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

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.

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• Shift current in 2D Janus Transition-Metal Dichalcogenides: the role of excitons
• Enabling the bulk photovoltaic effect in centrosymmetric materials through an external electric field
• \textit{Ab initio} \textit{GW}-BSE theory of optical activity in $\alpha$-quartz
• Perturbative second-order optical susceptibility of bulk materials: a symmetry-enforced return to non-orthogonal localized basis sets