Diffractive optics can be used to accurately control optical wavefronts, even in situations where refractive componentssuch as lenses are not available. For instance, conventional Fresnel zone plates (ZPs) enable focusing of monochromaticradiation. However, they lead to strong chromatic aberrations in multicolor operation. In this work, we propose the con-cept of spatial entropy minimization as a computational design principle for both mono- and polychromatic focusingoptics. We show that spatial entropy minimization yields conventional ZPs for monochromatic radiation. For polychro-matic radiation, we observe a previously unexplored class of diffractive optical elements, allowing for balanced spectralefficiency. We apply the proposed approach to the design of a binary ZP, tailored to multispectral focusing of extremeultraviolet (EUV) radiation from a high-harmonic tabletop source. The polychromatic focusing properties of these ZPsare experimentally confirmed using ptychography. This work provides a new route towards polychromatic wavefrontengineering at EUV and soft-x-ray wavelengths.© 2

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EUV Generation & Imaging

Loetgering, L., Liu, X., de Beurs, A., Du, M., Kuijper, G., Eikema, K., & Witte, S. (2021). Tailoring spatial entropy in extreme ultraviolet focused beams for multispectral ptychography. Optica, 8(2), 130–138. doi:10.1364/optica.410007