Incoherent phonon population and exciton-exciton annihilation in monolayer WS2 revealed by time-resolved spontaneous Raman scattering

TL;DR

This study investigates the ultrafast dynamics of incoherent phonons and excitons in monolayer WS2 using time-resolved Raman scattering, revealing rapid phonon population changes and exciton-exciton interactions crucial for understanding 2D material optoelectronics.

Contribution

It provides the first detailed measurement of phonon and exciton population dynamics, including relaxation times and exciton-exciton annihilation rates, in monolayer WS2.

Findings

Optical phonon population builds up instantly with a 4 ps relaxation time.

Exciton relaxation rate is approximately 70 ps^-1.

Exciton-exciton annihilation rate is 0.1 cm^2s^-1.

Abstract

Atomically thin layer transition metal dichalcogenides have been intensively investigated for their rich optical properties and potential applications in nano-electronics. In this work, we study the incoherent optical phonon and exciton population dynamics in monolayer WS2 by time-resolved spontaneous Raman scattering spectroscopy. Upon excitation of the exciton transition, both the Stokes and anti-Stokes optical phonon scattering strength exhibit a large reduction. Based on the detailed balance, the optical phonon population is retrieved, which shows an instant build-up and a relaxation lifetime of around 4 ps at an exciton density E12 cm-2. The corresponding optical phonon temperature rises by 25 K, eventually, after some 10s of picoseconds, leading to a lattice heating by only around 3 K. The exciton relaxation dynamics extracted from the transient vibrational Raman response shows a strong excitation density dependence, signaling an important bi-molecular contribution to the decay. The exciton relaxation rate is found to be (70 ps)-1 and exciton-exciton annihilation rate 0.1 cm2s-1. These results provide valuable insight into the thermal dynamics after optical excitation and enhance the understanding of the fundamental exciton dynamics in two-dimensional transition metal materials.