Research On Nonlinear Optical Processes At Ferroelectric Domain Walls And Nonlinear Interfaces

The artificially poled ferroelectric crystal has been widely used in nonlinear optics,because of its controllable optical superlattice structures.Researchers use periodical or non-periodical domain inversion structures to enhance and modulate nonlinear harmonic waves.In the study of ferroelectric crystals,it is found that the domain walls serve as the interfaces of domains with different polarization directions,and exhibit particular physical properties different from the bulk media,such as enhanced conductivity,dielectric properties and photovoltage effect.In nonlinear processes,the domain wall has local,enhancement and low-dimensional modulation effects on the nonlinear polarization wave,which can be regarded as a specific nonlinear interface.In this dissertation,the properties of the nonlinear Cherenkov radiation are analyzed,the nonlinear properties and symmetry of the domain walls are investigated,and the interfaces with discontinuous nonlinear susceptibilities are studied.Based on the local response of the nonlinear polarization wave at domain walls,the secondorder coefficients of the domain wall in lithium niobate are measured by means of nonlinear Cherenkov radiation.We obtain the complete second-order nonlinearity coefficient matrix,and analyze the symmetry of domain wall,which is different from that of bulk medium.This provides a new way to study the properties of domain walls and achieve non-invasive domain wall imaging.Using the modulation effect of nonlinear polarization wave,we realize the nonlinear Cherenkov radiation in the anomalous-like dispersion environment.And the self-imaging of domain walls is realized by using the degenerate nonlinear Cherenkov radiation.The enhanced nonlinear Talbot effect is observed.The anomalous-like dispersion environment in birefringent crystals has been studied.Under this condition,the methods of scattering assistant and reflection assistant are used to realize the non-collinear phase matching.The experimental results show that the enhancement of nonlinear Cherenkov radiation in ferroelectric crystal with domain inversion is directly related to the domain wall.In addition to the different nonlinear coefficients with bulk medium,the domain wall also represents the interface with discontinuous nonlinear coefficients.We use the paraxial coupled wave equation to analyze the effect of discontinuous nonlinear coefficients,and propose a new configuration utilizing the auxiliary medium to enhance the conversion efficiency at the boundary of crystal.Both in the visible and ultraviolet regions,the intensity of harmonic waves are enhanced.This provides a flexible and tunable phase matching method for the generation of ultraviolet coherent light.The applications of coupled wave equation have been developed from one-dimensional interfaces to two-dimensional structures,and the transmission of nonlinear harmonic wave has been analyzed.In the experiment,multiple nonlinear Cherenkov radiations are observed at the interface of the periodically poled lithium niobate crystal.The complete phase matching is achieved in the anomalous-like dispersion environment,and the enhancement of Cherenkov second harmonic is obtained.Using the coupled wave equation to analyse the nonlinear processes at the domain wall,we find that the radiations of nonlinear harmonic waves satisfy the nonlinear Snell's law.This result is applied to the multi-layer structure.The phases and coherence of refracted second harmonics are analyzed.This has been verified experimentally.The concept of nonlinear metasurface is proposed,and the directions of harmonic waves are analyzed by nonlinear Snell's law.The modulation of the nonlinear polarization wave from radiative to non-radiative state is discussed.The modulation of polarization wave at nonlinear interfaces and nonlinear metasurfaces can better facilitate the usage and control of light.