Exciton resonance of second harmonic generation in ZnSe

We have shown that the normally forbidden second harmonic generation signal is enhanced drastically when twice the incident photon energy is resonant with the excitation energy of a 2P exciton in ZnSe/GaAs thin films. Using this resonant effect and a novel spectroscopic technique which utilizes a phase-locked laser pulse pair, we have investigated the dynamical behavior of 2P excitons. The interferogram of exciton polarization has a tail of a few picoseconds. Beating structure was observed in the decay profile. This structure can be explained by polarization interference between the heavy hole and light hole excitons split by strain. Further, we have studied the fine structure of the 2P exciton in ZnSe films grown on GaAs substrates using this resonance phenomenon. The 2P exciton line is found to show distinct fine structure caused by envelope-hole coupling. in addition to splitting of the exciton state due to strain effects.