Free-electron interactions with van der Waals heterostructures: a source of focused X-ray radiation

The science and technology of X-ray optics have come far,enabling the focusing of X-rays for applications in highresolution X-ray spectroscopy,imaging,and irradiation.In spite of this,many forms of tailoring waves that had substantial impact on applications in the optical regime have remained out of reach in the X-ray regime.This disparity fundamentally arises from the tendency of refractive indices of all materials to approach unity at high frequencies,making X-ray-optical components such as lenses and mirrors much harder to create and often less effcient.Here,we propose a new concept for X-ray focusing based on inducing a curved wavefront into the X-ray generation process,resulting in the intrinsic focusing of X-ray waves.This concept can be seen as effectively integrating the optics to be part of the emission mechanism,thus bypassing the effciency limits imposed by X-ray optical components,enabling the creation of nanobeams with nanoscale focal spot sizes and micrometer-scale focal lengths.Specifically,we implement this concept by designing aperiodic vdw heterostructures that shape X-rays when driven by free electrons.The parameters of the focused hotspot,such as lateral size and focal depth,are tunable as a function of an interlayer spacing chirp and electron energy.Looking forward,ongoing advances in the creation of many-layer vdw heterostructures open unprecedented horizons of focusing and arbitrary shaping of X-ray nanobeams.