  We present a novel design for an ultra-compact, passive light source capable
of generating ultraviolet and X-ray radiation, based on the interaction of free
electrons with the magnetic near-field of a ferromagnet. Our design is
motivated by recent advances in the fabrication of nanostructures, which allow
the confinement of large magnetic fields at the surface of ferromagnetic
nanogratings. Using ab initio simulations and a complementary analytical
theory, we show that highly directional, tunable, monochromatic radiation at
high frequencies could be produced from relatively low-energy electrons within
a tabletop design. The output frequency is tunable in the extreme ultraviolet
to hard X-ray range via electron kinetic energies from 1 keV-5 MeV and
nanograting periods from 1 {\mu}m-5 nm. Our design reduces the scale, cost, and
complexity of current free-electron-driven light schemes, bypassing the need
for lengthy acceleration stages in conventional synchrotrons and free-electron
lasers and driving lasers in other compact designs. Our design could help
realize the next generation of tabletop or on-chip X-ray sources.

Optics