Observation of Anisotropic Second Harmonic Generation in Two-Dimensional Niobium Diselenide

The intrinsic anisotropy of NbSe₂ provides a favorable prerequisite of second harmonic generation (SHG) and rich possibilities for tailoring its nonlinear optical (NLO) properties. Here we report the highly efficient SHG of mechanically exfoliated NbSe₂ flakes. The nonlinear optical response changes with excitation wavelengths, layer thicknesses, and polarizations of the excitation laser. The anisotropic SHG response further exhibits the intrinsic non-centrosymmetric crystal structure and could effectively assign the crystalline orientation of NbSe₂ flakes. Interestingly, although NbSe₂ flakes with tens of nanometers thickness experience attenuations in SHG performance, more efficient SHG anisotropy ratios were obtained, which are around 4 times higher than that of the 5-layer counterpart. The determined second-order nonlinearities of NbSe₂ flakes (monolayer: ∼1.0 × 10³ pm/V; 3-layer: ∼73 pm/V) are comparable to those of the commonly reported two-dimensional materials (e.g., MoS₂, WSe₂, graphene) with the same number of layers and much higher than those of commercial nonlinear optical crystals.