# Marrying excitons and plasmons in monolayer transition-metal dichalcogenides

## TL;DR

This paper reveals how shortwave plasmons couple with excitons in monolayer transition-metal dichalcogenides, explaining observed optical sidebands and highlighting conditions for strong exciton-plasmon interactions in electron-doped tungsten-based monolayers.

## Contribution

It introduces a novel mechanism linking shortwave plasmons and excitons in monolayer TMDs based on their unique band structure.

## Key findings

- Shortwave plasmons couple with excitons in monolayer TMDs.
- Optical sidebands in WSe₂ and WS₂ are explained by exciton-plasmon coupling.
- Strong coupling conditions are specific to electron-doped tungsten monolayers.

## Abstract

Just as photons are the quanta of light, plasmons are the quanta of orchestrated charge-density oscillations in conducting media. Plasmon phenomena in normal metals, superconductors and doped semiconductors are often driven by long-wavelength Coulomb interactions. However, in crystals whose Fermi surface is comprised of disconnected pockets in the Brillouin zone, collective electron excitations can also attain a shortwave component when electrons transition between these pockets. Here, we show that the band structure of monolayer transition-metal dichalcogenides gives rise to an intriguing mechanism through which shortwave plasmons are paired up with excitons. The coupling elucidates the origin for the optical side band that is observed repeatedly in monolayers of WSe$_2$ and WS$_2$ but not understood. The theory makes it clear why exciton-plasmon coupling has the right conditions to manifest itself distinctly only in the optical spectra of electron-doped tungsten-based monolayers.