Second-harmonic generation in strained transition metal dichalcogenide monolayers

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 an immense impact on the SHG intensity along different polarization directions. The impact of strain on the SHG can be modelled with a second-order nonlinear photoelastic tensor. We determined the photoelastic tensors for four different transition metal dichalcogenide (TMD) monolayers: MoS₂, MoSe₂, WS₂ and WSe₂. We find that the photoelastic tensors depend highly on the excitation wavelength. Therefore, we identified the fundamental energies where strain has the highest impact on the SHG. This allows for highly sensitive, noninvasive strain matrix imaging at the submicron scale in all investigated TMDs.