Nonlinear Generation And Steering Of Light In Second-Order Nonlinear Optical Materials

Nonlinear generation and steering of light is one of the most important research interests of nonlinear optics.This thesis focuses on the nonlinear generation and steering of light in second order nonlinear optical material(like LiNbO3,LiTaO3 and KTP).Several phase-matching types,like birefringence phase-matching(BPM)and quasi-phase-matching(QPM),have been reported since 1960s.Now the phase-matching technique has been widely used to realize frequency conversion process with high efficiency and nonlinear steering,especially after the development of superlattice material.On the other hand,the beams with special polarization or phase distribution also attract people's interest around the world in recent years.Among them,the beam with spiral phase wavefront(also called vortex beam),which was proved to carry optical orbital angular momentum(OAM)in 1992,became extremely popular for its unique dynamic and quantum properties.In this thesis,we mainly study on the conversion of OAM during nonlinear processes involving with vortex beam,and Cherenkov frequency down conversion based on phase velocity modulation in nonlinear photonics crystals.The contents are organized as follows:1.A brief overview of development of laser and nonlinear optics,including the concept of BPM,QPM,nonlinear Cherenkov radiation,Raman-Nath diffraction and Bragg diffraction.Application of QPM in nonlinear engineering is discussed.Besides,we also introduce the basic concepts related to OAM,including derivation of OAM from Maxwell equations,the methods to generate beams carrying OAM and ways to test the topological charge of vortex beam.2.In this part,we investigate the second harmonic generation(SHG)of beams carrying fractional OAM.It is not only possible to generate states of light with integer OAM,but also with a fractional OAM.Based on the nonlinear coupled equations,we theoretically predict the OAM distribution of the generated second harmonic(SH)waves of fractional states.In experiment,we use the periodically poled lithium niobate(PPLN)to realize the SHG of beams carrying fractional OAM,whose results show that the different components with integer OAM of the fundamental beam will interact with each other during the nonlinear optical process,further verifying the superimposed nature of fractional OAM.3.Studying the properties of beams carrying OAM in SHG process.Based on the far field Green function solution of nonlinear wave equations,we theoretically study the intensity and OAM distribution of SH wave.Our discussion considered several general phase-matching conditions,including collinear process,Cherenkov radiation process,Raman-Nath diffraction process,Bragg diffraction process and phase-mismatching conditions.Theoretical simulation shows that the OAM always conserves during the collinear SHG process,while non-collinear nonlinear process will disturb the symmetry of fundamental waves,which shall violate the OAM conservation law.In experiment we utilize the one-dimensional periodically poled LiNbO3(PPLN)and KTP(PPKTP)to study the several nonlinear processes respectively and the results match the theoretical simulation well.4.Cherenkov frequency down conversion process in nonlinear photonics crystals.Nonlinear Cherenkov frequency up conversion process can automatically occur.However,the frequency down conversion process is still a challenge because of the Cherenkov phase velocity threshold,which requires the phase velocity of nonlinear polarization wave must be larger than that of harmonic wave.Firstly,we utilize the birefringence property of KTP to realize the degenerate 532nm-1064nm?1064nm Cherenkov difference frequency generation(CDFG).In this scheme,we choose the proper polarizations of pump and signal waves to fulfill the Cherenkov phase-matching condition.Then we expand the concept of birefringence into waveguide to realize the compactable CDFG,where the mode dispersion engineering becomes more flexible.Here we theoretically explain the feasibility of CDFG in Ti:diffused lithium niobate optical planar waveguide.After that,we get the plane wave solution of CDFG in waveguide based on the nonlinear coupled mode equations.The theoretical conversion efficiency of CDFG is also given.