Degenerate four-wave-mixing spectroscopy on epitaxially laterally overgrown GaN: Signals from below the fundamental absorption edge

We performed photoluminescence (PL) and degenerate four-wave-mixing (FWM) spectroscopy in epitaxially laterally overgrown GaN at 10 K. Optical transitions based on exciton complexes such as biexciton emission, exciton–exciton scattering, electron–hole plasma, and so on, were revealed by PL under a wide range of excitation densities. The FWM signals were observed from states below the fundamental excitonic absorption edge, showing that nonlinear photoswitching can be performed with a transmittance geometry. The origin of such nonlinearlity was discussed by correlating with feasible many-body effects between excitons, biexcitons, and free carriers.