Ultrafast Exciton Decomposition in Transition Metal Dichalcogenide Heterostructures

TL;DR

This paper introduces a first-principles method to analyze ultrafast phonon-induced exciton decomposition in TMD heterostructures, revealing rapid exciton relaxation dynamics affecting their optical and spin properties.

Contribution

It provides a novel theoretical framework for understanding ultrafast exciton relaxation in TMD heterostructures, highlighting the role of phonon scattering in exciton dynamics.

Findings

Rapid reduction in bright state optical activity within femtoseconds

Exciton interlayer delocalization occurs immediately after photoexcitation

Ultrafast changes in exciton momentum, spatial, and spin properties observed

Abstract

Heterostructures of layered transition metal dichalcogenides (TMDs) host long-lived, tunable excitons, making them intriguing candidates for material-based quantum information applications. Light absorption in these systems induces a plethora of optically excited states that hybridize both interlayer and intralayer characteristics, providing a distinctive starting point for their relaxation processes, in which the interplay between generated electron-hole pairs and their scattering with phonons play a key role. We present a first-principles theoretical approach to compute phonon-induced exciton decomposition due to rapid occupation of electron-hole pairs with finite momentum and opposite spin. Using the MoSe$_2$/WSe$_2$ heterostructure as a case study, we observe a reduction in the optical activity of bright states upon phonon scattering already in the first few femtoseconds proceeding the photoexcitation, driving exciton interlayer delocalization and subsequent variations in the exciton spin. Our results reveal an unexpected and previously unexplored starting point for exciton relaxation dynamics, suggesting increased availability for coherent interactions and non-radiative processes through ultrafast changes in exciton momentum, spatial, and spin properties upon light excitation.