Motion And Force Analysis Of Mie Particles In The Evanescent Field Of Optical Fibers

Near-field optical tweezers based on evanescent field can trap particles with sub-contact and it has wide applications in many fields such as biology,medical science and physics.And optical fiber tweezers based on evanescent field attract the attention of researchers because of the advantages of space flexibility,low insertion and high drive efficiency.In this thesis,we have in-depth discussion and research about both the fiber probe optical tweezers and nanofiber optical tweezers.Firstly,we introduce the research background and development status of optical tweezers in recent years.We mainly introduce several different types of near-field optical tweezers and explain the advantages of optical fiber tweezers by comparison.Meanwhile,we start with light pressure and explain the basic principle of optical tweezers and the scattering forces and gradient forces on particle.Then we establish the corresponding computational models and computing methods,especially the ray tracing method for Mie particle,according to the size of particle.In the section of simulation analysis,we firstly simulate the evanescent field of optical fiber probe with Finite Element Method.Then we analysis the trapping for particle by optical fiber probe with different taper angles(include the first taper and the second taper)and the forces exerted on different size particles with same optical fiber probe.We finally obtain the optimum angles of double-tapers which exert the largest axial force on particle.For the part of optical nanofiber,we simulate the evanescent field based on the wave theory.Then we establish the computational model by ray tracing which can calculate the scattering forces,gradient forces and velocity of Mie sphere.We also discuss the relation between forces and several parameters such as the distance of sphere and optical nanofiber,the radius of sphere and optical nanofiber.And we obtain the optimum radius of optical nanofiber for particles trapping.In the section of experiment,we respectively use the static etch method and melt-pulling method to fabricate optical fiber probe and optical nanofiber.The incident wavelength of optical fiber is 980 nm.In the part of probe fabrication,we measure the etching rates of both the core and cladding of optical fiber and we explore the factors,such as the temperature,the time,the concentration of hydrofluoric acid and the types of confining liquid,for the probe fabrication of double-tapers.We obtain the optimum fabricated scheme of double-tapers optical fiber probe based on these experiments.In the end we set up both the manipulation systems of optical fiber probe and optical nanofiber.The manipulation system of optical fiber probe finally realized the stabilized trapping of yeast which is the basis of application for optical fiber tweezers in biological field.