The invention is a method and apparatus for generating terahertz radiation. The terahertz source is a versatile terahertz device that can be configured to transmit a plurality of wavelengths, thereby facilitating the detection of multiple contaminants using a single source device. In one embodiment, the Smith-Purcell radiation effect is exploited by passing an electron beam over a modulated conducting surface, wherein the spacing of the periods of the modulated surface is varied. The variations in the modulated surface enable the source to produce light of varying wavelengths.