Propagation Properties And Optical Trapping Of Special Vortex Beams

Since vortex beam has a spiral phase structure and carries orbital angular momentum,the propagation and application of vortex beam has always been an important topic in the struc-tured light field.With the enrichment of light-field control methods in recent years,some special structure vortex beams have been proposed.Based on these,in this thesis we have investigated the propagation characteristics of fractional vortex beam,multiramp fractional vortex beam,and vortex-pair beam as well as optical trapping of vortex-pair beam.The contents of this thesis are organized as follows.In Chapter 1,we have introduced the research background of vortex beams including integer vortex beam,fractional vortex beam,and some other special vortex beam.The generation meth-ods of vortex beams and the detection techniques of optical vortices have also beam introduced.Finally,the calculation and experimental methods used in this thesis have also been exhibited.In Chapter 2,we have derived the analytical results for the Fresnel diffraction of a practical Gaussian beam passing through a spiral phase plate of arbitrary integer order.Experimental results on the propagation of light passing through a spiral phase using spatial light modulator have also been discussed.In Chapter 3,we have experimentally demonstrated the distribution and number of vortices contained in fractional vortex beams(FVBs)at the Fraunhofer diffraction region.It shows that the jumps of total vortex strength for FVBs happen only when non-integer topological charge is before and after any even integer number,and there are two different mechanisms for the generation and movement of vortices on the focal plane(i.e.,the Fraunhofer diffraction region).Meanwhile,we have also measured the beam propagation factor(BPF)of such FVBs and have found that their BPF values almost increase linearly in the x component(along the initial edge dislocation)and oscillate increasingly in the y component(vertical to the initial edge dislocation).Our experimental results are in good agreement with the numerical results.In Chapter 4,we have further experimentally investigated the evolution properties of mul-tiramp fractional vortex beams(MFVBs)including FVBs in free space,and we have revealed the dependence of the total vortex strength on both the non-integer topological charge a and the multiramp number m,which are the parameters of the initial multiramp phase structures of MFVBs.In near-field regions,the vortices contained in MFVBs are unstable,and they change and evolve as the propagation distance increases,therefore it is hard to effectively confirm their vortex strength.However,in far-field regions,the evolution of vortices in fields becomes sta-ble and the value of the vortex strength can be confirmed experimentally via measuring vortex structures by the interference method.Our result have shown that the vortex strength for prac-tical MFVBs(including FVBs)has the different behavior compared with the ideal plane-wave MFVBs.These findings give us an understanding of such light fields of both FVBs and MFVBs and may be useful to potential applications related to light signal process and propagation.In Chapter 5,we have proposed and experimentally demonstrated the use of a vortex-pair beam in optical trapping and manipulation.Using the focal properties of such vortex-pair beams,we have successfully trapped two spherical microparticles simultaneously,and obtained the pre-cisely position-controllable manipulation by adjusting the off-axis distance of the vortex pair on the initial phase plane.Furthermore,like an optical wrench due to two bright spots at the focal plane of objective lens,the high-precision angular-controllable rotation of cylindrical microrods have also been realized by rotating the initial phase structure.Our result can provide an alterna-tive manipulation of microparticles and may have potential applications in biological area,and optically driven micromachines or motors.Last,it gives the main conclusions and innovations of this thesis,and also presents a future plan.