Optical nonlinearities in the excited carrier density of atomically thin transition metal dichalcogenides

TL;DR

This paper presents a theoretical study of optical nonlinearities affecting excited carrier densities in atomically thin transition metal dichalcogenides, revealing the limitations of linear models at high excitation levels.

Contribution

It combines ab-initio electronic calculations with many-body theory to analyze nonlinear absorption and carrier dynamics in TMD monolayers, identifying the range of validity for linear approximations.

Findings

Nonlinear effects significantly alter carrier density at high pump fluences.

Linear absorption models underestimate achievable carrier densities.

Many-body renormalizations impact optical properties at elevated excitations.

Abstract

In atomically thin semiconductors based on transition metal dichalcogenides, photoexcitation can be used to generate high densities of electron-hole pairs. Due to optical nonlinearities, which originate from Pauli blocking and many-body effects of the excited carriers, the generated carrier density will deviate from a linear increase in pump fluence. In this paper, we use a theoretical approach that combines results from ab-initio electronic-state calculations with a many-body treatment of optical excitation to describe nonlinear absorption properties and the resulting excited carrier dynamics. We determine the validity range of a linear approximation for the excited carrier density vs. pump power and identify the role and magnitude of optical nonlinearities at elevated excitation carrier densities for MoS2, MoSe2, WS2, and WSe2 considering various excitation conditions. We find that for above-band-gap photoexcitation, the use of a linear absorption coefficient of the unexcited system can strongly underestimate the achievable carrier density for a wide range of pump fluences due to many-body renormalizations of the two-particle density-of-states.