Terahertz emission in SnS2 single crystals: ultrafast shift current

We report on THz emission in single-crystalline SnS₂ in response to above bandgap excitation. Symmetry properties of THz generation suggest that its origin is an ultrafast surface shift current, a 2nd order nonlinear effect that can occur as a result of above-gap photoexcitation of a non-centrosymmetric semiconductor. Multilayer SnS₂ can exist in several polytypes that differ in the layer stacking. Of those polytypes, 2H and 18R are centrosymmetric while 4H is not. While Raman spectroscopy suggests that the single crystalline SnS₂ in our experiments is 2H, its THz emission has symmetry that are fully consistent with the P3m1 phase of 4H polytype. We hypothesize that the stacking disorder, where strain-free stacking faults that interrupt regions of 2H polytype, can break inversion symmetry and result in THz emission. These results lay the foundations for application of SnS₂ as an efficient, stable, flexible THz source material, and highlight the use of THz spectroscopy as a sensitive tool for establishing symmetry properties of materials.