Mapping complex mode volumes with cavity perturbation theory

Microcavities and nanoresonators are characterized by their quality factors (Q) and mode volumes (V ). While Q is unambiguously defined, there are still questions on V and, in particular, on its complex-valued character, whose imaginary part is linked to the non-Hermitian nature of open systems. Helped by cavity perturbation theory and near-field experimental data, we clarify the physics captured by the imaginary part of V and show how a mapping of the spatial distribution of both the real and imaginary parts can be directly inferred from perturbation measurements. This result shows that the mathematically abstract complex mode V , in fact, is directly observable.