# Intrinsically chiral exciton polaritons in an atomically-thin semiconductor

## TL;DR

Researchers created chiral exciton polaritons in a thin semiconductor by coupling light and electron-hole pairs, enabling control over their spin with polarized light.

## Contribution

A novel method for inducing strong coupling between chiral photons and valley excitons in atomically-thin semiconductors is demonstrated.

## Key findings

- Intrinsically chiral exciton polaritons are formed with enhanced circularly polarized photoluminescence.
- The spin alignment of polaritons can be controlled using σ+ and σ− circularly polarized optical excitation.
- A microscopic model explains the energy relaxation dynamics via Brillouin zone folding.

## Abstract

Photonic bound states in the continuum (BICs) have emerged as a versatile tool for enhancing light-matter interactions by strongly confining light fields. Chiral BICs are photonic resonances with a high degree of circular polarisation, which hold great promise for spin-selective applications in quantum optics and nanophotonics. Here, we demonstrate a novel application of a chiral BIC for inducing strong coupling between the circularly polarised photons and spin-polarised (valley) excitons (bound electron-hole pairs) in atomically-thin transition metal dichalcogenide crystals (TMDCs). By placing monolayer WS2 onto the BIC-hosting metasurface, we observe the formation of intrinsically chiral, valley-selective exciton polaritons, evidenced by circularly polarised photoluminescence (PL) at two distinct energy levels. The PL intensity and degree of circular polarisation of polaritons exceed those of uncoupled excitons in our structure by an order of magnitude. Our microscopic model shows that this enhancement is due to folding of the Brillouin zone creating a direct emission path for high-momenta polaritonic states far outside the light cone, thereby providing a shortcut to thermalisation (energy relaxation) and suppressing depolarisation. Moreover, while the polarisation of the upper polariton is determined by the valley excitons, the lower polariton behaves like an intrinsic chiral emitter with its polarisation fixed by the BIC. Therefore, the spin alignment of the upper and lower polaritons (↑↓ and ↑↑) can be controlled by σ+ and σ− circularly polarised optical excitation, respectively. Our work introduces a new type of chiral light-matter quasi-particles in atomically-thin semiconductors and provides an insight into their energy relaxation dynamics.

The authors observe strong coupling between valley excitons in a monolayer WS2 and intrinsically chiral photons shaped by a metasurface. The polaritons display enhanced chiral emission with their spin configuration controlled by the light source.