Sphere of arbitrarily polarized exceptional points with a single planar metasurface.

Abstract

Exceptional points (EPs) are spectral singularities of non-Hermitian systems and represent the coalescence of eigenvalues and eigenstates. Traditional photonic systems typically exhibit coalesced eigenstates that correspond to circular polarizations of a specific handedness, thereby restricting their applicability to only the poles of the Poincaré sphere. Here, by judiciously combining optical anisotropy, chirality and non-Hermiticity of diffractive plasmonic metasurfaces with basis transformation, we achieve a continuum of EPs for which the corresponding coalesced eigenstates can access any point on the Poincaré sphere, greatly alleviating the strict requirement of approaching EP degeneracy. Our theoretical proposal and experimental implementation overcome the main practical limitation of EPs, extending the applicability of the topological phase to arbitrarily polarized state within the diffraction region. The emergence of these non-conventional EPs not only contributes to applications in wavefront engineering and optical multiplexing, but also brings in new fundamental properties of topological systems in general.