# Single-crystalline gold nanodisks on WS$_2$ mono- and multilayers: Strong coupling at room temperature

## TL;DR

This study demonstrates room-temperature strong light-matter coupling between gold nanodisks and WS$_2$ monolayers and multilayers, achieving record-high Rabi splitting values that advance nanophotonic applications.

## Contribution

It introduces ultra-thin single-crystalline gold nanodisks as a novel platform for strong coupling with WS$_2$, achieving the highest Rabi splitting reported for TMDCs with open plasmonic cavities.

## Key findings

- Strong coupling at room temperature with 108 meV Rabi splitting.
- Enhanced coupling with multilayer WS$_2$, reaching 175 meV Rabi splitting.
- Record-high Rabi splitting for TMDCs coupled to plasmonic nanostructures.

## Abstract

Engineering light-matter interactions up to the strong-coupling regime at room temperature is one of the cornerstones of modern nanophotonics. Achieving this goal will enable new platforms for potential applications such as quantum information processing, quantum light sources and even quantum metrology. Materials like transition metal dichalcogenides (TMDC) and in particular tungsten disulfide (WS$_2$) possess large transition dipole moments comparable to semiconductor-based quantum dots, and strong exciton binding energies allowing the detailed exploration of light-matter interactions at room temperature. Additionally, recent works have shown that coupling TMDCs to plasmonic nanocavities with light tightly focused on the nanometer scale can reach the strong-coupling regime at ambient conditions. Here, we use ultra-thin single-crystalline gold nanodisks featuring large in-plane electromagnetic dipole moments aligned with the exciton transition-dipole moments located in monolayer WS$_2$. Through scattering and reflection spectroscopy we demonstrate strong coupling at room temperature with a Rabi splitting of $\sim$108 meV. In order to go further into the strong-coupling regime and inspired by recent experimental work by St\"uhrenberg et al., we couple these nanodisks to multilayer WS$_2$. Due to an increase in the number of excitons coupled to our nanodisks, we achieve a Rabi splitting of $\sim$175 meV, a major increase of 62%. To our knowledge, this is the highest Rabi splitting reported for TMDCs coupled to open plasmonic cavities. Our results suggest that ultra-thin single-crystalline gold nanodisks coupled to WS$_2$ represent an exquisite platform to explore light-matter interactions.