Polarization-Independent Second Harmonic Generation in 2D Van Der Waals Kagome Nb₃ SeI₇ Crystals

Second harmonic generation (SHG) of 2D crystals has been of great interest due to its advantages of phase-matching and easy integration into nanophotonic devices. However, the polarization-dependence character of the SHG signal makes it highly troublesome but necessary to match the laser polarization orientation relative to the crystal, thus achieving the maximum polarized SHG intensity. Here, it is demonstrated a polarization-independent SHG, for the first time, in the van der Waals Nb₃ SeI₇ crystals with a breathing Kagome lattice. The Nb₃ triangular clusters and Janus-structure of each Nb₃ SeI₇ layer are confirmed by the STEM. Nb₃ SeI₇ flake shows a strong SHG response due to its noncentrosymmetric crystal structure. More interestingly, the SHG signals of Nb₃ SeI₇ are independent of the polarization of the excitation light owing to the in-plane isotropic arrangement of nonlinear active units. This work provides the first layered nonlinear optical crystal with the polarization-independent SHG effect, providing new possibilities for nonlinear optics.