Substrate interference and strain in the second harmonic generation from MoSe$_2$ monolayers

TL;DR

This study investigates how substrate interference and biaxial strain independently influence second harmonic generation in MoSe2 monolayers, revealing that both factors can significantly enhance nonlinear optical responses.

Contribution

It provides experimental and theoretical evidence that substrate interference and strain independently modulate the SHG response in MoSe2 monolayers, clarifying conflicting reports.

Findings

15-fold increase in SHG intensity on SiO2 substrate

Substrate interference accounts for a seven-fold enhancement

Biaxial strain doubles the nonlinear susceptibility 

Abstract

Nonlinear optical materials of atomic thickness--such as non-centrosymmetric 2H transition metal dichalcogenide monolayers--have a second order nonlinear susceptibility ($\chi^{(2)}$) whose intensity can be tuned by strain. However, whether $\chi^{(2)}$ is enhanced or reduced by tensile strain is a subject of conflicting reports. Here, we grow high-quality MoSe$_2$ monolayers under controlled biaxial strain created by two different substrates, and study their linear and non-linear optical responses with a combination of experimental and theoretical approaches. A 15-fold overall enhancement in second harmonic generation (SHG) intensity is observed on MoSe$_2$ monolayers grown on SiO$_2$ when compared to its value when on a Si$_3$N$_4$ substrate. A seven-fold enhancement was ascertained to substrate interference, and a factor of two to the enhancement of $\chi^{(2)}$ arising from biaxial strain: substrate interference and strain are independent handles to engineer the SHG strength of non-centrosymmetric 2D materials.