Optical polarization and intervalley scattering in single layers of MoS2 and MoSe2

TL;DR

This study investigates how phonon-assisted intervalley scattering affects optical polarization in single-layer MoS2 and MoSe2, revealing a threshold energy related to LA phonons that causes depolarization.

Contribution

It identifies the kinetic energy threshold for depolarization due to intervalley scattering, providing a unified understanding of the process in these materials.

Findings

Depolarization occurs above twice the LA phonon energy.

Almost complete depolarization within ~100 meV above threshold.

Kinetic energy of excitons is key to depolarization process.

Abstract

Single layers of MoS2 and MoSe2 were optically pumped with circularly polarized light and an appreciable polarization was initialized as the pump energy was varied. The circular polarization of the emitted photoluminescence was monitored as function of the difference between the excitation energy and the A-exciton emission at the K-point of the Brillouin zone. Our results show a threshold of twice the LA phonon energy, specific to the material, above which phonon-assisted intervalley scattering causes depolarization. In both materials this lead to almost complete depolarization within ~100 meV above the threshold energy. We identify the extra kinetic energy of the exciton (independent of whether it is neutral or charged) as the key parameter for presenting a unifying picture of the depolarization process.