# Chiral nonlinear polaritonics with van der Waals metasurfaces

## TL;DR

A new chiral metasurface enables selective polariton formation and chiral light interactions for advanced photonic applications.

## Contribution

A monolithic TMDC metasurface with broken symmetry enables maximal chirality for polaritonics and nonlinear optics.

## Key findings

- The metasurface supports chiral exciton-polaritons with maximal chirality up to 20° oblique incidence.
- Chiral strong coupling is observed in third-harmonic generation experiments.
- The platform is suitable for nonreciprocal photonic devices and valleytronics.

## Abstract

Chiral optical cavities are crucial for the development of nonequilibrium quantum materials by discriminating and selectively coupling to light of a specific circular polarization, but fundamentally cannot be realized with conventional mirror cavities. Here, we demonstrate this unique functionality by developing a monolithic transition metal dichalcogenide (TMDC) metasurface with broken out-of-plane symmetry, allowing for the selective formation of self-hybridized chiral exciton-polaritons. Our metasurface maintains maximal chirality for oblique incidence up to 20°, thereby outperforming all previously known designs. Moreover, we study the chiral strong-coupling regime in nonlinear experiments and reveal polaritonic signatures in chiral third-harmonic generation. Our results position maximally chiral van der Waals (vdW) metasurfaces as a versatile platform for tunable chiral polaritonics with applications in nonreciprocal photonic devices and valleytronics.

A maximally chiral metasurface enables polariton-driven third-harmonic generation with controllable light helicity.