Optical imaging of strain in two-dimensional crystals

TL;DR

This paper introduces a novel optical second harmonic generation technique capable of mapping the full strain tensor in two-dimensional crystals with sub-diffraction spatial resolution, advancing strain imaging methods.

Contribution

It demonstrates the use of SHG to extract the full strain tensor in 2D materials, surpassing traditional optical methods in resolution and detail.

Findings

SHG allows full strain tensor extraction in 2D crystals.

The method achieves spatial resolution below the diffraction limit.

Photoelastic tensor elements of MoS2 were determined.

Abstract

Strain engineering is widely used in material science to tune the (opto-)electronic properties of materials and enhance the performance of devices. Two-dimensional atomic crystals are a versatile playground to study the influence of strain, as they can sustain very large deformations without breaking. Various optical techniques have been employed to probe strain in two-dimensional materials, including micro-Raman and photoluminescence spectroscopy. Here we demonstrate that optical second harmonic generation constitutes an even more powerful technique, as it allows to extract the full strain tensor with a spatial resolution below the optical diffraction limit. Our method is based on the strain-induced modification of the nonlinear susceptibility tensor due to a photoelastic effect. Using a two-point bending technique, we determine the photoelastic tensor elements of molybdenum disulfide. Once identified, these parameters allow us to spatially image the two-dimensional strain field in an inhomogeneously strained sample.