Coherent dynamics of excitons in GaN studied by femtosecond four-wave-mixing spectroscopy

Femtosecond four-wave-mixing (FWM) is used to study the coherent dynamics of excitons in thin epilayers of GaN grown by metalorganic chemical vapor deposition on sapphire substrates. Temperature dependent FWM is used to accurately measure the exciton homogeneous linewidth and it is shown that the exciton-LO phonon interaction is larger in GaN than in other III-V materials. Furthermore, the excitonic resonances in our samples are shown to be very nearly homogeneously broadened even at low temperature. We have observed strong beating behavior in the FWM signal corresponding to the energy separation of the A and B free exciton transitions. The beats were studied as a function of relative position across the B exciton linewidth in order to determine that the beating is due to a coherent exchange of population between the A and B excitons and not to, so called, polarization interference. The quantum beats were further studied as a function of polarization geometry and a phase shift of 180 degrees was observed when changing from collinear to cross-linear polarization geometries. The FWM signal was calculated in the ultrashort pulse limit in order to theoretically model the observed phase change.