Creation of tunable multiple 3D dark spots with cylindrical vector beam.

We present a method to generate multiple three-dimensional (3D) dark focal spots along the optical axis by focusing a cylindrical vector beam. The formation of uniform 3D dark spots is determined by optimizing three parameters of the pupil filter in the wavefront of the lens and the polarized state of the incident cylindrical vector beam. By adjusting the three parameters of the pupil filter, the number and position of dark spots can be controlled willingly, and the uniformity of dark spots can be adjusted by modulating the polarized state of the incident cylindrical vector beam. A single dark spot with uniform surrounding intensity is formed and double dark spots are also obtained with the proper parameters and can be separated with an equal distance away from the geometric focus. Moreover, multiple dark spots numbering up to six are formed along the optical axis. The size of each 3D dark spot is almost the same, and the central light intensity of the dark spots is nearly zero while the light intensity surrounding around is almost uniform. This work may find valuable application in particle trapping, microscopes, optical engineering, and so on.