We propose a new diffractive optical element, called a spiral Dammann zone plate (SDZP), to generate a series of dipole vortices along the optical axis in the focal region of a focusing objective. By combining this SDZP and another Dammann grating, we describe the generation of three-dimensional dipole vortex arrays in the focal volume of an objective. For experimental demonstration, a 1×5 SDZP with base charge of l=1 is fabricated by using lithography and wet-etching techniques, and a 1×5 coaxial dipole vortex array is achieved for an objective of NA=0.127. Furthermore, by combining the 1×5 SDZP and another 5×5 Dammann grating, a 5×5×5 dipole vortex array is also experimentally demonstrated. The results show that topological charges of these 5×5 vortex arrays on five coaxial planes could be tunable by selecting a vortex beam carrying different charge as the incident field.
© 2012 Optical Society of AmericaPDF Article