Theory and Ab Initio Computation of the Anisotropic Light Emission in Monolayer Transition Metal Dichalcogenides

TL;DR

This paper presents a first-principles theoretical framework for understanding the anisotropic light emission in monolayer TMDCs, explaining experimental observations and predicting new emission behaviors.

Contribution

It introduces an ab initio method to compute direction- and polarization-dependent photoluminescence in monolayer TMDCs, accounting for exciton valley superpositions.

Findings

Excitons emit light anisotropically depending on valley superpositions.

The model explains observed PL anisotropy and polarization.

It predicts new emission regimes in monolayer TMDCs.

Abstract

Monolayer transition metal dichalcogenides (TMDCs) are direct gap semiconductors with unique potential for ultrathin light emitters. Yet, their photoluminescence (PL) is not completely understood. We compute the radiative recombination rate in monolayer TMDCs as a function of photon emission direction and polarization, and obtain polar plots of the PL for different excitation scenarios using the ab initio Bethe-Salpeter equation. We show that excitons in a quantum superposition state of the K and K' inequivalent valleys emit light anisotropically upon recombination. Our results can explain the PL anisotropy and polarization dependence measured in recent experiments, and predict new light emission regimes. When averaged over emission angle and exciton momentum, our new treatment recovers the temperature dependent radiative lifetimes we previously derived. Our work provides a first-principles approach to study light emission in two-dimensional materials.