Coherent terahertz Smith–Purcell radiation from laser-wakefield-accelerated electron bunches

Radiation from undulating currents that charged-particle bunches induce when passing over a grating, known as Smith–Purcell radiation (SPR), becomes coherent for bunches shorter than the grating period. Millimeter-long bunches from radio-frequency accelerators generate coherent SPR only at mm-wave and low-THz frequencies. Here, we generate strong coherent 30 THz pulses by passing micrometer-long electron (e) bunches from a laser-wakefield accelerator within a millimeter of a grating without damaging it or perturbing the e-bunches. Such radiation promises non-invasive, single-shot diagnosis of femtosecond e-bunch profiles, including inside free-electron lasers, and a highly configurable source of synchronized THz/hard X-ray pulses for ultrafast high-field THz spectroscopy.