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Semiconductor lasers have formal analogies to Bardeen-Cooper-Schrieffer (BCS) superconductors. This work shows that, in analogy to gapless superconductivity, a gapless lasing parametric regime, in which the frequency gap in the fluctuation spectrum is closed, exists for steady-state semiconductor lasers. The gap opens when the laser intensity exceeds a threshold. This gapless-to-gapped transition occurs at a third-order exceptional point. 

Abstract Spectrally narrow optical resonances can be used to generate slow light, i.e., a large reduction in the group velocity. In a previous work, we developed hybrid 2D semiconductor plasmonic structures, which consist of propagating optical frequency surface-plasmon polaritons interacting with excitons in a semiconductor monolayer. Here, we use coupled exciton-surface plasmon polaritons (E-SPPs) in monolayer WSe 2 to demonstrate slow light with a 1300 fold decrease of the SPP group velocity. Specifically, we use a high resolution two-color laser technique where the nonlinear E-SPP response gives rise to ultra-narrow coherent population oscillation (CPO) resonances, resulting in a group velocity on order of 10 5  m/s. Our work paves the way toward on-chip actively switched delay lines and optical buffers that utilize 2D semiconductors as active elements.