Diffractive optics can be used to accurately control optical wavefronts, even in situations where refractive components such as lenses are not available. For instance, conventional Fresnel zone plates (ZPs) enable focusing of monochromatic radiation. However, they lead to strong chromatic aberrations in multicolor operation. In this work, we propose the concept of spatial entropy minimization as a computational design principle for both mono- and polychromatic focusing optics. We show that spatial entropy minimization yields conventional ZPs for monochromatic radiation. For polychromatic radiation, we observe a previously unexplored class of diffractive optical elements, allowing for balanced spectral efficiency. We apply the proposed approach to the design of a binary ZP, tailored to multispectral focusing of extreme ultraviolet (EUV) radiation from a high-harmonic tabletop source. The polychromatic focusing properties of these ZPs are experimentally confirmed using ptychography. This work provides a new route towards polychromatic wavefront engineering at EUV and soft-x-ray wavelengths.
Bibliographical noteFunding Information:
European Research Council (637476); Nederlandse Organisatie voor Wetenschappelijk Onderzoek (13934). The authors thank F. Campi (ARCNL) for useful comments on the manuscript. L.L. gratefully acknowledges the support of NVIDIA Corporation with the donation of the Tesla K40 GPU used for this research.
Acknowledgment. The authors thank F. Campi (ARCNL) for useful comments on the manuscript. L.L. gratefully acknowledges the support of NVIDIA Corporation with the donation of the Tesla K40 GPU used for this research.
© 2021 Optical Society of America
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