Control The Orbital Angular Momentum In Third-harmonic Generation Using Quasi-phase-matching

Nonlinear frequency conversion of laser is an important subject of nonlinear optics.The quasi-phase matching technique satisfies the phase matching conditions by periodically modulating the second order nonlinear coefficients of nonlinear crystals,thus realizing a variety of high efficiency nonlinear optical processes,which greatly broadens the working band range of the existing lasers,In recent years,structured beams with special field distribution have been a hot topic in the world.The Laguerre-Gaussianl g)beam has a wave front with a helical structure,a hollow intensity distribution and a set of complete orthogonal bases.Because of the above properties of LG beam,it can be used in the fields of particle trapping,super-resolution imaging,large-capacity optical communication,quantum information and so on.In this paper,combining the nonlinear frequency conversion of laser and carrying orbital angular momentum beam,in the process of triple frequency doubling,the two degrees of freedom of photon polarization and orbital angular momentum are used to design the superlattice microstructure.The tripling process of incident light is realized,and the orbital angular momentum of triple-frequency light is regulated.The main contents of this paper include the following aspects:1.In the introduction part,two kinds of matching methods,birefringent phase matching and quasi-phase matching,and the basic concepts of optical superlattices based on quasi-phase matching are introduced.In addition,the origin of photon orbital angular momentum and the background of nonlinear frequency conversion for carrying orbital angular momentum beams are also introduced.2.The second chapter mainly introduces the basic theory of photon orbital angular momentum,including the basic characteristics of intensity and phase distribution of orbital angular momentum beam(also called vortex beam),and deduces the magnitude of orbital angular momentum.This paper introduces several common methods for generating and detecting orbital angular momentum of photons and its applications in the fields of micro-manipulation,optical communication,quantum information and precision measurement.3.In the third chapter,we design the scheme of controlling the angular momentum of photon orbit in the process of tripling frequency:by inputting the horizontally polarized vortex control light,and the vertical polarization of the Gaussian wave,we carry out the Type-0 type frequency doubling and the Type-II class summation.Two cascaded second-order nonlinear processes are used to control the angular momentum of photon orbits in the triple-frequency doubling process.For the cascaded nonlinear processes,we compare the quasi-periodic optical superlattices with the cascaded sector periodic optical superlattices,and determine the optimal design scheme of the superlattices.We also solve the coupled wave equations of nonlinear processes based on Maxwell equations.It is proved theoretically that the proposed scheme is feasible.4.In the fourth chapter,we verify the scheme.The sample used in the experiment is a cascade sector periodic lithium tantalate optical superlattice prepared by room temperature electric field polarization technique.The incident base wave includes two 1342nm near-infrared light beams with the same frequency,different polarization and different orbital angular momentum.In the experiment we observed that the orbital angular momentum carried by triple-frequency light is the same as that carried by the incident LG beam,which proves that the designed scheme is effective.In addition,the frequency doubling,the temperature tuning curve of the triple frequency,the conversion efficiency of the triple frequency,the spot of the triple frequency light and the orbital angular momentum carried by it are studied in detail.The experimental results are in agreement with the theoretical expectation.