Second harmonic generation in strained transition metal dichalcogenide monolayers: MoS2, MoSe2, WS2, and WSe2

Second-harmonic generation (SHG) is a powerful measurement technique to analyze the symmetry properties of crystals. Mechanical strain can reduce the symmetry of a crystal and even weak strain can have a considerable impact on the SHG intensity along different polarization directions. The impact of strain on the SHG can be modeled with a second-order nonlinear photoelastic tensor. In this work, we determined the photoelastic tensors at a fundamental wavelength of 800 nm for four different transition metal dichalcogenide (TMD) monolayers: MoS₂, MoSe₂, WS₂, and WSe₂. Strain is applied using a three-point bending scheme, and the polarization-resolved SHG pattern is measured in backscattering geometry. Furthermore, we connected the strain dependent SHG with the strain dependence of the A-exciton energy. With the second-order nonlinear photoelastic tensor, full strain information can be accurately extracted from polarization-resolved SHG measurements. Accordingly, uniaxial strain, induced by polydimethylsiloxan (PDMS) exfoliation and transfer, is measured. We find that TMD monolayers fabricated with PDMS are strained by ∼0.2%. With the experimentally determined nonlinear photoelastic tensors, it will be possible to optically probe arbitrary strain fields in TMD monolayers.