Intervalley scattering of excitons and trions in monolayer WSe$_{2}$ under strong excitation

TL;DR

This study investigates how exciton and trion valley relaxation dynamics in monolayer WSe₂ are affected by strong optical excitation, revealing faster valley scattering and detailed decay processes through time-resolved photoluminescence measurements.

Contribution

It provides new insights into exciton and trion valley dynamics under strong excitation, including quantification of valley scattering times and PL decay mechanisms in monolayer WSe₂.

Findings

Inter valley scattering time decreases from 50 ps at low power to less at high power.

Exciton PL decay time is approximately 10 ps.

PL decay is bi-exponential, with short radiative and long reservoir transfer components.

Abstract

Monolayer transition metal dichalcogenides offer the possibility of optical control of the valley degree of freedom. In order to asses the potential of these materials in applications, detailed knowledge of the valley dynamics is essential. In this work, we apply low temperature time-resolved photoluminescence (PL) measurements to investigate exciton valley relaxation dynamics and, in particular, its behavior under strong excitation. At the lowest excitation powers the inter valley scattering time is $\simeq 50$ ps, but shortens by more than a factor of two at the highest powers. We attribute this acceleration to either heating of the exciton system or the presence of a dense exciton gas, which could influence the exciton valley properties. Furthermore, we analyze the PL dynamics of excitons and trions. We find that the PL decays for all peaks are bi-exponential and approximately independent of the excitation power. We attribute the short decay to radiative recombination and escape to a reservoir of dark states. The long decay is ascribed to a transfer of excitons back from the reservoir. For the first time, we evaluate the exciton PL decay time of $\simeq$ 10 ps. The latter process is valley-conserving and occurs on a timescale of $\simeq$ 50 ps.