Frequency Conversion In KDP Crystal Assisted By Optical Elastic Scattering

The interaction between applied optical fields and nonlinear optical medium results in various nonlinear optical phenomena.For example,multiphoton absorption,surface plasmon resonance,stimulated Raman scattering,and optical Kerr effects,etc..Several nonlinear optical processes generated simultaneously in nonlinear optical medium induce spatial conical emission.The projection of spatial conical emission on two-dimensional?2D?plan can be observed as a single ring or series of rings.Conical emission provides us an efficient method for exploring properties of crystal structure and optical parameters of optical interaction.Previous work has already demonstrated that conical emission can be realized in periodically poled medium,such as PPLN,PPLT,PPKTP etc..Generally,conical emission in periodically poled crystal is assisted by the compensation by reciprocal lattice vector,in order to satisfy the condition of quasi-phase-matching.In our work,we observed the phase-matched conical frequency conversion processes in bulk medium.Compared with the micro photonic structure,phase-matching condition in bulk medium was able to be satisfied without the assistance of reciprocal lattice vector.Experimentally,elastic-scattering assisted conical frequency processes were achieved in bulk medium.In principle,whether it was micro photonic structure or bulk medium,the incidence of laser beam and defects in materials caused scattering light propagating along various directions.Owning to the help of the poly-directional scattering light,the fundamental wave vector was always able to find a particular direction of scattering wave vector which could assist the generation of conical frequency conversion.In this paper,we proposed the scheme of scattering-assisted phase-matching conical frequency conversion generation inside KDP crystal.In KDP crystal,a 42m symmetry crystal,we experimentally observed the existence of second harmonic generation,sum-frequency generation,and difference-frequency generation using the second-order susceptibility?^?7?2?8?of KDP crystal.Furthermore,the third-harmonic generation and four-wave-mixing generation using the third-order susceptibility?^?7?3?8?was observed simultaneously.Particularly,in KDP crystal,the projection of scattering-assisted phase-matched conical second harmonic generation in 2D plan displayed a double-ring pattern second harmonic emission.This kind of double-ring pattern,to some degree,revealed the polarization states of scattering wave.Besides,the inhomogeneous intensity distribution of second harmonic ring could be related to different effective nonlinear coefficients.Furthermore,the effective nonlinear coefficient was dependent on the applied optical fields and symmetry structure of crystal.Therefore,the distribution of intensity on the second harmonic ring could help us expound the symmetry structure of crystal.