  In photonics, Dispersive Quasi-Normal Modes (DQNMs) refer to optical resonant
modes, solutions of spectral problems associated with Maxwell's equations for
open photonic structures involving dispersive media. Since these DQNMs are the
constituents determining optical responses, studying DQNM expansion formalisms
is the key to model the physical properties of a considered system. In this
paper, we emphasize the non-uniqueness of the expansions related to the
over-completeness of the set of modes and discuss a family of DQNM expansions
depending on continuous parameters that can be freely chosen. These expansions
can be applied to dispersive, anisotropic, and even non-reciprocal materials.
As an example, we particularly demonstrate the modal analysis on a 2-D
scattering model where the permittivity of a silicon object is drawn directly
from actual measurement data.
