis is made of the influence of spatial dispersion on the spectral distribution of a reflected second harmonic. It is shown that the spectrum of the harmonic may exhibit, near the exciton absorption line, an additional maximum at a frequency corresponding to the phase matching between the first (fundamental-frequency) and second harmonic waves. An analysis is made of the possibility of determination of the effective mass of an exciton from the spectral distribution of the intensity of a reflected harmonic.
We present a thorough investigation of the different relaxation channels opened in a strongly interacting system of excitonic quasiparticles in CuCl for density regimes ranging up to 50 MW/
cm2. A spectrally narrow pump beam consisting of 30-ps pulses is tuned around one-half the biexciton energy. It creates an initial distribution of biexcitons through resonant two-photon absorption. A spectrally broad probe beam made of pulses that also have 30-ps duration monitors the excitation-induced variation of the transmission spectrum of CuCl samples near the
Z3 exciton resonance. Coherent processes corresponding to parametric emissions of various orders are observed. Absorption and gain bands that reflect the evolution of biexciton and exciton populations on their respective dispersion curves are followed as functions of the time delay between pump and probe pulses.
This 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.