  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.
