Exciton enhanced nonlinear optical responses in monolayer h-BN and MoS2: Insight from first-principles exciton-state coupling formalism and calculations

TL;DR

This paper develops an ab initio exciton-state coupling formalism to analyze nonlinear optical responses in monolayer h-BN and MoS2, revealing how excitonic effects enhance second harmonic generation and shift currents.

Contribution

It introduces a detailed first-principles method to understand excitonic effects in nonlinear optics of 2D materials, emphasizing exciton coupling and resonance effects.

Findings

Strong excitonic enhancement depends on exciton coupling strength.

Resonant bright excitons significantly boost nonlinear responses.

A and B excitons in MoS2 show minimal enhancement despite being bright.

Abstract

Excitons are vital in the photophysics of materials, especially in low-dimensional systems. The conceptual and quantitative understanding of excitonic effects in nonlinear optical (NLO) processes is more challenging compared to linear ones. Here, we present an ab initio approach to second-order NLO responses, incorporating excitonic effects, that employs an exciton-state coupling formalism and allows a detailed analysis of the role of individual excitonic states. Taking monolayer h-BN and MoS2 as two prototype 2D materials, we calculate their second harmonic generation (SHG) susceptibility and shift current conductivity tensor. We find strong excitonic enhancement requires that the resonant excitons are not only optically bright themselves, but also be able to couple strongly to other bright excitons. Our results explain the occurrence of two strong peaks in the SHG of monolayer h-BN and why the A and B excitons of MoS2 unexpectedly exhibit minimal excitonic enhancement in both SHG and shift current generation.