Interfacial Cherenkov radiation from ultralow- energy electrons

Cherenkov radiation occurs only when a charged particle moves with a velocity exceeding the phase velocity of light in that matter. This radiation mechanism creates directional light emission at a wide range of frequencies and could facilitate the development of on-chip light sources except for the hard- to- satisfy requirement for high- energy particles. Creating Cherenkov radiation from low- energy electrons that has no momentum mismatch with light in free space is still a long- standing challenge. Here, we report a mechanism to overcome this challenge by exploiting a combined effect of interfacial Cherenkov radiation and umklapp scattering, namely the constructive interference of light emission from sequential particle-interface interactions with specially designed (umklapp) momentum- shifts. We find that this combined effect is able to create the interfacial Cherenkov radiation from ultralow- energy electrons, with kinetic energies down to the electron-volt scale. Due to the umklapp scattering for the excited high- momentum Bloch modes, the resulting interfacial Cherenkov radiation is uniquely featured with spatially separated apexes for its wave cone and group cone.