Line Waves in Non-Hermitian Metasurfaces

Planar junctions between reactive surface impedances with dual character (capacitive/inductive) can sustain line waves localized both in-plane and out-of-plane around the discontinuity, which propagate unattenuated along one-dimensional paths. Due to their attractive properties, these waves are of potential interest in applications ranging from integrated photonics to optical sensing. Here, we introduce and explore a non-Hermitian platform supporting these exotic modes based on parity-time symmetry. Specifically, we show that line waves can occur in the presence of a uniform surface reactance (either capacitive or inductive) and a symmetric resistance discontinuity from negative to positive values (i.e., gain and loss). We study analytically and numerically the propagation properties of these waves and the underlying physical mechanisms and also illustrate their intriguing properties in terms of confinement, reconfigurability, spin-momentum locking and lasing. Finally, we address possible practical implementations based on photoexcited graphene. Our results hold intriguing potentials for applications in flat optics and reconfigurable photonics.