Optical imaging of strain in two-dimensional crystals

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 extraction of 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.

Nonlinear optics of two‐dimensional transition metal dichalcogenides

“…Copyright 2017, American Chemical Society. E, Strain‐modified SHG of monolayer MoS 2 . Copyright 2018, Nature Publishing Group.…”

“…Mechanic strain is an effective way to alter the electronic and optical properties by deforming the crystal lattice . Recently, strain has been proved as an effective method to engineer the NLO . For instance, a monolayer MoS 2 under a strain is no longer belongs to D 3 h space group and the χ (2) tensor has more than one independent components.…”

“…For instance, a monolayer MoS 2 under a strain is no longer belongs to D 3 h space group and the χ (2) tensor has more than one independent components. The modified χ (2) tensor under strain can be rewritten by including the photoelastic tensor caused by strain . Figure E shows the angle‐dependent SHG of monolayer MoS 2 changed dramatically with 1% tensile strain, in which SHG reduced along the uniaxial strain .…”

“…It was reported that TMDs could sustain a stain larger than 10% due to the excellent mechanic properties, indicating the promising applications in stretchable and flexible nonlinear devices. In addition, SHG can serve as a powerful technique to probe the magnitude and direction of strain by measuring the polarization‐dependent SHG . Recently, Liang et al reported that THG could serve as a universal method to image the strain tensor because THG does not require the absence of inversion symmetry …”

“…155 Recently, strain has been proved as an effective method to engineer the NLO. 78,153,156,157 For instance, a monolayer MoS 2 under a strain is no longer belongs to D 3h space group F I G U R E 5 A, Charge-induced SHG of bilayer WSe 2 . 42 Copyright 2015, American Chemical Society.…”