Optical Forces Of Focused Femtosecond Laser Pulses On Nonlinear Optical Rayleigh Particles

Maxwell's equations based on theory of electromagnetism showed that when an electromagnetic wave interacts with the induced polarization of an object,the force on the object due to electromagnetic wave scattering and absorption is exerted.One of the most effective usages of this mechanism is a particle trapped with a single focused laser beam demonstrated by Ashkin in 1986,also known as optical tweezers.With the development of laser technology,optical tweezers are widely used in the control of non-contact particles in physics,chemistry,and biology.Compared with the traditional continuous wave laser as the light source of optical tweezers technology,the high-repetition-rate femtosecond laser pulse train has a narrow pulse width,a high peak power,a high temporal and spatial resolution,which can not only capture nano-sized particles,but also act on biological tissues almost without harming the surrounding tissue.In addition,the nonlinear optical process of the interaction between femtosecond laser pulses and particles brings many novel particle trapping effects and dynamic behaviors.In view of the unique advantages and application prospects of high-repetition-rate femtosecond laser pulse in optical tweezers technology,this thesis focuses on the interaction between high-repetition-rate femtosecond laser pulse and dielectric Rayleigh particles,and studies the dependence of optical force distribution on nonlinear optical characteristics and structure of particles,etc.The main contents of this thesis are as follows:Firstly,based on the dipole approximation model,the optical force theory of high-repetition-rate femtosecond laser pulses acting on dielectric Rayleigh particles is developed.Different from the traditional dipole approximation theory,the nonlinear polarization correction of particles should be considered when the high-repetition-rate femtosecond laser pulse interacts with the immersed dielectric Rayleigh particles.Based on Clausius-Mossotti equation and polarizability scattering correction theory,the linear and nonlinear polarizability expressions of particles are derived.According to Richards-Wolf vectorial diffraction theory and energy conservation,the dependence between the intensity distribution of focal field and the average power of incident light is improved.According to the dipole approximation model,the calculation formula of the average optical force on a single particle is derived.The analytical expressions of the average optical forces of nonlinear optical solid particles immersed in surrounding solutions with and without the third-order optical nonlinearity are derived,respectively.Secondly,the effects of the third-order optical nonlinear effect on both the optical force and particle trapping dynamics are studied.The results show that the self-focusing effect of the particle increases the trapping force on the particle and enables it to be trapped in the center of the beam more stably.The self-defocusing effect of particles not only dissipates the gradient force,causing the captured particles to exit along the propagation direction of the beam,but also gives rise to the phenomenon of potential well splitting in the transverse plane and can stabilize the capture of multiple particles at the same time.Then the effects of various physical parameters on the optical force are analyzed,and a variety of control methods are provided.Thirdly,the optical force distribution and dynamic behavior of Rayleigh particles in the solution with the nonlinear optical effect are further studied.Interestingly,immersing in the solution with strong two-photon absorption effect,Rayleigh particles can be continuously pulled towards the light source along the opposite direction of the beam propagation.In order to explain this physical phenomenon,the physical mechanism of pulling force is analyzed carefully,and the selection range of particles and solutions that can satisfy the conditions of pulling force is presented.Finally,the optical force distributions of the hollow-core structure nanoparticles with nonlinear optical effect are investigated.The results show that the volume ratio of the core and the shell directly affects the kinetic behavior of particles.Simplistically,hollow-core nanoparticles with air in the middle are studied.Such as the third-order nonlinear optical effect of the shell can affect the gradient force,and the nonlinear absorption effect will influence the radiation force.The dynamics behavior of the particle is eventually the result of the combination of volume and nonlinear effects.Finally,the dependence between the velocity at the focus and the incident light power is calculated,and a method for characterizing the two-photon absorption coefficient of hollow-core nanoparticles by the capture dynamics experiments is theoretically presented.This thesis establishes and develops the nonlinear polarization theory of the interaction between laser and particle,analyzes and identifies the novel optical force originated from nonlinear polarization,and develops and improves the nonlinear optical characterization technology of a single particle.The results of this thesis provide theoretical support for the potential applications of nonlinear optical capture in particle sorting,characterization of two-photon absorption coefficients of individual nanoparticles,biophotonics and superresolution imaging,ect.