Focusing hard x rays to nanometer dimensions using Fresnel zone plates

The question is addressed of what is the smallest spot size that hard x rays can be focused to using Fresnel zone plates. A thick tilted zone plate optic with large numerical aperture is considered in numerical simulations and is shown to efficiently focus hard x rays down to below 1 nm, well below the theoretical limit for reflective optics such as waveguides and that of refractive optics. The focal spot size is ultimately limited by the atomic structure of matter. The practical realization of these optics will require a significant technological effort, but would enable hard x-ray nanoprobe studies with close to atomic resolution at current and future x-ray sources, such as x-ray free electron lasers and energy recovery linacs.