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.

中文翻译：

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均质和核壳非球形颗粒中依赖于形态的共振

我们制定了一种微扰理论，用于确定介电非球形核壳粒子中准正规模式的频率。该理论基于扩展边界条件法（也称为零场法），用于用矩阵表示内部场$问$. 在这个框架中，形态相关共振的激发对应于消失的特征值$问$矩阵，可以使用微扰技术来确定，从而为相应的特征频率生成明确的公式。我们将我们的结果专门用于已变形为球体的核壳球形粒子的情况，并提供数值模拟与微扰理论之间的比较。对于这里考虑的小扰动，发现了极好的一致性。此外，我们针对广泛的参数范围研究了核壳颗粒的特征频率。