Coherent-transient four-wave-mixing studies of excitons in a high quality GaN epilayer

Abstract

Spectrally-resolved and time-integrated four-wave-mixing measurements are performed in the femtosecond regime on excitons in GaN epilayers. A comparison between spectrally-resolved and time-integrated data indicates that the excitons in our samples are nearly homogeneously broadened even at low temperature. Strong beating has been observed between the A and B free excitonic resonances. Spectrally-resolved four-wave mixing is used to show that the beats are due to true quantum beating. A phase shift 180 degrees in the quantum beats is observed when changing from a collinear polarization geometry to a cross-linear geometry. The FWM signal is calculated for each polarization geometry in the ultrashort-pulse limit in order to theoretically model the observed phase change.