K. El Sayed

Exceptionally slow rise in differential reflectivity spectra of excitons in GaN: Effect of excitation-induced dephasing

Femtosecond differential reflectivity spectroscopy (DRS) and four-wave mixing (FWM) experiments were performed simultaneously to study the initial temporal dynamics of the exciton line-shapes in GaN epilayers. Beats between the A-B excitons were found only for positive time delay in both DRS and FWM experiments. The rise time at negative time delay for the DRS was much slower than the FWM signal or differential transmission spectroscopy at the exciton resonance. A numerical solution of a six band semiconductor Bloch equation model including nonlinearities at the Hartree-Fock level shows that this slow rise in the DRS results from excitation induced dephasing, that is, the strong density dependence of the dephasing time which changes with the laser excitation energy.

Femtosecond pump-probe and four-wave mixing studies of excitons in GaN

Femtosecond pump-probe (P-P) and four-wave mixing (FWM) experiments were performed simultaneously at 11 K on gallium nitride epilayers to study the initial temporal line-shapes of the exciton. A-B exciton beats were found in both P-P and FWM experiments near the exciton resonance. However, the differential reflection spectra showed a much slower rise time that persisted at longer negative time delay than the FWM signal or differential transition spectra at the exciton resonance. A numerical solution of a six band semiconductor Bloch equation model including all Hartree Fock nonlinearities shows that this slow rise results from excitation induced dephasing, that is, the strong density dependence of the dephasing time which changes with the laser excitation energy.

Interband coherence in semiconductors; excitons and beyond

Abstract Spectrally resolved transient four-wave mixing is studied in GaAs/AlGaAs quantum wells for coherent excitation of exciton and continuum states. A distinct contribution from the continuum states is observed in the signal at the exciton resonance. From quantum beat experiments, the biexciton binding energy is determined as a function of well and barrier widths.

Photocarrier relaxation in the quantum kinetics regime

Summary form only given. We present a study of carrier relaxation in bulk GaAs at room temperature in the quantum kinetics regime. We use a modified frequency-resolved pump-probe technique with pump and probe durations independently adjustable from 30-100 fs. Differential absorption spectra (DAS) for 70-fs pump and probe at low excess energy is presented.

Early stage of Coulomb screening and kinetics under femtosecond excitation within the non-equilibrium RPA

Abstract During and shortly after a femtosecond laser pulse excitation of a semiconductor, the carrier kinetics is drastically altered. At this early stage the Coulomb potential is still unscreened and there is essentially no energy conservation in the two-particle scattering events. This explains in a natural way the results of recent femtosecond spectroscopy experiments.