Impact of Strain on the Second-Harmonic Generation in Transition Metal Dichalcogenide Monolayers

Second-harmonic generation (SHG) is a powerful measurement technique to analyse the symmetry properties of crystals. Mechanical strain can reduce the symmetry of a crystal and even weak strain can have an immense impact on the SHG intensity along different polarization directions [1]. The impact of strain on the SHG can be modelled with a second-order nonlinear photoelastic tensor [2,3]. We find that the photoelastic tensors depends highly on the excitation wavelength. We investigated the influence of strain on SHG for MoS ₂ , MoSe ₂ , WS ₂ , and WSe ₂ monolayers. In order to quantify the strain induced anisotropy in the polarization resolved SHG we define two parameters. The Distortion D = ([(A-B)/(A+B)]) ² , which is the ratio of the minima to the maxima of the envelope function of the polarization resolved SHG and the Intensity I = π(A ² + B ² ), which is the integral of the SHG intensity over all polarization directions.