Manipulating Second-harmonic Waves In High-dimensional Nonlinear Photonic Crystal

The nonlinear photonic crystal(NPC)has space-dependent second-order nonlinear coefficients which has been widely used in optical frequency conversion,nonlinear imaging,nonlinear wave-front modulation,and nonlinear orbital angular momentum(OAM)conversion.Lithium niobate(LN)crystal is one the most popular materials from which to fabricate NPC structures because of its excellent optical properties.Generally,1D and 2D NPC are fabricated by electric poling,crystal growth,probe poling,etc.But these traditional methods can not be utilized in three dimensional NPC.In 2018,however,this problem has been solved by femtosecond laser writing.In this thesis,we focus on manipulating second-harmonic(SH)waves in high-dimensional nonlinear photonic crystal,and the main content includes the followings:(1)Controlling the diffraction patterns of second harmonic OAM beams by designing nonlinear fork gratings with supercell structures in LN crystals.An electrically poled z-cut LN crystal was used as the binary nonlinear grating,for which the SH waves generated by the positive and negative domains have a phase difference of ?.By tailoring the Fourier components of the supercell,i.e.,large-period gratings consisting of small unit cells,the Fourier spectra of these gratings can be reconstructed which determine their optical diffraction properties.For example,We experimentally demonstrate that the energy can be concentrated in the ±3-order SH vortex beams,carrying topological charge +3 and-3.We can also generate diffracted SH vortex beams with equal energies up to the first seven orders.(2)By using fs laser directly writing,3D nonlinear photonic crystal structure has been firstly fabricated in LN crystal.The ferroelectric domains are designated erased inside LN crystal by a computer-controlled focused laser beam,which makes the 3D modulation of second-order nonlinear coefficients come true.This scheme is no longer limited by the traditional technique of trying to realize the polarization reversal of adjacent domains,but uses femtosecond laser to erase or reduce the amplitude of nonlinear coefficients,and successfully realizes the fabrication of 3D NPC.We experimentally demonstrate the satisfaction of 3D quasi phase matching(QPM)and,in the case of nonlinear QPM,as the input fundamental wavelength decreases,the emission angle of the noncollinear SH spot increases because a higher-order reciprocal vector is required to compensate for the phase mismatch.