Excitons in nonlinear optical responses: shift current in MoS$_2$ and GeS monolayers

TL;DR

This paper introduces a computational method to evaluate excitonic effects on the nonlinear optical shift current in 2D materials, revealing significant contributions from dark excitons in MoS$_2$ and GeS monolayers.

Contribution

The study develops a novel ab initio approach combining Wannier interpolation and Bethe-Salpeter equation to accurately calculate excitonic nonlinear optical responses in 2D materials.

Findings

Dark $2p$-like excitons significantly contribute to shift current.

Excitonic effects increase predicted photocurrent by an order of magnitude.

In-gap photogalvanic currents of ~10 nA are predicted under high-intensity radiation.

Abstract

It is well-known that exciton effects are determinant to understand the optical absorption spectrum of low-dimensional materials. However, the role of excitons in nonlinear optical responses has been much less investigated at an experimental level. Additionally, computational methods to calculate nonlinear conductivities in real materials are still not widespread, particularly taking into account excitonic interactions. We present a methodology to calculate the excitonic second-order optical responses in 2D materials relying on: (i) ab initio tight-binding Hamiltonians obtained by Wannier interpolation and (ii) the Bethe-Salpeter equation with effective electron-hole interactions. Here, in particular, we explore the role of excitons in the shift current of monolayer materials. Focusing on MoS$_2$ and GeS monolayer systems, our results show that $2p$-like excitons, which are dark in the linear response regime, yield a contribution to the photocurrent comparable to that of $1s$-like excitons. Under radiation with intensity $\sim 10^{4} $W/cm$^2$, the excitonic theory predicts in-gap photogalvanic currents of almost $\sim 10$ nA in sufficiently clean samples, which is typically one order of magnitude higher than the value predicted by independent-particle theory near the band edge.