Coherent radiation at visible wavelengths from sub-keV electron beams

The Smith–Purcell effect allows for coherent free-electron-driven compact light sources over the entire electromagnetic spectrum. Intriguing interaction regimes, with prospects for quantum optical applications, are expected when the driving free electron enters the sub-keV range, though this has until now remained an experimental challenge. Here, we demonstrate the Smith–Purcell light emission from UV to visible using engineerable, fabricated gratings with periodicities as low as 19 nm and with electron energies as low as 300 eV. Our findings constitute a major step toward broadband, highly tunable, on-chip light sources, observation of quantum recoil effects, and tunable EUV and x ray sources from swift electrons.