Multifocal array with controllable orbital angular momentum modes by tight focusing

Multifocal spots arrays with controllable three-dimensional (3D) position, orbital angular momentum (OAM) and the number of focal planes along longitudinal direction have been proposed and investigated by using a special designed phase-only multi-zone plate at the back aperture of a high numerical aperture (NA) objective. By using Debye diffraction integral, the focusing field could be rewritten as a fast Fourier transform (FFT), which is the theory and principle for generating the controllable OAM-modes focusing spots. According to this theory, a hybrid multi-zone plate at the back aperture of a high NA objective is designed for the illuminations of the radially polarized Bessel Gaussian beam. Each focusing spot in the focal plane is generated by independent parts, so the multiplexing OAM-modes could be realized by focusing two or more spots at the same position with different orthogonal eigenstates. It is a practical and flexible OAM multi-focus technique with dynamically controllable 3D position, number of focal planes, corresponding topological charges and focal lengths to meet different requirements, such as 3D optical data storage, air-core fiber coupler and parallel optical manipulation. (C) 2016 Elsevier B.V. All rights reserved.