Generation of multiple focal spots with tunable axial distribution

Multi-focal Fresnel zone plates with the ability to generate a specific number of foci in predefined positions, providing three-dimensional (3D) light structures with high uniformity, are highly interesting in optical imaging, as well as in positioning and manipulation of multiple micro-particles in 3D space. However, the simultaneous control and manipulation of particles in 3D working space, requires the focus tuning along the propagation direction. Alvarez-Lohmann lenses are one kind of tunable lenses, in which the focal length tuning is achieved by slightly shifting two combined cubic lens elements with respect to each other. The diffractive versions of Alvarez-Lohmann lenses are advantageous light-weight elements with adjustable and fast refocusing ability. We present a tunable multi-focal zone plate, based on the Alvarez-Lohmann approach, which generates multiple focus spots along the optical axis and provides precise adjustment of the focus positions along the propagation direction. The distance of the focal spots can be tuned accurately, which is advantageous for dynamic 3D positioning, imaging and manipulations.