# Breakdown of Raman Selection Rules By Fr\"{o}hlich Interaction in Few-Layer WS$_2$

## TL;DR

This study demonstrates that strong electron-phonon coupling via Fröhlich interaction can break traditional Raman selection rules in few-layer WS$_2$, revealing forbidden modes and altering polarization dependence, with implications for optical detection and manipulation.

## Contribution

It reveals the breakdown of Raman selection rules caused by intraband Fröhlich interaction in resonant conditions, a novel insight into electron-phonon coupling effects in 2D materials.

## Key findings

- Infrared active and forbidden modes observed under resonance.
- All phonon modes show strongest intensity with parallel polarization.
- Selection rules depend on exciton symmetry, not phonon symmetry.

## Abstract

The Raman selection rules arise from the crystal symmetry and then determine the Raman activity and polarization of scattered phonon modes. However, these selection rules can be broken in resonant process due to the strong electron-phonon coupling effect. Here we reported the observation of breakdown of Raman selection rules in few-layer WS$_2$ by using resonant Raman scattering with dark A exciton. In this case, not only the infrared active modes and backscattering forbidden modes are observed, but the intensities of all observed phonon modes become strongest under paralleled-polarization and independent on the Raman tensors of phonons. We attributed this phenomenon to the interaction between dark A exciton and the scatted phonon, the so-called intraband Fr\"{o}hlich interaction, where the Raman scattering possibility is totally determined by the symmetry of exciton rather than the phonons due to strong electron-phonon coupling. Our results not only can be used to easily detect the optical forbidden excitonic and phononic states but also provide a possible way to manipulate optical transitions between electronic levels.