Resonance Expansion of Quadratic Quantities with Regularized Quasinormal Modes

Resonance expansions are an intuitive approach to capture the interaction of an optical resonator with light. Herein, a quasinormal mode expansion approach for quadratic observables is presented. The approach exploits the rigorous Riesz projection method. A numerical implementation of a state‐of‐the‐art quantum light source demonstrates the applicability with emphasis on modal expansions outside the underlying nanophotonic resonator.