Line Waves in Non-Hermitian Metasurfaces

Abstract

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.

Non-Hermitian Electromagnetic Metasurfaces at Exceptional Points (Invited Review)

“…Line waves, a new type of surface waves localized both out-of-plane and in-plane around the discontinuity with dual (inductive/captive) surface reactances, can propagate unattenuated along a 1-D track [98][99][100]. The peculiar properties of line waves, such as propagation-dependent polarization, field enhancement and robustness, enable a range of exotic applications in integrated optics and optical sensing.…”

“…Owning to the intriguing properties of surface waves, a PT-symmetric metasurface, composed of a gain-loss discontinuity characterized by the same reactance but opposite signed resistances, has been put forward to support the line-wave propagation (Fig. 7(b)), which differs from the conventional line waves associated with the duality principle [98]. In particular, it was the first time to modulate the in-plane gain-loss constituents to illustrate the line waves, while in the other scenarios the gain-loss modulations were stacked out-of-plane [38,91,101].…”

Exceptional Points in the Flatland: A Non-Hermitian Line-Wave Scenario

“…In particular, we focus on certain aspects that are distinctive of LW physics, and differ from typical non-Hermitian coupled-waveguide systems that are known to exhibit EPs. Furthermore, we illustrate how the phase transition at an EP embraces the known classes of conventional [27] and non-Hermitian [28] LWs, and we offer additional physical insight into how different 1-D waveguiding mechanisms can be continuously transformed into one another.…”

“…Another waveguiding scenario of significant recent interest is the case of "line waves" (LWs), which exhibit both in-plane and out-of-plane confinement and can be supported by metasurfaces at planar discontinuities of the surface reactance [26,27] or resistance [28].…”

“…This enables a truly one-dimensional (1-D) energy flow, with a wealth of attractive features in terms of singular field enhancement, near-field structure, chiral coupling and topologicallike robustness, which can be tailored and reconfigured by resorting to graphene [29] and/or gain-based [28] implementations. These characteristics are of great interest for "flat" optics and photonics scenarios, with potential applications ranging from subdiffractive sensing and near-field imaging to optical and quantum computing.…”