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 11K 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 transmition 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 (EID), that is, the strong density dependence of the dephasing time which changes with the laser excitation energy.