Interaction-Induced Transparency for Strong-Coupling Polaritons.

The propagation of light in strongly coupled atomic media takes place through the formation of polaritons-hybrid quasiparticles resulting from a superposition of an atomic and a photonic excitation. Here we consider the propagation under the condition of electromagnetically induced transparency and show that a novel many-body phenomenon can appear due to strong, dissipative interactions between the polaritons. Upon increasing the photon-pump strength, we find a first-order transition between an opaque phase with strongly broadened polaritons and a transparent phase where a long-lived polariton branch with highly tunable occupation emerges. Across this nonequilibrium phase transition, the transparency window is reconstructed via nonlinear interference effects induced by the dissipative polariton interactions. Our predictions are based on a systematic diagrammatic expansion of the nonequilibrium Dyson equations which can be controlled, even in the nonperturbative regime of large single-atom cooperativities, provided the polariton interactions are sufficiently long-ranged. Such a regime can be reached in photonic crystal waveguides thanks to the tunability of interactions, allowing us to observe the interaction-induced transparency transition even at low polariton densities.