Fractal zone plates for wideband imaging with low chromatic aberration and extended depth of field

Abstract

Fresnel zone plates are diffractive elements that are essential to form images in many scientific and technological areas, especially where refractive optics is not available. One of the main shortcomings of Fresnel zone plates, which limit their utility with broadband sources, is their high chromatic aberration. Recently presented, Fractal Zone Plates (FZPs) are diffractive lenses characterized by an extended focal depth with a fractal structure. This behaviour predicts an improved performance of FZPs as image forming devices with an extended depth of field and a reduced chromatic aberration. Here we report the achievement of the first polychromatic images obtained with a FZP that confirm these predictions. We show that the polychromatic modulation transfer function (MTF) of a FZP affected by defocus is about two times better than one corresponding to a Fresnel zone plate. This result opens the possibility to improve imaging techniques that use conventional Fresnel zone plates over a wide range of the electromagnetic spectrum, ranging from x-rays to THz.