Cascades of pi-phase plates: a transparent diffractive focusing system.

Typically, refractive lenses are used to focus rays of light, but an alternative way can be found by exploiting diffraction of light. It is well known that cascades of hard-edge apertures are able to focus light but with the great drawback of absorption losses. In this paper, we demonstrate that replacing hard-edge apertures with pi-phase plates within a cascade greatly improves the focusing of collimated Gaussian beams. In addition, we propose a simple model to design this cascade, in particular to find the locations and the radii of the different optics once the focal length has been chosen. This model deduced from numerical simulation is useful for sizing cascades consisting of a high number of components and characterized by a strong focusing ability, without requiring a time-consuming optimization process.

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