Exceptional points in polaritonic cavities and subthreshold Fabry–Perot lasers

Abstract

We show that concept of parity-time (PT) symmetry can be expanded to include mixed photon-exciton modes by demonstrating that eigenmodes of active (pumped) strongly coupled cavity polaritons with population inversion exhibit characteristics that are remarkably akin to those of coupled photonic structures with parity-time symmetry. The exceptional point occurs when the Rabi splitting of polariton branches inherent in passive polaritonic systems decreases with increase in pumping, leading to population inversion, and eventually two polaritonic modes merge into a single mode, thus manifesting the frequency pulling effect inherent to all lasers. But, remarkably, this exceptional point occurs below the lasing threshold. Furthermore, unlike most manifestations of PT symmetry in optics, which are observed in the interaction between two analogous photonic modes in waveguides or cavities, in this work the exceptional point is found in interaction between two very dissimilar modes—one photonic and one material excitation (exciton). Aside from fundamentally noteworthy expansion of the concept of PT symmetry to new systems, there is a prospect of using the exceptional point in polaritons for practical applications, such as sensing.