Integrated Cherenkov radiation emitter eliminating the electron velocity threshold

Cherenkov radiation(1-4) has played a key role in the discovery of some fundamental particles and physical phenomena, including anti-protons(5), J particles(6) and neutrino oscillations(4). The electron energy (velocity) threshold required to generate Cherenkov radiation in a natural medium is greater than hundreds of keV (refs 3,4). Although various approaches have been adopted, high-energy electrons (tens of keV)(7) are still required to generate Cherenkov radiation experimentally. Here, we demonstrate, in hyperbolic metamaterial, that the electron velocity threshold for Cherenkov radiation can be eliminated. Based on this threshold-less Cherenkov radiation, the first integrated free-electron light source has been realized. Cherenkov radiation covering lambda(0) approximate to 500-900 nm is obtained with an electron energy of only similar to 0.25-1.4 keV, which is two to three orders of magnitude lower than in previous reports(3,7-9). This work provides a way to achieve threshold-less Cherenkov radiation, opens up the possibility of exploring high-performance integrated free-electron light sources and optoelectronic devices, and offers a platform to study the interaction of flying electrons with nanostructures on chip.