Photonic Spin Hall Effect In Nonlinear Anisotropic Media

For space-constrained linearly polarized light,reflection/refraction will occur when it is transmitted in a non-uniform medium.In order to ensure the conservation of total angular momentum during the reflection/refraction process,spin-orbit interactions will occur,resulting in the reflected/refracted beam produces splitting,namely the photonic spin Hall effect.Due to its unique physical properties,the photonic spin Hall effect has great application potential in precision metrology,micro-nano photonic device research,and optical communication,so it has become a hot research topic in the optics field.Based on the special properties of the photonic spin Hall effect in anisotropic media,this paper studies the photonic spin Hall effect in linear and nonlinear anisotropic crystals,respectively.The main research work of this paper is summarized as follows:(1)A novel photonic spin Hall effect at the air-uniaxial crystal interface is demonstrated both theoretically and experimentally.First,we constructed the reflection model of the horizontally polarized Gaussian beam at the air-anisotropic crystal interface in theory,and analyzed the photon spin Hall effect occurring on this model.When the horizontally polarized light is reflected on the crystal surface,due the anisotropy of to the crystal,the incident polarization state occurs partial cross polarization conversion,where the conversion efficiency is related to the transverse wave vector K_x,and the linear polarization angle of the reflected light changes linearly with the vector K_x.The photons with opposite spin directions are spatially separated in the incident plane,that is,the photonic spin Hall response is generated.In the experiment,the relationship between the spin displacement and the optical axis of the crystal and the incident angle of the beam was measured.When the incident angle is close to Brewster’s angle,the optical axis of the crystal is rotated,and the maximum spin displacement is 29.5μm,which is approximately equal to half of the beam waist;and the spin shift direction is reversed when the optical axis is 90°.Therefore,in this novel photonic spin Hall effect induced by anisotropy,the spin displacement of the reflected beam can be adjusted by the incident angle and the orientation of the crystal optical axis.Based on this method,the photonic spin Hall effect can be flexibly manipulated.(2)The photonic spin Hall effect at the air-nonlinear crystal interface is verified in in theory and experiment.Firstly,starting from the coupled wave equation,we established and the model of beam transmission in nonlinear anisotropic crystals,and analyzed the generation process of the spin Hall effect in nonlinear light fields.It is found that under the combined effect of linear spin-orbit coupling and nonlinear frequency mixing effect,it is found that the spin splitting of the nonlinear optical field depends on the position of the incident light focal point in the crystal under the combined effect of linear spin-orbit coupling and nonlinear mixing effect.Unlike the nonlinear optical field,the spin splitting of the fundamental frequency optical field is completely independent of the position of the focal point.When the focal point moves from the front face to the back face of the crystal,the displacement direction of the nonlinear spin photons changes,and the nonlinear photons with opposite spin directions are completely separated.Experimentally,we verify the nonlinear spin Hall effect by taking the frequency doubling effect of a BBO crystal as an example,where the length of the crystal is 20 mm.It is found that when the focus position changes from 0 to 20 mm,the centroid displacement of the right circularly polarized(RCP)component in the frequency-doubled light changes from positive to negative,and the centroid displacement of the left circularly polarized(LCP)component changes from negative to positive,the positions of the centroids of the two components are interchanged;when the focus position is3 to 19mm,the spin displacement in the beam varies substantially linearly with the movement of the crystal.However,when the focus position is changed from 0 to 20 mm,the fundamental frequency light does not have any change.The experimental results are basically consistent with the theoretical expectations.The above research work and achievements have deepened the understanding of the spin-orbit interaction in the photonic spin Hall effect,provided a new method for the manipulation of the photonic spin Hall effect,and further promoted the photonic spin Hall effect.practical application.