An apparatus for generating Smith-Purcell radiation having at least one spectral component at a wavelength A includes a periodic structure including a dielectric material and an electron source, in electromagnetic communication with the periodic structure, to emit an electron beam propagating within about 5λ from a surface of the periodic structure to induce emission of the Smith-Purcell radiation. The electron beam has an electron energy tunable between about 0.5 keV and about 40 keV so as to change a wavelength of the Smith-Purcell radiation.