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 (NbOCl 2 ) fork holograms. The polarization‐resolved Raman measurements are performed on the NbOCl 2 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 NbOCl 2 flakes. Furthermore, the Curie temperature of the ferroelectric NbOCl 2 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.