Non-Hermitian chiral degeneracy of gated graphene metasurfaces

TL;DR

This paper investigates non-Hermitian degeneracies, specifically chiral exceptional points, in gated graphene metasurfaces, revealing their effects on polarization states and transmission properties through electrical control.

Contribution

It introduces an electrical and spectral method to resolve chiral EPs in a non-Hermitian metasurface, providing new insights into polarization dynamics and topological charges.

Findings

Quantification of nonorthogonality of polarization eigenstates.

Observation of enhanced asymmetric polarization conversion.

Identification of missing dimension at chiral EP.

Abstract

Non-Hermitian degeneracies, also known as exceptional points (EPs), have been the focus of much attention due to their singular eigenvalue surface structure. Nevertheless, as pertaining to a non-Hermitian metasurface platform, the reduction of an eigenspace dimensionality at the EP has been investigated mostly in a passive repetitive manner. Here, we propose an electrical and spectral way of resolving chiral EPs and clarifying the consequences of chiral mode collapsing of a non-Hermitian gated graphene metasurface. More specifically, the measured non-Hermitian Jones matrix in parameter space enables the quantification of nonorthogonality of polarisation eigenstates and half-integer topological charges associated with a chiral EP. Interestingly, the output polarisation state can be made orthogonal to the coalesced polarisation eigenstate of the metasurface, revealing the missing dimension at the chiral EP. In addition, the maximal nonorthogonality at the chiral EP leads to a blocking of one of the cross-polarised transmission pathways and, consequently, the observation of enhanced asymmetric polarisation conversion. We anticipate that electrically controllable non-Hermitian metasurface platforms can serve as an interesting framework for the investigation of rich non-Hermitian polarisation dynamics around chiral EPs.