Femtosecond four-wave-mixing studies of nearly homogeneously broadened excitons in GaN

Femtosecond degenerate four-wave-mixing (FWM) is used to study coherent dynamics of excitons in GaN epilayers. Spectrally resolved (SR) FWM data are dominated by the $A$ and $B$ intrinsic excitonic resonances. SR FWM combined with time-integrated (TI) FWM demonstrates that the excitonic resonances are nearly homogeneously broadened even at low temperature. The temperature-dependent dephasing rate is used to deduce exciton-phonon interaction rates. TI FWM shows a strong beating between the $A$ and $B$ excitons and the beats are shown to be true quantum beats. In addition, a $180°$ phase shift in the quantum beating was observed when the polarization geometry was changed from collinear to cross linear.