Morphology-dependent resonances in homogeneous and core-shell nonspherical particles

Abstract

We formulate a perturbation theory for determining the frequencies of quasinormal modes in dielectric nonspherical core-shell particles. The theory is based on the extended boundary condition method (also known as the null-field method), which is used to represent the internal field in terms of a matrix $Q$. In this framework, the excitation of a morphology-dependent resonance corresponds to a vanishing eigenvalue of the $Q$ matrix, which can be determined using perturbation techniques to yield explicit formulas for the corresponding eigenfrequency. We specialize our results to the case of a core-shell spherical particle that has been deformed into a spheroid and provide a comparison between numerical simulations and the perturbation theory. Excellent agreement is found for the small perturbations considered here. Further, we investigate the eigenfrequencies of core-shell particles for an extensive range of parameters.