The Research On Nonlinear Optical Properties Of Ferrofluids

As an emerging and important branch of modern optics,the research field of nonlinear optics is the nonlinear phenomenon and its application of the medium under the action of strong coherent light.Because of the rapid development of various ultrafast laser technologies,non-linear optics has ushered in new development opportunities.At the same time,pulsed laser technology has higher and higher performance requirements for nonlinear optical materials.At present,there are few new materials with tunable nonlinear optical properties,so it is very urgent to find new tunable nonlinear optical materials and optical devices.In recent years,it has been discovered that liquid suspensions of spherical medium nanoparticles can exhibit very excellent nonlinear optical properties.Therefore,the nonlinear optical properties of nanoparticle suspensions have been extensively studied by scholars.However,there is currently a lack of relevant research on the nonlinear optical properties of the ferrofluids,especially the study of tunable nonlinear optical properties of magnetic nanoparticle suspensions with ultrafast lasers on picosecond or femtosecond time scales is still blank.Based on the above analysis,this article focuses on various nonlinear optical characteristics excited by the ferrofluids under the action of ultrafast lasers with different time scales.The main research contents are as follows:(1)Water-based ferrofluids with different concentrations were prepared and characterized in linear optics.The linear refractive index of the low-concentration ferrofluids was measured using a triangular prism total reflection method.A test device for measuring the permeability of the ferrofluids was built.The linear transmittance of the ferrofluids under the action of a tunable high-power semiconductor laser was measured.The bending diffraction phenomenon of the ferrofluids grating under oblique incident angle was studied.(2)The spatial self-phase modulation effect of ferrofluids converging and diverging continuous Gaussian laser beams at high power was systematically studied.The spatial self-phase modulation process of the ferrofluids to the laser was studied by changing the sample position,concentration,incident laser power,and wavelength,and other variables.For the first time,the phenomenon of spatial self-phase modulation,which was different from the thermal lens effect of the ferrofluids,is discovered.A simple and convenient method was proposed to determine the self-defocusing characteristics of the ferrofluids under the action of a continuous laser.It can be judged by the bright fringe of the diffraction center of the far-field.The relationship between the nonlinear phase shift caused by the convergence and divergence of Gaussian beams and the number of far-field diffraction rings was deeply studied from both theoretical and experimental aspects.The law of the diffraction pattern changes with the incident power of the laser was summarized.The nonlinear refractive index coefficient and the third-order nonlinear polarizability of the sample at different positions were calculated by the calculation formula of the number of diffraction rings and the incident light intensity.Finally,the optical limiting phenomenon of the divergent and convergent Gaussian beams by the ferrofluids was studied.(3)The Z-scan experimental platform was used to study the nonlinear optical properties of the ferrofluids in the picosecond and femtosecond time domains.And the nonlinear absorption coefficient and nonlinear refraction coefficient of the sample under the excitation of two different pulse time-domain lasers were calculated.At the same time,related phenomena were explained through the theory of electronic transition.The nonlinear optical properties of the ferrofluids were controlled by changing the incident power.Theoretically explain the phenomenon of the modulation inversion of the nonlinear characteristics of the ferrofluids.The application of ferrofluids as a saturable absorber in mode-locking was studied,and a stable output pulse was obtained.(4)Z-scan was used to study the formation process of Fe₃O₄ nanoparticle clusters in ferrofluids under high-power picosecond laser incident conditions.By adjusting the incident power of the laser from large to small,important information such as the formation conditions and stable duration of nanoparticle clusters was studied,and a physical model based on light force-induced dynamic migration and clusters of nanoparticles was established.The evolution of nonlinear optical properties of Fe₃O₄nanoparticle colloidal suspension after laser-induced clusters was studied.The reverse saturation absorption coefficient and nonlinear refractive index of the ferrofluids were measured,and the relevant parameters were compared with the dried film samples of the ferrofluids.There are 82 figures,16 tables,and 217 references in this paper.