Strong and Anisotropic Third Harmonic Generation in Monolayer and Multilayer ReS$_2$

TL;DR

This paper demonstrates strong, anisotropic third harmonic generation in monolayer and multilayer ReS₂, revealing that lattice distortions significantly influence nonlinear optical properties and can be characterized via THG measurements.

Contribution

It reports the first observation of strong, anisotropic THG in ReS₂ and links lattice distortions to nonlinear optical behavior, expanding understanding of 2D material properties.

Findings

THG susceptibility in ReS₂ is about ten times higher than in MoS₂.

THG intensity depends on incident light polarization direction.

Lattice distortions cause the observed anisotropy in nonlinear optical response.

Abstract

We report observation of strong and anisotropic third harmonic generation (THG) in monolayer and multilayer ReS$_2$. The third-order nonlinear optical susceptibility of monolayer ReS$_2$, $\left| \chi^{(3)} \right|$ is on the order of $10^{-18} $ m$^2$/V$^2$, which is about one order of magnitude higher than reported results for hexagonal-lattice transition metal dichalcogenides such as MoS$_2$. A similar magnitude for the third-order nonlinear optical susceptibility was also obtained for a multilayer sample. The intensity of the THG field was found to be dependent on the direction of the incident light polarization for both monolayer and multilayer samples. A point group symmetry analysis shows that such anisotropy is not expected from a perfect $1T$ lattice, and must arise from the distortions in the ReS$_2$ lattice. Our results show that THG measurements can be used to characterize lattice distortions of two-dimensional materials, and that lattice distortions are important for the nonlinear optical properties of such materials.