Sandwich-structure-modulated photoluminescence enhancement of wide bandgap semiconductors capping with dielectric microsphere arrays.

Here we investigated the effect of substrate and film thickness on photoluminescence (PL) enhancement of wide bandgap semiconductor (i.e. ZnO) by dielectric microsphere array/luminescence film/substrate (MLS) sandwich structures. The PL enhancement channels in the sandwich structure were revealed, for the first time, including the focusing property of microsphere array (MSA) distinctly enhancing free-exciton recombination, anti-reflection effect of MSA increasing excitation cross-section area, MLS-supported TW-/SW-WGMs inducing ASE and Purcell's effect, and optical directional antenna effect for high equivalent NA of objective lens as well as out-coupling efficiency. The enhancement ratio of intensity (ERI) for ZnO UV-PL from free-exciton recombination in the sandwich structure was found to be strongly dependent upon the refractive index of substrate and luminescence film thickness. In order to achieve high ERI for PL emission, the refractive index of substrate should differ from luminescence film and the film thickness needs to be chosen to support WGMs in the sandwich structure. The maximum ERI beyond one order of magnitude for ZnO UV-PL was therefore predicted theoretically and validated experimentally, where 11.25-fold UV PL enhancement ratio was achieved in ~650-nm-thick ZnO film grown on SiC substrate and capped with 5.06-μm-diameter MSA. The ERI could further be increased by improving above-mentioned enhancement channels. The present work provides a novel platform to manipulate light by low-loss dielectric microstructures for enhancing photon-matter interaction, which would be employed for other semiconductors achieving energy-saving luminescence and high-sensitivity photoelectric detection in future.