Optical Field Regulation Based On Ultra-fast Optical Nonlinear Of Modified Oxide Materials

Femtosecond laser has the characteristics of narrow pulse width and high intensity,which can reduce the influence of reaction time,thermal effect and other factors in the process of interaction with materials.Using femtosecond laser to excite the nonlinearity of materials,it can be better applied in the fields of super-resolution imaging,optical limiting and ultrafast optical devices.In order to further expand the application of materials,it is necessary to improve their nonlinear properties,which are generally realized by changing the polarization state of incident light and the properties of materials.In this paper,graphene oxide（GO）and SnO₂ are successfully modified by processing under laser and controlling the growth time of materials preparation,and the nonlinear measurement is carried out by using ultrafast femtosecond laser.Super-resolution optical field is widely used in imaging,micro nano processing,particle manipulation and high-density storage.The micro and nano technology is developing more precise,but limited by the light diffraction.The manufacturing technology of large numerical aperture objective lens has reached the extreme and the shortest wavelength of visible light is about 400nm.In this paper,the super-resolution light field is obtained by cutting the light spot through the nonlinear characteristics of GO film.Bottle light field has unique and novel properties,such as barrel light intensity distribution,small dark spot size,etc.It is widely used in optical tweezers,laser trapping and other fields.Based on the nonlinear characteristics of SnO₂ samples,the bottle field and local bottle field are obtained.In this paper,the enhancement of optical nonlinearity of GO and SnO₂ and the regulation of optical field are studied by adjusting the polarization of incident light and modifying materials.GO films with excellent uniformity and flatness are prepared by vacuum filtration method.Grating and concentric circle structures with 50μm,125μm and 200μm are fabricated by CO₂laser marking machine.The results of SEM,XRD,XPS and Raman show the integrity of processing structure,the accuracy of processing linewidth and the effective reduction of GO.The third-order nonlinear properties of the film,including nonlinear absorption and refraction,are measured by Z-scan technique under the excitation of liner,radial and angular polarization of the laser with 532nm and 330fs.By changing the polarization state of incident light and reducing GO by laser processing,the nonlinearity of the film is obviously enhanced.Under the existing experimental conditions,the nonlinear absorption coefficient changes from-8×10^-8m/W to-7.5×10^-7m/W,and the corresponding nonlinear absorption effect increases to9.4 times;the nonlinear refractive index increases from 6×10^-17m²/W to4.3×10^-16m²/W,and the corresponding self-focusing effect increases to 7.2 times.The polarization state of incident light,processing size and processing structure have different effects on the nonlinear properties.By controlling the above three factors,the nonlinear properties can be flexibly controlled.It can be seen from the nonlinear properties of the film that the high-intensity light will pass through with low loss,and the low-intensity light will be absorbed largely.Using this property,the spot can be cut.Under the radial and angular polarized light,the tight focusing light field is controlled by the measured nonlinear properties,and the minimum spot size is 75 nm（～λ/7）,which is beyond the diffraction limit.SnO₂ samples with different morphology are prepared by chemical vapor deposition,and the differences in morphology structure and bond state composition are proved by SEM,AFM,XPS and Raman.The nonlinear absorption and refraction properties of the samples are measured under the excitation of liner and radial polarized light.The experimental results show that the nonlinear properties are closely related to the polarization state of the incident light and the structure of the samples.The nonlinear absorption coefficient increases from 6×10^-8m/W to 2.5×10^-7m/W,and the corresponding nonlinear absorption effect increases to 4.2 times;the nonlinear refractive index changes from-5×10^-17m²/W to-9.5×10^-17m²/W,and the corresponding self-defocusing effect increases to 1.9 times.At the same time,the optical limiting characteristics of the sample under two polarized light are studied,and the minimum optical limiting threshold is 1.9×10¹²W/m².It can be seen from the nonlinear properties of the sample that the high-intensity light will be absorbed largely,and the low-intensity light will pass through with low loss.Finally,in the simulation and experiment,not only the bottle light field is successfully obtained,but also the local bottle light field is regulated.The size of dark spot,the radius and width of light field are increased by 4.3 times,2 times and 1.5times respectively.