Megaelectronvolt electron acceleration driven by terahertz surface wave

Particles at relativistic energies form the basis of acceleration science. The emergence of terahertz-driven acceleration promises vastly smaller and cost-efficient accelerators; however, the field strength inside the acceleration structure has hitherto prevented the energy gain from reaching the megaelectronvolt range. Here, we demonstrate a record electron energy gain as high as 1.1 MeV and an effective acceleration gradient up to 210 MV/m driven by terahertz surface waves, using their strong confinement to the waveguide and the fundamental transverse magnetic mode that is favorable for acceleration. The discrepancy in the velocity between the terahertz surface waves and electrons allows for potential phase-space control, including temporal compression and spatial focusing. We expect these proof-of-principle results to enable the development of a tunable and highly efficient electron accelerator driven by terahertz surface waves for application in compact microscopy, radiation sources, and cancer therapies.