Theoretical Study On Optical Trapping And Application Of Optical Tweezers

Since the first demonstration of optical manipulation of micro-particles byAshkin in1970s, the optical tweezers, as a novel optical manipulating technology,has been rapidly developed and applied in variety of fields due to its inherentadvantages, such as non-contact, no mechanical damage, and accurate manipulation.Centered on improving trapping efficiency of optical tweezers and applications inmicro-devices and micro-fluidic chips, the following research works are completedin this thesis.1. The formulas for optical trapping forces in the Ray-optics model and theElectromagnetic scattering model are derived. The dependences of the opticaltrapping efficiency on the numerical aperture of objective, the polarization stateof incident light and the refractive index of particles are calculated and comparedin the two models. The shortage of Ray-optics model and the advantage ofElectromagnetic scattering model are analyzed.2. The trapping efficiencies of cylindrically symmetric vector beams for dielectricspheres with different sizes are calculated by using the Electromagnetic scatteringmodel. It is found that the azimuthally polarized beam is able to traplow-refractive-index particles steadily and also can trap multiplehigh-refractive-index small particles simultaneously. The trapping efficiencies ofcylindrically symmetric vector beams for ellipsoid particles are calculated andfound that the azimuthally polarized beam has an advantage on the transversetrapping and the radially polarized beam has an advantage on the axial trapping.Using the liquid crystal polarization convertor to generate the radially andazimuthally polarized beams, it is verified that the radially polarized beam has astronger axial trapping efficiency than those of azimuthally polarized and linearlypolarized beams.3. With hydrodynamic method, the maximal transverse trapping forces on threekinds of micro-particles (yeast cells, silica beads and PMMA beads) are measured by dragging these particles in water. In the freely falling experiment of PMMAbead into the optical trap, the effective trapping distance and the dependence oftrapping force on the particle position are measured and fitted. The relationshipbetween the size of yeast cells and the maximal transverse trapping force is alsoinvestigated.4. By spatially modulating intensity distribution in cross section of beam with acoating mirror, the axial trapping efficiency of optical tweezers is effectivelyimproved benefiting from the redistribution proportion of the large-angle andsmall-angle incident light rays. Stable three-dimensional trapping of yeast cellswith low numerical aperture objective is implemented. A method is proposed forchanging the ratio between axial and transverse trapping efficiencies by varingthe transmittance of the coating mirror, whcih is more effective than the one usingan obstructed plate.5. Two methods, i.e., laser-induced crystallization method and laser-assistedprecipitation method, are proposed and demonstrated for fabrication ofmicro-devices. Using the laser-induced crystallization method, micro-devicescomposed of copper sulfate are prepared and separated from the solution. Usingthe laser-assisted precipitation method, more stable micro-devices composed ofcalcium carbonate is fabricated, and the condition of precipitation reaction forthis method is discussed.6. Double line optical traps are proposed for micro-particles optical sorting.Y-shaped and11-shaped optical traps are designed and applied to sorting of yeastcells with different sizes in a micro-fluidic chip. On the basis of double lineoptical traps, a micro-particle conveyer is designed and used for transportingtarget yeast cells automatically.