Patterned Domain-engineering LiNbO₃ Functional Device And Its Application In Visible Detection Of Near Infrared Orbital Angular Momentum

Ferroelectric crystals represented by lithium niobate are gradually becoming one of the important materials studied in nonlinear optics because of their ferroelectric properties and high nonlinear characteristic coefficients.Through Czochralski method,the external electric field polarization method,and femtosecond laser direct writing,the spontaneous polarization direction in the lithium niobate crystal can be controlled to form special domain structures,such as periodic polarization and pattern polarization.By combining the theoretical basis of quasi-phase matching,nonlinear optical diffraction,nonlinear volume holography,etc.,researchers can flexibly manipulate the frequency-doubled light generated by the nonlinear process that occurs in domainengineering lithium niobate crystals,thereby achieving beam shaping in multidimensions.Based on this,a large number of domain-engineered lithium niobate functional devices have been demonstrated.This paper mainly studies and develops a processing method for self-supporting,patterned domain-engineering LiNbO₃ with its thickness of about 300-350 ?m.Furthermore,based on the nonlinear diffraction theory,the visible detection of near-infrared(NIR)orbital angular momentum(OAM)by designed domain-engineering lithium niobate crystals is proposed and experimentally demonstrated.This paper includes the following two parts:1.This paper reviews the unique nonlinear optical properties of ferroelectric crystals represented by lithium niobate,as well as its application in the field of nonlinear beam shaping.Three different nonlinear diffraction processes are discussed considering that the wave vectors of the fundamental-frequency optical filed and the sum-frequency optical filed,and the reciprocal lattice vector(provided by the domain engineering ferroelectric crystal)are non-collinear.The structural characteristics and polarization principle of lithium niobate,and several common processing methods of domain-engineered lithium niobate are introduced,respectively.The principle and process flow of the external electric field polarization method is introduced,which is the most common polarization scheme.In particular,in order to optimize the current external electric field polarization method,a set of 300-350 ?m thick patterned domain-engineered Mg-doped lithium niobate processing plan is proposed.Then its specific process details and protentional applications in nonlinear optics are discussed.2.This paper introduces the application prospects of NIR OAM in the field of optical communication are introduced,and the shortcomings of several existing detection schemes.A new scheme for visible detection of NIR OAM are theoretically proposed and experimentally demonstrated.A nonlinear Daman vortex grating is designed and manufactured by the nonlinear diffraction theory and a machinelearning algorithm,which can generate a specific visible vortex beam array with uniform energy distribution after being irradiated by NIR beams.Specific NIR vortex could be detected and recognized by the detector that works in visible wavelength region,due to its transform to Gaussian mode in corresponding diffraction order.Finally,through experiments,the feasibility of this detection scheme for single-order vortex and mixed multi-order vortex is verified.This new method is also proved to be applicative for visible detection of NIR vortices in broadband wavelength region.