Non-Hermitian landau damping and bulk Fermi arc in superlattice-gate-tuned graphene plasmons

Landau damping, in which a plasmon emits an electron-hole pair, has been considered as a hindering factor for low-loss nanophotonic applications of graphene surface plasmon-polaritons (GSP), and is usually avoided by Pauli-blocking at high doping. In contrast to this simple view against landau damping, here we unravel its ironic utility and show how significantly this loss mechanism can extend the existing framework of graphene nanophotonics. Given a properly designed spatial pattern, landau damping becomes a key element for exotic non-Hermitian band structure engineering of GSPs, without damaging their quality factor severely. We propose that periodic damping textures can be assigned by field-effect gating through a combination of two electrodes; a metagate, thin and periodically punctured, provides a superlattice photonic crystal structure to GSPs, and a backgate, placed far below the metagate, controls local carrier densities on graphene selectively on the regions above holes in the metagate. This setting allows us to use the backgate voltage as a switch for the bulk Fermi arc, a topological signature of an exceptional point pair in the band structure. Our work thus opens up a new regime of graphene plasmonics involving open systems and non-Hermitian topological physics.