High gradient terahertz-driven ultrafast photogun

Terahertz (THz)-based electron acceleration has potential as a technology for next-generation cost-efficient compact electron sources. Although proof-of-principle demonstrations have proved the feasibility of many THz-driven accelerator components, THz-driven photoguns with sufficient brightness, energy and control for use in demanding ultrafast applications have yet to be achieved. Here we present a novel millimetre-scale multicell waveguide-based THz-driven photogun that exploits field enhancement to boost the electron energy, a movable cathode to achieve precise control over the accelerating phase as well as multiple cells for exquisite beam control. The short driving wavelength enables a peak acceleration gradient as high as ~3 GV m⁻¹. Using microjoule-level single-cycle THz pulses, we demonstrate electron beams with up to ~14 keV electron energy, 1% energy spread and ~0.015 mm mrad transverse emittance. With a highly integrated rebunching cell, the bunch is further compressed by about ten times to 167 fs with ~10 fC charge. High-quality diffraction patterns of single-crystal silicon and projection microscopy images of the copper mesh are achieved. We are able to reveal the transient radial electric field developed from the charged particles on a copper mesh after photoexcitation with high spatio-temporal resolution, providing a potential scheme for plasma-based beam manipulation. Overall, these results represent a new record in energy, field gradient, beam quality and control for a THz-driven electron gun, enabling real applications in electron projection microscopy and diffraction. This is therefore a critical step and milestone in the development of all-optical THz-driven electron devices, validating the maturity of the technology and its use in precision applications.