Non-Hermitian Topological Metasurfaces Based on Photonic-Plasmonic Hybrid Resonators

In recent years, intense research efforts have been undertaken to understand the roles of symmetry and topology in non-Hermitian physics, with photonics acting as a convenient testbed. Manipulating topology in photonics is enticing due to the promise of devices that are robust against defects. Such devices would have behaviors encoded in their topology rather than their geometry. Already, topological photonics in Hermitian systems has yielded remarkable demonstrations of unidirectional edge states and topological photonic insulators. Combining topology and non-Hermitian physics holds further promise for the observation of rich physical phenomena.<br/>The synergy between topology and non-Hermiticity in photonics holds immense potential for next-generation optical devices that are robust against defects. However, most demonstrations of non-Hermitian and topological photonics have been limited to super-wavelength scales due to increased radiative losses at the deep-subwavelength scale. By carefully designing radiative losses at the nanoscale, we demonstrate a non-Hermitian plasmonic-dielectric metasurface in the visible with non-trivial topology. The metasurface exhibits higher-order (&gt;2) parity-time symmetry. The device exhibits an exceptional concentric ring in its momentum space, indicating a non-Hermitian Z₃ topological invariant of V = -1. Additionally, the metasurface can be tuned through a PT phase transition using only the in-plane angle. Fabricated devices are characterized using Fourier-space imaging for single-shot bandstructure measurements. Our results demonstrate the potential of combining topology and non-Hermiticity for nanophotonic devices with a highly tailored directional response and topological protection.