# In-plane Propagation of Light in Transition Metal Dichalcogenide Monolayers: Optical Selection Rules

## TL;DR

This study investigates the optical selection rules in monolayer transition metal dichalcogenides, revealing polarization-dependent emission and providing direct measurements of bright-dark exciton splitting.

## Contribution

It provides experimental validation of optical selection rules in TMD monolayers and measures bright-dark exciton splitting using edge-emission polarization-resolved photoluminescence.

## Key findings

- Polarization-dependent emission lines observed
- Bright-dark exciton splitting measured as 40 meV for WSe2 and 55 meV for WS2
- Experimental results agree with group theory analysis

## Abstract

The optical selection rules for inter-band transitions in WSe2, WS2 and MoSe2 transition metal dichalcogenide monolayers are investigated by polarization-resolved photoluminescence experiments with a signal collection from the sample edge. These measurements reveal a strong polarization-dependence of the emission lines. We see clear signatures of the emitted light with the electric field oriented perpendicular to the monolayer plane, corresponding to an inter-band optical transition forbidden at normal incidence used in standard optical spectroscopy measurements. The experimental results are in agreement with the optical selection rules deduced from group theory analysis, highlighting the key role played by the different symmetries of the conduction and valence bands split by the spin-orbit interaction. These studies yield a direct determination on the bright-dark exciton splitting, for which we measure 40 $\pm 1$ meV and 55 $\pm 2$ meV for WSe2 and WS2 monolayer, respectively.