Quasinormal Mode Basis for Open Floquet Photonic Systems

Quasinormal modes universally describe resonances in open non-Hermitian systems. However, a first-principles scattering theory bridging internal Floquet dynamics with external excitation in time-varying open systems has remained elusive. Here, we develop a generalized ab initio Floquet-quasinormal mode theory for open Floquet photonic systems, revealing complex phenomena inaccessible to phenomenological temporal coupled-mode theory. It enables rigorous analysis of the mode coupling induced by time modulation and more importantly, the measurable response of eigenmodes under external excitation. As an example, we calculate the scattering cross section of a spherical particle with a time-modulated shell, demonstrating a case study of photonic Autler-Townes splitting. Our framework allows efficient first-principles simulations by decoupling spectral and spatial evaluations, avoiding costly full-wave resimulations.