Double resonant Raman scattering and valley coherence generation in monolayer WSe2

TL;DR

This paper investigates double resonant Raman scattering in monolayer WSe2, revealing how phonon-assisted exciton transitions generate valley coherence, with implications for valleytronics and quantum information.

Contribution

It demonstrates the role of multi-phonon resonance in generating valley coherence through 2s to 1s exciton transitions in monolayer WSe2.

Findings

Double resonant Raman peaks shift with laser energy.

Valley superposition states are generated by 2s exciton excitation.

High valley coherence is observed in 1s exciton photoluminescence.

Abstract

The electronic states at the direct band gap of monolayer WSe2 at the $K^+$ and $K^-$ valleys are related by time reversal and may be viewed as pseudo-spins. The corresponding optical interband transitions are governed by robust excitons. In double resonant Raman spectroscopy, we uncover that the 2s exciton state energy differs from the 1s state energy by exactly the energy of the combination of several prominent phonons. Superimposed on the exciton photoluminescence (PL) we observe the double resonant Raman signal. This spectrally narrow peak shifts with the excitation laser energy as incoming photons match the 2s and outgoing photons the 1s exciton transition. The multi-phonon resonance has important consequences: Following linearly polarized excitation of the 2s exciton a superposition of valley states is generated which can relax fast via phonon emission and with minimal loss of coherence from the 2s to 1s state. This explains the high degree of valley coherence measured for the 1s exciton PL.