Terahertz Surface Wave Compression for Low-Energy Electron Diffraction and Imaging

Low-energy electrons, with their large scattering cross sections and exceptional sensitivity to electric fields, have attracted considerable attention for probing ultrafast surface structural and electronic dynamics, particularly through techniques such as low-energy electron diffraction and imaging. However, the significant dispersion of low-energy electron pulses during transport poses a critical challenge to achieving high temporal resolution in time-resolved experiments. In this Letter, we present an at-the-source compression method for low-energy electron pulses using terahertz surface waves on a micrometer-sized tip cathode. Simultaneous electron acceleration and compression have been achieved directly at the emitter surface within a few tens of micrometers. A low-energy (1.5 keV) electron beam with near femtocoulomb charge is temporally compressed by a factor of 3.5, producing 74-femtosecond (FWHM) electron bunches. The quality of the compressed electron bunches is validated through high-quality diffraction patterns of few-layer graphene and projection imaging of a copper mesh. Furthermore, the compressed electrons are applied to investigate the transient electric field propagation generated by photoexcited charged particles on metal surfaces, demonstrating the enhanced temporal resolution. This advancement presents the most compact solution for an electron gun that integrates electron generation, acceleration, and compression into a unified system, paving the way for investigating surface dynamics and complex material phenomena with unprecedented precision.