We propose a method to determine the topological charge (TC) of a perfect vortex. With the phase shift technique, the perfect vortex and its conjugate beam exactly overlap and interfere. Consequently, the TC of a perfect vortex is determined by counting the number of interference fringes. This proposed method enables in situ determination of the TC of the perfect vortex without the need for additional optical elements, and it is immune to environmental vibration and parasitic interference.
We report a novel method to freely transform the modes of a perfect optical vortex (POV). By adjusting the scaling factor of the Bessel-Gauss beam at the object plane, the POV mode transformation can be easily controlled from circle to ellipse with a high mode purity. Combined with the modulation of the cone angle of an axicon, the ellipse mode can be freely adjusted along the two orthogonal directions. The properties of the "perfect vortex" are experimentally verified. Moreover, fractional elliptic POVs with versatile modes are presented, where the number and position of the gaps are controllable. These findings are significant for applications that require the complex structured optical field of the POV.
For optical tweezers, a tiny focal spot of the trapping beam is necessary for providing sufficient intensity-gradient force. This condition results in a limited small trapping range to guarantee stable trapping of the particle. Exploiting structured light, i.e., an optical vortex beam, the trapping range can be enlarged by adjusting its doughnut ring diameter. However, the trapped particle scarcely remains static due to the optical spanner action of the orbital angular momentum of the vortex beam. To enlarge the trapping range and simultaneously ensure stable trapping, we propose a beam, referred to as a mirror-symmetric optical vortex beam (MOV). Essentially, MOV is constructed by using two opposite optical spanners and a pair of static optical tweezers. The optical spanners attract the particle to the site of the static optical tweezers, which realizes long-range optical trapping. Through detailed force-field analysis, it is found that MOV could perform these setting functions. In experiments, yeast cells are manipulated in a long range of ∼25 μm, which is 3 times longer than that of the Gaussian beam. Further, the trapping range is easily adjusted by changing a parameter as desired. This technique provides versatile optical tweezers, which will facilitate potential applications for particle manipulation.
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