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.