Interference-Induced Cavity Resonances and Imaginary Rabi Splitting.

Polaritons are usually described within single-mode cavity QED models. However, nanophotonic environments typically involve several modes that spectrally overlap and interfere, giving rise to sharp dip features, such as Fano profiles, in the electromagnetic spectral density. Here, we identify these features as interference-induced resonances, effective electromagnetic modes with complex non-Hermitian couplings to quantum emitters. We show that these modes hybridize with emitters to form polaritons even when the system parameters do not satisfy the single-mode strong-coupling criterion. Moreover, the resulting polaritons differ in their decay rates, a phenomenon we term imaginary Rabi splitting. Extending the analysis to ensembles, we find that coupling to interference-induced resonances produces long-lived polaritons that can outlast excitonic dark states. Numerical simulations of a realistic hybrid metallodielectric platform confirm these predictions and demonstrate their robustness against disorder and loss. Our results reveal a new polaritonic regime beyond the single-mode description, offering new opportunities for controlling light-matter interactions in complex electromagnetic environments.