Optical Force Theory And Optical Manipulation Based On The Linear And Nonlinear Optical Polarization

Optical tweezers,also known as optical trapping or optical micromanipulation,use optical traps created by tightly focused laser beams to trap and manipulate objects in microns,nanometers and atoms.So far,two Nobel Prizes in physics had been awarded for researches directly related to optical force and optical manipulation.The first was awarded to Steven Chu,Claude Cohen-Tannoudji,and William D.Philp in 1997 for their work on cooling atoms using lasers.The second one was awarded to Ashkin in 2018 for inventing optical tweezers and promoting their applications in biology.Compared with the traditional mechanical operations,optical tweezers have the advantages of no physical contact,high resolution,good penetrability,and flexible structure.Therefore,optical tweezers are widely used in chemistry,biology,physics,and medicine and other fields.Optical tweezers technique has been developed by substituting a continuous wave with femtosecond laser pulses.Therefore,using femtosecond laser pulses to trap micro/nano particles or smaller particles is an inevitable trend in the development of optical tweezers technology.In addition,the nonlinear optical process of femtosecond laser pulses interacting with the trapped particles leads to many new dynamic behaviors.In view of the unique advantages and widely application prospects of femtosecond laser pulses in optical tweezers,this thesis focuses on the interaction between femtosecond laser pulses and Rayleigh particles,and develops the optical force theory based on the linear and nonlinear polarization effects.In addition,the effects of optical nonlinearities,such as Kerr nonlinearity,thermally induced nonlinearity,two-photon absorption,and second-harmonic generation,on the trapping dynamics in femtosecond optical tweezers are also studied.In this theis,the basic applications of optical tweezers with optical nonlinearity in particle trapping,two-photon absorption coefficient characterization of a single particle,and three-dimensional particle orientation measurement are studied,and innovative results are obtained in the following four aspects:First,based on the dipole approximation theory,we consider the optical force of Rayleigh particles,when the second-and third-order nonlinear optical effects are excited by the interaction of femtosecond laser pulses with particles.Different from the traditional theory of optical force based on the linear polarization,the optical nonlinearities of Rayleigh particles and surrounding medium can be excited when femtosecond laser pulses interact with Rayleigh particles immersed in solvent.Therefore,a formula of optical force based on the nonlinear polarization is needed.Based on the dipole approximation theory,the expression of time-averaged optical force on a single nanoparticle is derived.The nonlinear optical force theory in this part provides the foundation and preliminary preparation for the trapping dynamics of femtosecond optical tweezers in the following chapters.Secondly,the effect of third-order nonlinear refraction on optical micromanipulation and particle dynamics behavior is investigated when Kerr and/or thermo-optical nonlinearity exists in femtosecond optical tweezers.The results show that the thermal-optical nonlinearity of the surrounding solvent can change the relative refractive index of the particles,so a femtosecond pulsed Gaussian beam can be used to achieve the 3D stable trapped of the particles with slightly low refractive index,such as polystyrene particles immersed in carbon disulfide.Furthermore,the effect of thermo-optical nonlinearity on optical trapping of high refractive index particles is also studied.Due to thermo-optical nonlinearity,the trapping stiffness increases nonlinearly with increasing power.This work provides the possibility for Gaussian beams to trap low refractive index particles beyond the linear optics regime and successfully explains some reported experimental observations.Thirdly,the motion dynamics of two-photon absorpting nanoparticles trapped by circularly polarized vortex beams were studied.By measuring the dependence of the orbital rotational frequency of the trapped particles and the incident power,a method was proposed to characterize the two-photon absorption coefficient of a single nanoparticle based on femtosecond optical tweezers.A vortex beam carrying optical angular momentum can drive particles in orbiting motions around the optical axis.Different from the trapping dynamics of the particles without two-photon absorption,the orbital motion of the particles with the twophoton absorption accelerates nonlinearly with the increase of incident power.The results of this study provide new ideas for characterization of nonlinear optical coefficients of individual nanoparticles,angular momentum manipulation,and particle transport in nonlinear optical systems.Finally,the optical micromanipulation and particle dynamics behavior of anisotropic nanoparticles with the second-order optical nonlinearity are studied.Based on the linear and second-order nonlinear polarization theory,the time-average optical force theory of anisotropic Rayleigh particles with the second-order optical nonlinearity in femtosecond optical tweezers is developed.The relationship between the depth of the optical escape potential and the 3D direction angles of particles is studied.It is found that the second-order optical nonlinearity polarization of the nano-object leads to the 3D orientation of the spherical object,while the linear optical effect of the nano-particle only leads to the 2D orientation.The results provide a new dimension for the nonlinear control of nanoparticles and a new control method for the optical driving of nano-targets.In this thesis,the linear and nonlinear polarization theory of femtosecond laser interacting with nanoparticles with high repetition rate is established and developed,and the spatial manipulation and nonlinear characterization techniques of single nano-particles are perfected and developed.The results of this study provide theoretical support for optical tweezers in nonlinear characterization of single nanoparticles,angular momentum manipulation,and super-resolution imaging.