Threshold-less Cherenkov radiation in hyperbolic metamaterial

To generate Cherenkov radiation (CR) in natural medium, the electron energy threshold is higher than hundreds of keV. Even though various approaches were adopted, the high-energy electrons as high as tens of keV is still required in experiment. Here we proposed to eliminate the threshold of electron energy to generate CR with the help of hyperbolic metamaterial (HMM). The analytical and simulation results indicate that, even though electron energy is lower than 0.1keV, the CR could be obtained in HMM in a visible and near-infrared frequency region. Further, the on-chip integrated threshold-less CR source, consisted with a planar electron emitter, Au-SiO2 multilayers HMM, and periodic metal nano-slits, has been realized. It is demonstrated that, with low-energy electrons (0.25-1.4keV), the CR is generated covering λ0=500~900nm. The electron energy generating CR experimentally is two~three orders of magnitude lower than that in natural media and artificial structures. As we know, this is the first on-chip integrated free electron light source benefiting from the threshold-less CR. Although less than 1% of the light energy could be coupled to free space, the total output light power still reaches 200nW, which is two orders of magnitude higher than free electron light source by using other nanostructures.