Anisotropic and Temperature-Tunable Second-Harmonic Vortex Generation in Ferroelectric NbOCl₂ Holograms

Optical vortex beams with helical phase fronts have immense potential to enhance data capacity due to the unbounded values of orbital angular momentum. Chip-scale platforms for producing vortex beams are of paramount importance for a variety of applications. On the other hand, 2D materials with unique optical properties are essential for developing multifunctional ultrathin photonic devices. Here, anisotropic and temperature-tunable second-harmonic vortex beam generation is demonstrated with ultrathin ferroelectric niobium oxide dichloride (NbOCl₂) fork holograms. The polarization-resolved Raman measurements are performed on the NbOCl₂ crystal to understand the anisotropic behavior of the Raman modes. It is demonstrated that the anisotropic and temperature-tunable second-harmonic vortex beams can be generated regardless of the relative orientation of the fork gratings with respect to the crystallographic orientation of the NbOCl₂ flakes. Furthermore, the Curie temperature of the ferroelectric NbOCl₂ crystal is determined according to the measured temperature-dependent second-harmonic generation intensities. The results presented here create new opportunities for the development of advanced polarization-sensitive and temperature-switchable nonlinear photonic devices used for future applications in integrated photonic devices, quantum optical chips, and optical communication.