Nonlinear Manipulation Of Vortex Beam And Scattering Light In Periodically Poled Nonlinear Optical Crystals

Recently,many researchers have been dedicated to the field of vortex beams due to its widespread applications such as optical trapping,optical manipulation,imaging,astronomic detection,rotational speed detection,quantum communication,and so on.Different applications require vortex beams with special optical frequencies.As a result,it is necessary to generate vortex beams with various frequencies for practical applications.Based on the theory of quasi-phase matching(QPM),light frequency can be effectively converted in nonlinear photonic crystals,which can also be applied for nonlinear manipulation of vortex beams.However,there are obvious differences in QPM between vortex beams and Gaussian beams.For example,the orbital angular momentum(OAM)of vortex beam will change in nonlinear frequency conversion.In this thesis,the optical frequencies of vortex beams are efficiently converted through second harmonic generation(SHG)and third harmonic generation(THG)in various nonlinear photonic crystals.The nonlinear three-wave coupled equations for high-order spatial modes are developed,which are utilized to theoretically study Laguerre-Gaussian modes involved optical parametric amplification.Besides,we experimentally investigate the QPM-enhanced THG with scattering light.The main research results are listed below:First of all,we have experimentally studied QPM SHG and THG of OAM beams in one-dimensional periodically and quasi-periodically poled nonlinear crystals.The OAM conservation law holds in both nonlinear processes.While the topological charge of the input fundamental beam is 3,the topological charge of second harmonic and third hannonic waves are 6 and 9.In the quasi-periodically poled nonlinear crystal,the conversion efficiencies of SHG and THG of vortex beam can reach 10%and 1%,respectively.Second,we have demonstrated multiple copies of second harmonic vortex beams in a two dimensional periodically poled nonlinear crystal.The sample is a squarely poled nonlinear crystal with a period of 5.5 p.m.When the wavelengths of fundamental beams are 945 nm,926 mn,906 nm,collinear and noncollinear SH beams carrying OAMs are simultaneously generated with the assistance of various reciprocal vectors.Thirdly,we develop a simple method to detect the mode index of Laguerre-Gaussian beams through a single cylindrical lens,which is used to study the conversion of radial and azimuthal indices of higher-order Laguerre-Gaussian modes in a QPM SHG process.The experimental results show that the radial index of the second harmonic wave is as twice as that in the fundamental beam.Fourthly,we have theoretically studied the vortex beams involved optical parametric amplification process.We obtain the analytical solutions of high-order mode involved three-wave coupled equations,which is applied to investigate the optimal conditions for the efficient amplification of a pure Laguerre-Gaussian mode.At last,we have observed conical THG in a two dimensional periodically poled nonlinear crystal.The principle can be understood as elastic scattering light involved QPM THG.The random distribution directions of the scattering fundamental beam lead to the conical intensity distribution.Our results provide a new method to generate conical beams with short wavelengths.