Azimuthal angle-dependent curvature in nonlinear Raman–Nath diffraction arcs from a PPLN grating

Nonlinear Raman–Nath diffraction (NRND) is an intriguing second-order nonlinear optical process that enables versatile transverse phase matching and spatial diffraction. This phenomenon has been experimentally demonstrated in periodically poled lithium niobate (PPLN) nonlinear gratings and other complex nonlinear photonic structures. In this work, we observe a pronounced NRND effect with a parabolic distribution when a high-peak-power Ti:sapphire femtosecond pulse laser is obliquely incident on a single PPLN nonlinear grating, exhibiting diffraction behavior akin to that observed in linear grating diffraction. By rotating the crystal to vary the azimuthal angle φ 1 while keeping the polar angle α constant, it is ensured that the incident beam remains confined within the same angular cone. As the azimuthal angle φ 1 is adjusted, diffraction spots of different orders no longer align along a straight line but instead shift away from the horizontal, forming a mild curvature—an observation, to the best of our knowledge, previously unreported. These findings offer insights into the interaction between incident light and nonlinear media, providing a foundation for exploring a broader range of incident angles and geometrical configurations. The results not only deepen the understanding of nonlinear diffraction processes but also pave the way for the controlled manipulation of nonlinear optical interactions under diverse experimental conditions, thereby contributing to the optimization of nonlinear optical devices and processes.