Parametric study on photoluminescence enhancement of high-quality zinc oxide single-crystal capping with dielectric...

A dielectric microsphere is a multifunctional platform to manipulate light in microscale by nanofocusing, optical whispering gallery resonance, and unidirectional antenna. Dielectric microsphere arrays (MSAs) have demonstrated the capability for photoluminescence (PL) and Raman enhancement without plasmonics. In this work, we investigate the effects of excitation power, tilting angle, and temperature on PL enhancement of high-quality zinc oxide (ZnO) single-crystal capping with fused silica MSAs. The microsphere diameter is optimized to 3.5-5.5 μm, achieving the maximum UV-PL enhancement ratio of intensity (ERI) up to tenfold by strong focusing and unidirectional antenna effects. Under the excitation power <0.2  mW, the incident light focused by the MSA increases the localized exciton state density for a higher ERI of ∼15-fold. The angle-sensitive PL intensity from the MSA enhancer provides a simple approach achieving unidirectional UV emission from planar ZnO. The 16-fold enhancement for UV-PL near 130°C is also demonstrated, for the first time, owing to thermal ionization of hydrogen-related donor that increases free-exciton concentration. The high temperature stability and reproducibility of PL enhancement up to 400°C promote the nonplasmonic MSAs superior to surface plasmon-related metal nanostructures for ZnO-based highly efficient luminescence and highly sensitive photon detection above room temperature.