Tuning of SHG in Biaxially Strained Monolayer MoSe₂ and MoS₂

Second harmonic generation (SHG) is the two-photon non-linear optical process occurring in crystals with broken inversion symmetry such as in the case of monolayer (ML) transition metal dichalcogenides. The change in optical properties of uniaxially strained TMDCs is widely studied while the nature of the homogeneous (biaxial) strain has not been studied in detail. Here, we perform the SHG spectroscopy on ML MoSe₂ and MoS₂ grown at high temperatures in a homebuilt PVD system that yielded up to ~0.7% tensile strain in the MLs depending on the choice of substrates. Polarization-resolved SHG shows 60° period in peak intensities suggesting that the symmetry of MLs remains intact with strain. We discuss the possible enhancement of off-diagonal nonlinear susceptibility at exciton resonances and its strain-tuning effect.