${\Gamma}$-valley assisted intervalley scattering on monolayer and bilayer WS2 revealed by time-resolved Kerr rotation spectroscopy

TL;DR

This study uses time-resolved Kerr rotation spectroscopy to explore valley depolarization and carrier relaxation in monolayer and bilayer WS2, revealing the role of ${\Gamma}$-valley assisted intervalley scattering at low temperatures.

Contribution

It provides new insights into the valley depolarization mechanisms in WS2, highlighting the impact of ${\Gamma}$-valley assisted intervalley scattering under off-resonance excitation.

Findings

Ultrafast decay (<1 ps) linked to stimulated emission or Pauli-blocking.

Slow decay (~4 ps to 15 ps) associated with ${\\Gamma}$-valley assisted intervalley scattering.

Bilayer WS2 exhibits a longer decay (~48 ps) due to indirect band gap effects.

Abstract

We investigated the valley depolarization and carrier relaxation process of monolayer and bilayer WS2 at 10 K by using time-resolved Kerr rotation (TRKR) and differential reflectance measurement simultaneously. Two decay processes extracted from TRKR signals were observed on both monolayer and bilayer WS2. In monolayer WS2, the initial ultrafast decay component (less than 1 ps) was interpreted as the stimulated emission or Pauli-blocking of electrons and holes in one valley by comparing with carrier decay process. The relatively slow component was around 4 ps under low excitation energy (less than 2.21 eV) and then increases with excitation energies, which approaches a saturation value of 15 ps. The onset excitation energy of 2.21 eV suggests the ${\Gamma}$-valley assisted intervalley scattering between the K and K' valley play a critical part during the valley depolarization process under the off-resonance excitation condition. By contrast, the slow decay component (48 ps) of bilayer WS2 is comparable with the carrier lifetime (58 ps). It is attributed to irreversible scattering processes from K (or K') to ${\Gamma}$ valley due to its characteristic of indirect band gap semiconductor.