| As an important part of laser technology,the nonlinear optical frequency conversion technique expanded the available laser wave range utilizing the nonlinear polarization effect of optical medium in strong light field.In the nonlinear optical process,high efficient energy conversion between interacting light waves depends on the momentum conservation in the process,also called phase matching.However,the dispersion property of nonlinear optical materials produces the phase difference between fundamental wave and second harmonic wave,which makes it difficult to satisfy the phase matching condition.Birefringence phase matching was an effective phase matching method,but it required high anisotropy of crystal and polarization direction of incident light,so being difficulty to be practice applied in some nonlinear optical crystals.In 1962,N.Bloembergen proposed the concept of quasi-phase matching(QPM)in which the phase-mismatch caused by dispersion can be compensated by the nonlinear coefficient of periodic modulation in crystal.In 1987,E.Yablonovitch and S.John presented a material model,"Nonlinear photonic crystals",based on photonic crystals,which can effectively realize QPM.Because the nonlinear coefficients regulated by controlling the polarization direction of ferroelectric domains,ferroelectric crystals are also regarded as applicable nonlinear photonic crystal materials.After decades of development,with the maturity of crystal growth and preparation technology,a batch of practical nonlinear photonic crystals were developed by ferroelectric domains inversion induced by electric or light poling,such as lithium niobate(LN)and potassium titan oxide phosphate(KTP).The perovskite ferroelectric crystal owned ferroelectric domain structures with different polarization directions,which generate multi-dimensional reciprocal lattice vector to realize particular QPM process under the action of spontaneous polarization,is a potential nonlinear photonic crystal.In this paper,we show a natural potassium-tantalate-niobate(KTN)perovskite nonlinear photonic crystal with 3D spontaneous Rubik's domain structures at near room-Curie-temperature.These crystal supercells contain 3D rotated ferroelectric polarizations corresponding to the reconfigured second-order susceptibilities,which can provide rich reciprocal vectors for compensating phase mismatch along an arbitrary direction and polarization of incident light.This natural nonlinear photonic crystal directly meets the 3D quasi-phase-matching condition without external poling,which would establish a promising platform for all-optical nonlinear beam shaping and trigger newfangled optoelectronic applications for perovskite ferroelectrics.The main content of the paper is as follows:?.Characterization of the properties of KTN crystal1.According to the features of solid solution and phase transition in KTN crystal,we analyzed the composition which was conducive to form super-crystal structure.The photoelectron energy spectrum of the experimental KTN crystal was measured by XPS,the electron peak area of each element in the photoelectron energy spectrum was calculated to obtain the ratio of each element in crystal,and the crystal composition was KTa0.56Nb0.44O3.2.The Curie temperature of KTN crystal was analyzed according to the crystal composition and phase transition characteristics.Though the DSC curves of KTN crystal,we determined Curie temperature at a near room-temperature 40 0C.3.The ferroelectric property of KTN crystal was characterized by the hysteresis loop,and the saturated polarization intensity Ps of the crystal was measured as 16.98?C/cm2,which proved that the material had good ferroelectric property at room temperature.?.Characterization and study of internal domain structure of KTN super-crystalThe ferroelectric domain structure of supercell in KTN super-crystal was observed and characterized by a variety of testing methods.Firstly,we studied and characterized the macroscopical morphology and polarization distribution of the domain structure in the supercell by polarizing light microscopy and piezoresponse force microscopy.Later,in order to study the influence of the mesoscopic supercell structure on the optical properties of the crystal,linear and nonlinear optical response of KTN photonic crystal with different cutting direction was evaluated.Finally,it is concluded that the supercell structure is a three-dimensional cubic element formed by enmeshing 1800 ferroelectric domains perpendicular to each other in the polarization direction.Based on the above analysis,we established the three-dimensional super-crystal model of KTN crystal.?.Study on three dimensional quasi-phase matching of KTN super-crystalThe micrometric supercell structure of periodic distribution in KTN super-crystal provides rich superlattice reciprocal lattice vector to achieve the QPM.By employing high power infrared laser as fundamental light,we study frequency doubling experiment of KTN super-crystal.Combined with the experimental results and supercell ferroelectric domain structure of the KTN crystal,we systematically studied and analyzed the available QPM patterns in super-crystal,calculated the SHG spot distribution patterns in each QPM process and integrated them to facilitate comparison with the experimental results.According to the influence of the supercell structure on the crystal lattice orientation,we derived the nonlinear coefficients of different orientations and analyzed the influence of the supercell owned 3D rotated ferroelectric polarizations on the polarization properties of second-harmonic generated by QPM.When the pump power is 4.12 W,the SHG intensity is 103.67 ?W,relating to a collinear SHG conversion efficiency of 2.52×10-5. |
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