Effects of polycrystalline birefringent grains on the morphology dependent resonance modes of a spherical resonator
05 Aug 2019-Journal of Applied Physics (AIP Publishing LLCAIP Publishing)-Vol. 126, Iss: 5, pp 053102
TL;DR: In this paper, the effect of the grains on resonance modes was investigated for the perturbative effects due to nonsphericity and surface roughness and inhomogeneity in RI arising from polycrystallinity and annealing conditions.
Abstract: Morphology dependent resonance (MDR) modes in a dielectric sphere are highly sensitive to its shape, size, and refractive index (RI). Many dielectric materials with potential applications are birefringent in nature. Synthesis techniques of resonators of such materials may lead to polycrystalline morphologies. Due to its fascinating applications, titanium dioxide (TiO2) has attracted attention as a morphology dependent resonator. However, its high RI is accompanied by high birefringence. The effect of the grains on resonance modes is the interest of this study. Polycrystalline TiO2 microspheres with different grain distributions are synthesized by annealing at 500 °C and 700 °C. MDR modes in the photoluminescence spectrum of single spheres are found to exhibit mode splitting and mode shifting with respect to MDR modes of an equivalent isotropic sphere. The MDR spectral features of the near-perfect spheres have been investigated for the perturbative effects due to (a) nonsphericity and surface roughness and (b) inhomogeneity in RI arising from polycrystallinity and annealing conditions. Mode splits are demonstrated to arise from the perturbative effects of the larger grains through explicit computations using discrete dipole approximation for a Voronoi tessellated cell structure representing a polycrystalline sphere. Mode shifts are demonstrated to arise from radial inhomogeneity of the refractive index using the Aden-Kerker theory on spheres with core-shell morphology. The effects of surface roughness are not found to be significant. The present work extends the scope of model-assisted investigations in understanding morphology dependent resonators and indicates the possibility of designing resonators with prescribed refractive index features.
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