Reflections on the Spatial Exponential Growth of Electromagnetic Quasinormal Modes

Abstract A major research objective across various fields is to represent the response of open systems using quasinormal mode (QNM) expansions, akin to the treatment of normal modes in closed systems. In electromagnetism, QNM expansions effectively describe modal physics inside resonators and in their near field. However, challenges arise in the intermediate and far field, where QNM fields grow exponentially, posing mathematical issues and often being considered unphysical. How can a near‐field relevant concept lose its validity? Where does this transition occur? This perspective seeks to answer these questions by analyzing foundational concepts such as cavity perturbation theory and dissipative coupling using model problems. The analysis reveals no fundamental inconsistencies with exponential growth and sometimes yields surprising results, such as an increase in coupling coefficients between QNMs of two distant bodies as separation increases. These findings should be widely shared to prevent misunderstandings and enhance the understanding of contemporary electromagnetic QNM theories. The final section, intended for experts in electromagnetic QNMs, provides a thorough analysis of these theories.