Ultrafine Third-order Optical Nonlinearity Of Magnetically Doped GO Films And ZnO Crystals With Different Crystal Planes

Nonlinear optics is based on the strong coherent optical radiation and matter interaction in discipline,its development thanks to the application of laser technology from theory to practice,nonlinear optics and nonlinear optical materials in optical information processing,laser technology,optical limiter,optical switch,integrated optics and communication technology in the areas of application has a very important influence.So far,the research on nonlinear optical materials has been extended from solid to gas,liquid and various composite hybrid materials,And the excitation pulses have evolved from nanosecond lasers to ultra-fast picosecond and femtosecond lasers.With the continuous progress of photoelectric technology,the optical nonlinear effect of materials has gradually failed to meet the requirements of researchers for practical application.Therefore,it has become an important direction for researchers to explore new nonlinear optical materials and improve the optical nonlinear effect of materials.Graphene oxide（GO）,as an oxide of graphene,still presents a stable two-dimensional honeycomb structure of graphene,with excellent optical,electrical,magnetic and mechanical properties.After oxidation,hydroxyl and epoxy groups are added to the original surface,and carboxyl and carbonyl groups are added to the surface edge,which makes GO have good hydrophilicity,At the same time,it also has good surface activity,and can link various small molecules to GO through chemical bonds,so as to achieve the purpose of changing the properties of GO materials.This is also the main means for researchers to regulate the nonlinear optical properties of GO.Zinc oxide（ZnO）,as a wide band gap direct band gap semiconductor material,has many unique excellent optical properties.Traditional doped or co-doped ZnO materials have been used to improve its photoelectric properties.However,different crystal plane orientations of ZnO also affect the electrical and optical properties of ZnO materials.However,the nonlinear optical properties of ZnO materials and doped ZnO materials are rarely reported,especially the third-order optical nonlinear phenomena of materials excited by vector beam.In this paper,Fe₃O₄-GO hybrid films doped with Fe₃O₄magnetic nanoparticles were prepared by vacuum filtration method and ZnO crystals with different crystal planes were purchased,and their ultrafast third-order optical nonlinearity was studied under femtosecond pulse condition.The third-order optical nonlinearity of Fe₃O₄-GO hybrid films with different doping concentrations was investigated.Pure GO film and Fe₃O₄-GO hybrid film with doping concentration of 0.01m L,0.025m L,0.03m L and 0.05m L were prepared by vacuum extraction and filtration method,and the quality of the films was confirmed by SEM,AFM,EDS,Raman spectroscopy and UV-vis absorption spectroscopy.The film was then tested by Z-Scan and optical limiting and found that,With the increase of peak intensity of incident light,the nonlinear absorption and refraction of all GO films show an increasing trend;With the increase of the doping concentration of Fe₃O₄magnetic nanoparticles,the nonlinear absorption and nonlinear refraction of the hybrid film change from saturation absorption and self-focusing effect to reverse saturation absorption and self-defocusing effect at the doping concentration of 0.025m L;When the doping concentration is 0.05m L,the optical limiting effect of the film reaches the maximum,and the minimum optical limiting initial value is1.1×10¹⁰W/m²and the maximum limiting efficiency is about 55%.Secondly,the influence of external magnetic field on the third-order optical nonlinear effect of Fe₃O₄-GO hybrid film with different doping concentration was explored by adding magnetic field in Z-Scan optical path.The results show that the doping concentration of the third-order optical nonlinear effect of the hybrid film changes from 0.025m L to 0.01 m L in advance after increasing the magnetic field,and the reverse saturation effect and self-defocusing effect are also enhanced.Finally,the anisotropic optical nonlinear effect of the film was tested.The results show that pure GO films have no anisotropy of optical nonlinearity,while Fe₃O₄-GO hybrid films have about40%anisotropy of nonlinear absorption and about 16%anisotropy of nonlinear refraction.ZnO crystals with different crystal planes（ZnO[101],ZnO[110],ZnO[100]）and ZnO crystals doped with Ga（Ga:ZnO[101]）have been investigated for their enhancement and tunable ultra-fast third-order nonlinear effects under femtosecond linear/radially polarized light excitation.Firstly,the band gaps of the samples were analyzed by UV-vis absorption spectroscopy,and then the ultra-fast third-order optical nonlinear effects were studied based on Z-Scan technology.The results show that all samples exhibit saturation absorption and self-focusing effects under the excitation of linear/radially polarized light,which is caused by the direct transition of electrons from the defect state to the excited state in ZnO crystals.Under femtosecond linear polarized light excitation,the third-order optical nonlinearity of ZnO[101]is the strongest,which is caused by the third-order optical nonlinearity of near resonance enhancement caused by the narrowed energy band gap.Under the excitation of radially polarized light,the third-order optical nonlinearity of ZnO[110]is the strongest,and slightly stronger than that under the same conditions under femtosecond line polarized light.This is due to the anisotropic nonlinearity caused by the axisymmetrical polarization of femtosecond vector laser.Therefore,the harmonically enhanced third-order optical nonlinearity can be achieved by adjusting the energy band gap of ZnO crystals and combining the femtosecond vector beam with ZnO with specific crystal planes.