Ultrafast dynamics in monolayer TMDCs: the interplay of dark excitons, phonons and intervalley Coulomb exchange

TL;DR

This paper investigates the ultrafast valley dynamics in monolayer TMDCs, revealing how exchange interactions and phonon processes lead to unexpected bleaching effects without B exciton population, advancing understanding for valleytronic applications.

Contribution

It provides a microscopic explanation for ultrafast bleaching phenomena in TMDCs, highlighting the roles of exchange coupling and phonon-mediated dark exciton formation.

Findings

Bleaching at B transition occurs without B exciton population.

Exchange coupling and phonons facilitate dark exciton thermalization.

Intra- and intervalley interactions govern femtosecond valley dynamics.

Abstract

Understanding the ultrafast coupling and relaxation mechanisms between valleys in transition metal dichalcogenide semiconductors is of crucial interest for future valleytronic devices. Recent ultrafast pump-probe experiments showed an unintuitive significant bleaching at the excitonic $B$ transition after optical excitation of the energetically lower excitonic $A$ transition. Here, we present a possible microscopic explanation for this surprising effect. It is based on the joint action of exchange coupling and phonon-mediated thermalization into dark exciton states and does not involve a population of the B exciton. Our work demonstrates how intra- and intervalley coupling on a femtosecond timescale governs the optical valley response of 2D semiconductors.