Study On Axial Optical Force And Torque Exerted On Micro-particles In Optical Vortex

Optical tweezers are three-dimensional(3D) optical traps where micro-particles could be trapped and manipulated by focusing the laser beam using a high numerical objective. Rather than a fixed single optical trap which use traditional Gaussian ones,scientists and researchers indicated that some novel beams could be coupled into optical traps. Special trapping beam mode could either optimize the performance of optical system or implement the novel manipulation. And the study on the optical force of the sphere of intermediate range in vortex beams will improve the ability of special optical tweezers system; meanwhile, the study on the optical force and trapping behavors of the cylinder of intermediate range will promote the applications of micro/nano electronic devices. In this thesis, the systematical investigations have been made on the dynamic information of micro-particles in Laguerre-Gaussian vortex.The structure of 3D helical wave-front phase and ring-shaped intensity distribution of vortex beam were simulated using the numerical tools. And the study focused on the transverse intensity distribution of the Laguerre-Gaussian beams in different modes. The phase holograms for optical vortex was generated and vortex beams in different topological charge was generated by experiment using spatial light modulator.The axial optical trapping efficiency of isotropic microsphere in tightly focused Laguerre-Gaussian beams was studied. The transfer matrix method was introduced and multi-pole expansion of strongly focused laser beams was done by vector spherical wave-functions based on generalized lorenz mie theory. The transfer matrix for spherical particles was given,and effects of various related parameters on the axis trapping efficiency of micro-particles were calculated. Results shows that, the potential well formed by LG01 beam is deeper when the wavelength is shorter and the beam waist radius is smaller.The trapping behaviors of intermediate-scale cylinder micro-particles in vortex beams was studied. The extended boundary condition method was given, and thus the transfer matrix for cylinder was calculated. Then, the optical torque efficiency exerted on cylinders in Laguerre-Gaussian beams was calculated using transfer matrix method. Results shows that, the cylinders in high length-diameter ratio(L-D ratio) tend to parallel to the optical axis in LG01 vortex; the parameters of the beams, such as topological charge, waist radius and polarization state have different effects on the optical torque. The axis trapping efficiency exerted on the cylinders was studied when the orientation of the cylinders is clear. Results indicate that, for trapping ability, the LG01 optical traps is superior to the LG00 optical traps, and the LG01 optical traps formed by right-handed circular polarization is deeper than other polarization optical traps.The optical torque and optical axial trapping efficiency exerted on cylinders in counter propagating dual vortex traps was discussed. The study found that the torque exerted on the same cylinders in dual vortex traps and in single vortex traps is almost the same. When the two beams are incoherent, the total axial trapping efficiency in dual vortex traps is equivalent to the linear superposition of the axial trapping efficiency along +z direction and along –z direction in single vortex traps, and the optical potential well is symmetrical with respect to the center; when the two beams are coherent, the total axial trapping efficiency curve will have oscillating part. Furthermore, the dual traps could trap a particle of high reflective index, and the dual traps formed by the big waist radius could also trap a particle. The potential well formed by two beams with focuses overlapped is deeper than other potential well.