Modal optical gain and cavity mode analysis of unstructured and optically structured ZnO nanocrystalline thin films

Abstract

Deposition of high-crystal-quality ZnO nanocrystalline thin films (NCTFs) on a lattice-mismatched sapphire substrate was achieved using a pulsed laser deposition technique, and surface channel waveguides were fabricated by using a laser irradiation process. The optical structure defined on the film was characterized using a scanning electron microscope while the optical gain of unstructured and optically structured NCTFs was measured using the variable stripe length method (VSLM). We present a new analysis of a VSLM to study gain saturation for various stripe lengths, and its validity was confirmed by the consistent result obtained by using a widely used method for short stripe lengths. In the case of unstructured films, the inelastic exciton–exciton scattering dominates the stimulated emission (P-band) below Mott density while not only N-band due to the electron–hole plasma but also the exciton–exciton scattering contribute to the lasing beyond Mott density. These results suggest that the exciton–exciton scattering occurs in a localized space of a coherence scale, so the coexistence of the N-band and the P-band stimulated emission beyond the Mott density originates from the non-uniform spatial distribution. An induced cavity defined on a ZnO NCTF sample, showing constant periodic mode features, is confirmed which is attributed to local densification produced due to a refractive index change in a structured arc region.