  Optical resonators are widely used in modern photonics. Their spectral
response and temporal dynamics are fundamentally driven by their natural
resonances, the so-called quasinormal modes (QNMs), with complex frequencies.
For optical resonators made of dispersive materials, the QNM computation
requires solving a nonlinear eigenvalue problem. This rises a difficulty that
is only scarcely documented in the literature. We review our recent efforts for
implementing efficient and accurate QNM-solvers for computing and normalizing
the QNMs of micro- and nano-resonators made of highly-dispersive materials. We
benchmark several methods for three geometries, a two-dimensional plasmonic
crystal, a two-dimensional metal grating, and a three-dimensional nanopatch
antenna on a metal substrate, in the perspective to elaborate standards for the
computation of resonance modes.
