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Theoretical and Experimental Studies on Optical Tweezers with Special Mode Beams

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Tutor: YaoBaoLi
School: Xi'an Optics and Fine Mechanics
Course: Optics
Keywords: Optical tweezers,Cylindrically symmetric vector beam,Annular beam,4Pi focusing s
CLC: O439
Type: Master's thesis
Year:  2012
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Optical tweezers is an optical force trap by focusing the laser beam with a highnumerical aperture (NA) microscope objective, which can be employed to trap andmanipulate micro-particles. Since its first invention in1986by Ashkin, opticaltweezers has been developed quickly and served as a powerful tool for investigationin microscopic field. Now, optical tweezers is widely used in many research fields,such as in physics, biomedicine, colloid and fine machining. The studies of thesefields appeal for substantial improvement of optical tweezers technique, such ashigher trapping efficiency and lager trapping range. Focused on improving themanipulation capability of optical tweezers, this thesis covers the following researchworks1. The formulas for calculating optical trapping forces in the Ray-optics model andthe Electromagnetic scattering model are derived theoretically. The trappingefficiencies of cylindrically symmetric vector beam (radially polarized beam andazimuthally polarized beam) have been numerically simulated by T-matrix methodbased on the Electromagnetic scattering model. The trapping characteristics ofradially polarized beam and azimuthally polarized beam have been analyzed.2. We have proposed a long distance axial trapping approach with focused annularlaser beam based on a telescopic pair of axicon. By theoretically calculate thetrapping efficiencies of annular beams with different width of annulus, we findthat focused annular beam can improve axial trapping efficiencies compared withthe commonly used TEM00beam. Besides, there is an optimal annulus with d=0.5at which the maximal axial trapping efficiency will be obtained. Owing to thehigher axial trapping efficiency of focused annular beam, stably trapping andmanipulating particles in three dimensional space by a low NA objective with longworking distance may be realized, which is verified by successfully trapping SiO2sphere (5Pm in diameter) by focusing annular beam (generated by a telescopicpair of axicon) by objectives with NA=0.6and0.45. 3. We theoretically present a method of three-dimensional shifting of spherical focalspot generated in a4Pi focusing system. When the input fields at the pupil planesof two objectives of4Pi focusing system are suitably chosen, a spherical focalspot will be generated. By suitably modulating the input fields, continuousshifting of spherical focal spot in three dimensions in real-time in a4Pi focusingsystem can be realized. The phase modulation function can be obtained by thesimple technique we have presented.
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