Speaker
Description
Non-Hermitian and topological photonics have jointly sparked recent interest in the manifestation of spectral degeneracies and polarisation defects for radiation control. Here we study several types of non-Hermitian metaphotonic structures that support both topological singularities in polarisation fields, such as C-points (points of purely circular polarisation), and spectral singularities, such as exceptional points (branch points of the complex-valued spectrum), together with Fermi arcs connecting pairs of exceptional points. One example of such open photonic systems is a membrane metasurface with broken vertical symmetry (e.g. due to tilted slits or holes). We put forward a minimal two-band coupled-mode generalised model and demonstrate theoretically how polarisation and spectral singularities in non-Hermitian photonic systems are intrinsically intertwined. Starting from a perturbative expansion around exceptional points, we develop an effective theoretical framework that captures both the evolution of the eigenvalue splitting and the associated far-field polarisation structures. We show that the polarisation field develops nontrivial C-point textures whose trajectories can be traced in parameter space.
Using a feasible membrane metasurface design with conical holes, we further elucidate the formation of unidirectional guided-mode resonances with full radiation suppression towards one out-of-plane direction, which arises from the merging of C-points in the vicinity of a Fermi arc. Our theoretical description is supported by detailed numerical simulations, and it provides a unified picture of topological singularities with a hierarchy of perturbation scales in which exceptional points, Fermi arcs, and C-points emerge as distinct topological facets of a single underlying non-Hermitian photonic framework. We will also discuss its applications to generating highly directional, rainbow-free, vectorially polarised radiation at selected output angles.
| I am the presenting author | Yes |
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