Tunable X-rays from free electrons interacting with van der Waals materials

We theoretically predict and experimentally demonstrate highly tunable X-ray sources from free electrons - of kinetic energy ranging from a few kilovolts to a few hundred kilovolts - interacting with van der Waals (vdW) materials to generate Smith-Purcell radiation. The photon energy of the output X-rays is readily tuned via the energy of the free electrons, the composition of the vdW material target and the angle between the free electron beam and the vdW material target. As an application of this compact, tunable X-ray platform, we experimentally demonstrate the phenomenon of quantum recoil in free electron spontaneous emission.