Femtosecond coherent and incoherent spectroscopies on GaN

We have performed pump-probe (P-P) simultaneously with time-integrated-FWM as a function of the energy of excitation and the excitation intensity. This shows that the tendency of the quantum beats, as revealed in the previous study, are the same both in P-P and FWM but the change of trace in the negative delays in P-P is markedly different from that in FWM. The P-P signals both at below-gap excitation and at the low intensity excitation show another oscillatory behavior distinct from the A-B exciton beats.