  Highly asymmetric optical properties of b-Ga2O3 are investigated by
polarization-dependent photoluminescence (PL) excitation spectroscopy for
assessing its potential use for a novel ultraviolet emitter and nonlinear
optical material. Based on power-dependent excitation and two-photon depth
scan, it is demonstrated that the underlying mechanism for the PL occurring far
below the bandgap is not caused by intrinsic polaron effects, but by excitonic
recombination at extrinsic defect sites.
