Research On Manipulating Particle By Single Fiber Vortex Field Optical Tweezers

Optical tweezers is a technique that controls the motion of particles by conversing the momentum between photons and particles.It has the advantages of non-contact,non-damage,high accuracy,etc.Therefore,it has been widely used in many scientific fields such as medicine,biology,physics,etc,and effectively promoted the development of related fields.Vortex beams have attracted much attention because of their definite orbital angular momentum and dark hollow optical field distribution.In this paper,trapping forces of two kinds of optical fiber tweezers based on vortex evanescent fields and vortex focused fields are calculated theoretically.The motion states of particles in two different forms of vortex field also are analyzed.In the experiment,high-order vortex fields are stimulated by single-mode and multi-mode fibers coupling,and the axial capture and trapping forces of yeast cells are measured by single fiber optical tweezers capture system.Firstly,the background of optical tweezers is introduced,including its development and application in recent years.The main characteristics and trapping mechanism of optical tweezers with optical vortex are described,as well as the orbital angular momentum of vortex beams.Meanwhile,the basic structure and trapping of two kinds of single fiber optical tweezers are introduced in detail.The theoretical part consists of two parts:(1)The ray model is used to analyze trapping forces of the Mie particles in the vortex evanescent field.The results show that,the gradient forces and scattering forces in the cross section always decrease with the increase of radial distance.The direction of gradient force always points to the optical axis,which traps the particles onto the surface of optical fibers.However,under the action of the azimuthal scattering force,the particles will rotate around the optical axis.At the same time,the effects of the radii of micro-nano optical fibers and particles on the trapping forces are also investigated.It is found that both of gradient force and scattering force increase first and then decrease.(2)By T-matrix theory and the Maxwell stress tensor integralling,the forces and torques of single-layered and double-layered particles in the vortex focused field are calculated.The results show that,the optical field distribution of linear polarization field don't exhibit cylindrical symmetry.The trajectory of particles' orbital motion in linear polarization field is an ellipse,but trajectories in other polarization fields are a circle.At the same time,with the increase of the order of the vortex field,the rotation period of the particles increases gradually,and the elliptical orbit's eccentric rates decrease in the linear polarization field.The radial equilibrium position will gradually close to the optical axis with the increase of particle radius.However,when the particle radius is larger than a critical value,it will be captured on the optical axis and can not do orbital motion.For the double-layered particles,the double-layered particles with high inner refractive index have greater trapping forces,and the double-layered particles with low inner refractive have larger rotation frequency.In the experimental part,multi-mode optical fiber probes are fabricated by chemical etching method,the corrosion rates of optical fiber's cladding and fiber core at different temperatures are measured.The high-order modes are stimulated by single-mode and multi-mode fibers coupling.Then the axial capture of yeast cells is carried out by the single optical fiber tweezers capture system,and the trapping forces were measured.The results show that trapping forces of multi-mode fiber probe is much smaller than those of single-mode fiber probe.Besides,the impacts of particle size and core dislocation on the trapping forces are also investigated.