Gain spectra and stimulated emission in epitaxial (In,Al) GaN thin films

We measured the emission of (In,Al) GaN films under high intensity optical pumping both in the direction parallel and perpendicular to the film growth. In the edge emission geometry we determine the gain magnitude from the variable stripe length (VSL) method. We use the spontaneous emission collected perpendicular to the layer plane to calculate the spectral dependence of the gain. Theoretical calculations are in good agreement with those experimentally determined gain spectra. We also show that the observation of a stimulated emission peak perpendicular to the film is predominantly due to scattering of the in-plane stimulated emission but without ruling out contributions from microstructures in the films.