Studies On Twin Curvilinear Vortex Beams

In recent years,perfect optical vortex(POV)has many applications in optical micromanipulation and optical communication due to its excellent property that the ring radius is independent of the topological charge value.Especially,in optical micromanipulation,POV provides a phase gradient and a strong radial intensity gradient,which are helpful to trap and rotate high-refractive-index particles(particles with refractive index higher than the surrounding medium)well.However,POV with a single bright ring is difficult to trap and drive low-refractive-index particles.To solve this problem,researchers have proposed the concept of double-ring POV(DR-POV).The dark region in the middle of the DR-POV can be used to trap low-refractive-index particles.Although DR-POV provides an excellent solution for trapping and rotating low-refractive-index particles,it still lacks flexibility and versatility because it can only drive low-refractive-index particles moving on circular trajectories,which limits its application in optical micromanipulation.In this paper,the following research work is done to overcome this limitation of DR-POV with only circular trajectories.There are two methods reported to generate DR-POV,one is the Fourier transformation of an azimuthally polarized Bessel beam,and the other is the coherent superposition of two ?-dephased POVs.The first method has the advantage of being easy to generate and does not require frequent changes of the incident field when switching between trapping low and high refractive-index particles.The work in this paper is mainly based on the first method,which modulates the polarization state of the incident field and transforms its focusing field from ordinary curvilinear vortex beams(CVBs)to twin curvilinear vortex beams(TCVBs).The conventional CVBs have only one bright light curve with a preset phase gradient,while the proposed TCVBs consist of two light curves with the same shape and slightly different size.A dark region exists between two light curves,thus providing a more flexible trajectory design for micromanipulation of low-refractive-index particles.Furthermore,by rewriting the integral equation of the incident field,we generalize the two-dimensional TCVBs to the threedimensional one,which may have important applications in the three-dimensional motion of low-refractive-index particles.To verify the feasibility of TCVBs,we performed optical experiments.First,the incident field is obtained by spatial light modulator,then it is converted to cylindrical polarization by using the q-plate with q= 1/2,and finally it is focused to obtain the expected two-dimensional and threedimensional TCVBs.In order to represent the vortex properties of TCVBs,we also performed interference experiments,and the experimental results showed that the two light curves possess the same topological charge.Finally,the experimental scheme was improved to solve the problem that the intensity of the dark region of TCVBs is positively correlated with the phase topological charge.We believe that benefiting from their characteristic of arbitrarily predefined trajectory with independently tailored distribution of intensity and phase,the proposed TCVBs are promising and competitive in terms of application potential in scenarios of trapping and driving low-index particles.