Exciton-exciton interaction and heterobiexcitons in GaN

The formation of not only A biexcitons (XXAA) but also heterobiexcitons that consist of A and B excitons (XXAB) in a free-standing bulk GaN is identified by polarization-sensitive spectrally resolved FWM measurements. The FWM spectra and delay-time dependence show that the interaction between A and B exciton gives rise to the energy shifts of the spectra and the phase shifts of the quantum beating, which is considered as the effect of the unbound state of XXAB (i.e., XXAB*) and XXAB* is found to play an important role in the FWM signals for all polarizations. The unbound A biexciton (XXAA*) is also observed clearly in spectral and temporal domains.