Exceptionally Slow Rise in Differential Reflectivity Spectra of Excitons in GaN: Effect of Excitation-induced Dephasing

Femtosecond pump-probe (PP) 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 \textit{only for positive time delay} in both PP and FWM experiments. The rise time at negative time delay for the differential reflection spectra was much slower than the FWM signal or PP differential transmission spectroscopy (DTS) 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 (EID), that is, the strong density dependence of the dephasing time which changes with the laser excitation energy.Comment: 8 figure

Interband magnetoabsorption study of the shift of the Fermi energy of a 2DEG with an in-plane magnetic field

We investigate experimentally and theoretically the effects of an in-plane magnetic field on the two-dimensional (2D) electron gas via a shift of the Fermi energy in the interband magnetoabsorption. It is shown that the Fermi edge may either shift up (blue) or down (red) in an in-plane magnetic field. The shift depends on the relative strength of two components: (i) the diamagnetic shift of subband edge and (ii) an increase of the 2D density of states which lowers the Fermi energy with respect to the subband edge

Femtosecond four-wave mixing experiments on GaAs quantum wells using two independently tunable lasers

Femtosecond two beam and three beam (omega(1),omega(1);omega(2)) four-wave mixing (FWM) experiments on GaAs quantum wells have been performed using two partially synchronized, independently tunable lasers with external jitter compensation. Heavy and light hole beatings are observed with these two mutually incoherent lasers. FWM signals are observed when oz is completely below the exciton energies, with no spectral overlap with the absorption profile. These off-resonant signals are stronger than the interband continuum signals for equivalent detunings