Quasinormal modes of spheroidal resonators in the null-field framework and application to hybrid anapoles

A model for calculating the quasinormal modes (QNMs) of homogeneous and core-shell spheroidal resonators is formulated in the null-field framework. Relying on a multipole expansion of the electromagnetic fields, the Mittag-Leffler theorem is employed to extract the QNMs from the poles of a response matrix. Physical quantities such as the extinction and scattering efficiency and the total internal energy of a mode are expressed directly in terms of the multipole expansion coefficients. We apply the model to study hybrid anapoles, which are radiationless states characterized by an enhancement of the total internal energy. Interference between excited QNMs and the background comprising of all other QNMs leads to a suppression of the extinction efficiency. Further, we show this suppression is due to Fano-like interference between resonant and background multipoles.

Automated summary

A model for calculating the quasinormal modes (QNMs) of homogeneous and core-shell spheroidal resonators is proposed in this study. The model is applied to investigate hybrid anapoles, which are radiationless states characterized by an enhancement of the total internal energy.