Diffractive lenses exist for that (Fresnel zone plates are being used for that) - the problem is that they're absorptive diffractive lenses instead of phase mas

Diffractive lenses exist for that (Fresnel zone plates are being used for that) - the problem is that they're absorptive diffractive lenses instead of phase masks and of course they have limited manufacturing precision. It's a contributing factor to the application of ptychographic methods (i.e. image synthesis from multiple diffraction patterns) to arrive at a high-resolution X-Ray image (e.g. in X-Ray microscopy). Edit: Phase mask mirrors ('surface profile' fresnel zone plate) do exist though.