Rigorous Electromagnetic Quasinormal‐Mode Method Made Easy for Users

ABSTRACT Full‐wave numerical methods based on quasinormal modes (QNMs) offer valuable physical insights and computational efficiency for analyzing electromagnetic resonators. However, despite their advantages, many researchers in electromagnetism continue to favor real‐frequency domain or time‐domain approaches, often using finite element or finite‐difference time‐domain methods. This preference stems from various factors, including the perception that QNM theory is still developing or requires advanced mathematical tools from complex analysis. In this work, we combine numerical techniques with accurate approximations to simplify the computation of QNMs and enable ultrafast reconstructions using QNM expansions. The result is a new approach that is straightforwardly accessible to users familiar with real‐frequency methods. We demonstrate the practicality of our approach through an open‐source package [ https://doi.org/10.5281/zenodo.18708748 ] implemented within a widely‐used commercial photonics software.