X-ray creation by relativistic electrons in a nanophotonic vacuum*

The vanishingly small response of matter to light beyond ultraviolet (UV) frequencies makes it challenging to leverage concepts from nanophotonics to manipulate the emission and propagation of light at such frequencies. Here, we show that free-electron based two-quanta processes allow tunable light sources from UV to gamma ray frequencies, without the need for any external electromagnetic fields. We find that the intentional design of vacuum fluctuations by tuning the permittivity of a material at IR frequencies allows one to exert direct control over the spectrum and intensity of output X-ray radiation. The per-electron power of such X-ray emission is comparable with that of synchrotrons employing a 0.1-1 T magnetic field. We find that this powerful emission derives its strength from the strong Casimir-Polder forces in the extreme nanoscale of materials. Fundamentally, our results reveal a way to apply the tools of nanophotonics even at frequencies where materials do not respond. They also constitute a new mechanism by which vacuum fluctuations can be used to generate high frequency light, potentially enabling new concepts such as compact and tunable sources of photons from UV-to-gamma-ray energies.