Effects of defects and surface roughness on high-Q modes in ZnO microspheres

Abstract

Photoluminescence (PL) spectra of high-Q ZnO microspheres of various degrees of inhomogeneity are studied. Multiple sharp resonance features with distinct patterns associated with whispering gallery modes (WGM) of these microspheres are observed. Relative WGM peak strengths in these PL spectra are found to be closely related to defects and the surface roughness of the microspheres. A theoretical model based on Mie theory and Green’s function method is used to simulate the optical emission spectra of these microspheres. The effects of defects and surface roughness can be characterized by a frequency-dependent external quality factor (Q ex ) with a prefactor. By adjusting the prefactor in Q ex and a parameter describing the contribution of stimulated emission in the PL spectra, we can fit the observed PL spectra well. Through this study, we gain a better understanding of the relation between the diffusive scattering and the lineshapes of spectral features in PL.

Microwave-assisted in situ laser dye incorporation into high sensitivity whispering gallery mode microresonators

“…a slight deviation from a perfect spherical shape from the individual studied particle. Another possible reason for deviation is surface roughness or the presence of individual surface defects on the µRs [57,58]. With increasing diameter and thus increasing surface area, the probability of the presence of defects increase, as do possible effects of the surface roughness.…”