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Femtosecond pump-probe (PP) differential reflectivity spectroscopy (DRS) andfour-wave mixing (FWM) experiments were performed simultaneously to study theinitial temporal dynamics of the exciton line-shapes in GaN epilayers. Beatsbetween the A-B excitons were found \textit{only for positive time delay} inboth PP and FWM experiments. The rise time at negative time delay for thedifferential reflection spectra was much slower than the FWM signal or PPdifferential transmission spectroscopy (DTS) at the exciton resonance. Anumerical solution of a six band semiconductor Bloch equation model includingnonlinearities at the Hartree-Fock level shows that this slow rise in the DRSresults from excitation induced dephasing (EID), that is, the strong densitydependence of the dephasing time which changes with the laser excitationenergy.Comment: 8 figure

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