# Enabling valley selective exciton scattering in monolayer WSe$_2$ through upconversion

## TL;DR

This study demonstrates valley-specific exciton scattering in monolayer WSe₂, revealing that B-excitons predominantly scatter within the same valley, and introduces a selective upconversion technique to probe excitonic interactions.

## Contribution

It provides the first experimental evidence of valley-selective exciton scattering and employs a novel upconversion method to generate and study B-excitons in monolayer WSe₂.

## Key findings

- B-excitons scatter mainly within the same valley.
- Power-dependent negative polarization of B-exciton emission observed.
- Upconversion signal vanishes when laser detuning exceeds 4 meV.

## Abstract

Excitons, Coulomb bound electron-hole pairs, are composite bosons and their interactions in traditional semiconductors lead to condensation and light amplification. The much stronger Coulomb interaction in transition metal dichalcogenides such as WSe$_2$ monolayers combined with the presence of the valley degree of freedom is expected to provide new opportunities for controlling excitonic effects. But so far the bosonic character of exciton scattering processes remains largely unexplored in these two-dimensional (2D) materials. Here we show that scattering between B-excitons and A-excitons preferably happens within the same valley in momentum space. This leads to power dependent, negative polarization of the hot B-exciton emission. We use a selective upconversion technique for efficient generation of B-excitons in the presence of resonantly excited A-excitons at lower energy, we also observe the excited A-excitons state $2s$. Detuning of the continuous wave, low power laser excitation outside the A-exciton resonance (with a full width at half maximum of 4 meV) results in vanishing upconversion signal.