Fourth-harmonic generation via nonlinear diffraction in a 2D LiNbO3 nonlinear photonic crystal from mid-IR ultrashort pulses

Five conical harmonic beams are generated from the interaction of femtosecond mid-infrared (mid-IR) pulses at a nominal input wavelength of 1997 nm with a 2D LiNbO3 nonlinear photonic crystal with Sierpinski fractal superlattices. The main diffraction orders and the corresponding reciprocal vectors involved in the interaction are ascertained. Second and third harmonics emerging at external angles of 23.82° and 36.75° result from nonlinear ?erenkov and Bragg diffractions, respectively. Three path