Coherent Terahertz Smith-Purcell radiation from laser-wakefield-accelerated electron bunches

Smith-Purcell radiation from undulating currents that charged-particle bunches induce when passing over a grating becomes coherent when the grating period exceeds the bunch length. However, millimeter-long bunches from radio-frequency accelerators restrict such coherence to mm-wave and low-THz frequencies. Here, we generate coherent 30-THz Smith-Purcell radiation pulses with ~10 μJ energy by passing 3-micrometer-long electron (e) bunches from a laser-wakefield accelerator < 1 mm above a grating without damaging the grating or perturbing the e-bunches. Such radiation promises non-invasive diagnosis of femtosecond e-bunch profiles, including inside free-electron lasers, and a highly configurable source of synchronized THz/infrared/electron pulses for ultrafast spectroscopy.