Wavelength-dependent nonlinear wavefront shaping in 3D nonlinear photonic crystal

A 3D nonlinear photonic crystal containing four parallel segments of periodic χ⁽²⁾ grating structure is fabricated employing the femtosecond laser poling of ferroelectric Ca_0.28Ba_0.72Nb₂O₆ crystal. The second harmonic generation from this four-segment structure is studied with a fundamental Gaussian wave. By tuning the wavelength of the fundamental wave, the second harmonic varies from the Laguerre–Gaussian beam (topological charge l_c = 1) to the higher-order Hermite–Gaussian beam and Laguerre–Gaussian again (l_c = −1). This effect is caused by the wavelength-dependent phase delays introduced by the four-grating structure. Our study contributes to a deeper understanding of nonlinear wave interactions in 3D nonlinear photonic crystals. It also offers new possibilities for special beam generation at new frequencies and their control.