Abstract
Free-electron radiations prompt numerous advanced applications such as particle detectors, biological imaging, and light sources from microwave to X-ray. While their polarization, directionality, and phase could be shaped with prosperous metamaterials, their natural broad spectra have not been tailored to yield frequency combs, which are state-of-the-art technology in metrology, spectroscopy, and precision frequency synthesis. Here, the frequency comb directly emitted from the free-electron radiation is demonstrated by simultaneously exciting a series of modes with equidistant spectral lines. Both the offset and repetition frequencies are readily adapted by structural parameters, and relative spectrum intensities between the comb teeth are customizable based on selective coupling, thus permitting flexible tunability over broadband. Moreover, the repetition rate is experimentally verified at the microwave regime. The proposed methodology implies that swift electrons present a natural and versatile platform for generating frequency combs, facilitating the development of metrology and spectroscopy in the terahertz band.