This paper reports on a new method of creating flat, diffractive lens structures, an evolution beyond Zone Plates.
Apparently, by creating transparent photoresist patterns of varying thickness (on a scale of 3 microns - so virtually flat) on a glass substrate, they can generate lenses that are achromatized for multiple wavelengths, up to continuous bands.
A final comment suggest this could generalize to almost any electromagnetic wavelength. (RF to X-Ray+ ?)