Calculation Of Trapping Force Of Optical Tweezers Using T-Matrix Method

Optical tweezers is a new type of physical tools which based on mechanical effect of light. It is widely used in physics, medicine, biology, chemistry and materials, etc, because it can be used to capture and manipulate the micro-objects with non-contact and no invasion. A number of different methods of numerical calculation had been developed for various models of optical tweezers. The laguerre-gaussian beams(LGpl beams) can be used for constructing new type of optical tweezers, and has orbital angular momentum, which can be passed to the particles. The orbital angular momentum makes the captured particles move in the field. The electromagnetic model of optical tweezers fields study is comprehensive and complicated. Calculating the trapping force of the LGpl beams has substantial significance for optical tweezers applications using the electromagnetic model. In this thesis, trapping force of the LGpl beams is calculated using T matrix method. The influences of the size of the parameters, the orders of the LGpl beams, the numerical apertures of objective, particle characteristics(radius, refractive index, etc.) on the capture performance of optical tweezers is studied by numerical simulation. These provide theoretical support for micro particle manipulation and micro force measurement with optical tweezers.The main results are summarized as following:(1) According to the relationship between the particles size and incident beams, the models of optical tweezers are described, mainly including Ray-optics method, Rayleigh Scattering theory, Finite Difference Time Domain, Finite Element method, Generalized Lorentz-Mie theory and T-matrix method, which are used to calculate the trapping force. And we concluded that T-matrix method is the better calculation method to study the electromagnetic model of optical tweezers.(2) The trapping efficiency of optical tweezers, the orders of the LGpl beams, the refractive index and the size of the particles are studied using T-matrix method. The axial and radial trapping efficiency of optical tweezers using the LGpl beams and that of the TEM00-mode Gaussian beams are compared. The results show that the axial trapping efficiency of the LGpl beams with orders less than 4 are higher than that of the LGpl beams with orders more than 4. The trapping efficiency and the trapping regions of the LGpl beams are better than the trapping efficiency and the trapping regions of the Gaussian beams, and they increase with the increasing of the particles radii and the refractive indices of the particles.(3) Trapping force of the LGpl beams trapped core-shell particles is calculated using T-matrix method, the numerical apertures of objective, the refractive indices of particles cladding, the azimuthal model index and the radial model index of the LGpl beams are changed. The results show that, in a certain range, the trapping efficiencies and trapping regions of optical tweezers increase with the increasing of the numerical apertures of objective. However the trapping efficiencies reduce with the numerical apertures of objective are too big or too small. The trapping efficiencies of optical tweezers are increasing, when the refractive indices of particles core are closer to the refractive indices of particles cladding. The trapping efficiencies of optical tweezers are decreasing, when the refractive indices of particles core and the refractive indices of particles cladding have a bigger difference. The azimuthal model index and the radial model index of the LGpl beams have changed with the trapping efficiencies have different changes.