Quasinormal Mode Expansion Method for Resonators with Partial-fraction Material Dispersion

In this paper, we first establish a Quasinormal Mode (QNM) solver for open resonators made of materials with general dispersion which can be modeled by partial fractions, and develop the corresponding analytical QNM expansion method (QNMEM) for both discrete and periodic resonant structures. When the response of the resonators is dominant by several leading QNMs, a simplified QNMEM can be used to analyze their spectra in a reasonable accuracy. The simplified QNMEM is used to analyze the spectra of the metal-dielectric-metal perfect absorber, which has the advantages of both high computation speed and clear physical insight.