Strain-induced Giant Second-harmonic Generation in Monolayered $2H$-MoX$_2$ (X=S,Se,Te)

TL;DR

This study uses first-principles calculations to show that strain can significantly enhance the second-harmonic generation in monolayered MoX2 materials, with potential for tunable nonlinear optical applications.

Contribution

It provides the first detailed analysis of strain effects on the nonlinear susceptibilities of monolayer MoX2, revealing tunability and giant enhancements in SHG.

Findings

$ ext{Chi}^{(2)}$ reaches ~140 pm/V for MoTe$_2$ in static limit.

Strain induces direct-to-indirect band gap transitions affecting $ ext{Chi}^{(2)}$.

$ ext{Chi}^{(2)}$ can be enhanced to ~800 pm/V at specific wavelengths with strain.

Abstract

Dynamic second-order nonlinear susceptibilities, $\chi^{(2)}(2\omega,\omega,\omega)\equiv \chi^{(2)}(\omega)$, are calculated here within a fully first-principles scheme for monolayered molybdenum dichalcogenides, $2H$-MoX$_2$ (X=S,Se,Te). The absolute values of $\chi^{(2)}(\omega)$ across the three chalcogens critically depend on the band gap energies upon uniform strain, yielding the highest $\chi^{(2)}(0)\sim$ 140 pm/V for MoTe$_2$ in the static limit. Under this uniform in-plane stress, $2H$-MoX$_2$ can undergo direct-to-indirect transition of band gaps, which in turn substantially affects $\chi^{(2)}(\omega)$. The tunability of $\chi^{(2)}(\omega)$ by either compressive or tensile strain is demonstrated especially for two important experimental wavelengths, 1064 nm and 800 nm, where resonantly enhanced non-linear effects can be exploited: $\chi^{(2)}$ of MoSe$_2$ and MoTe$_2$ approach $\sim$800 pm/V with -2\% strain at 1064 nm.