Terahertz-driven,all-optical electron gun

Ultrashort electron beams with a narrow energy spread,high charge,and low jitter are essential for resolving phase transitions in metals,semiconductors,and molecular crystals. These accelerated beams,produced by phototriggered electron guns,are also injected into accelerators for x-ray light sources. The achievable resolution of these time-resolved electron diffraction or x-ray experiments has been hindered by surface field and timing jitter limitations in conventional RF guns,which thus far are <200 MV/m and >96 fs,respectively. A gun driven by optically generated single-cycle terahertz (THz) pulses provides a practical solution for enabling not only GV/m surface fields but also absolute timing stability,since the pulses are generated by the same laser as the phototrigger. Here,we demonstrate an all-optical THz gun yielding a peak electron energy approaching 1 keV,accelerated by >300 MV/m THz fields in a micrometer-scale waveguide structure. We also achieve quasimonoenergetic,sub-kiloelectron volt bunches with 32 fC of charge,which can already be used for time-resolved low-energy electron diffraction. Such ultracompact,easy-to-implement guns-driven by intrinsically synchronized THz pulses that are pumped by an amplified arm of the already-present photoinjector laser-provide a new tool with the potential to transform accelerator-based science.