A scheme to generate coherent terahertz (THz) radiation carrying orbital angular momentum (OAM) through down-frequency conversion of optical lasers and higher order mode coupling is proposed and analyzed. Based on the laser–electron interaction in the undulator under a linear regime, the scheme utilizes dual-frequency lasers resonating at the harmonics of two consecutive undulators and a dispersion section to impose a THz helical microbunching in the electron beam. By choosing appropriate parameters of lasers and dispersion section, the density modulation can be down-converted to a THz spectral-domain and the central frequency can be extensively tuned together with the helical mode number. Then, the helically microbunched beam can be used as a source of THz OAM radiation in a downstream radiator. We expect that the intense vortex beam in the terahertz band will bring new opportunities for the research of quantum topological materials.
