Quasinormal modes in dispersive photonic time crystals

Quasinormal modes characterize the transient response of static optical cavities. Here we introduce the notion of a Floquet quasinormal mode to describe transient responses in photonic time crystals. Contrasting their static counterparts, exceptional points associated with symmetry transitions are an inherent feature, as modes spontaneously and nonperturbatively lock their phase to the oscillations of the material. We further investigate the limiting behavior of the Floquet quasinormal modes in large cavities. Distinct nonperturbative behavior arises in time-modulated systems as increasingly large time-crystal cavities come closer to achieving the maximum gain predicted from a bulk wave number band gap.