High-energy high-density attosecond electron bunch from short-wavelength laser-nanofoil interaction.

Relativistic attosecond electron bunches are pivotal for ultrafast science, enabling direct observation of atomic-scale dynamics, efficient generation of attosecond X/gamma-ray pulses, and serving as advanced injectors for compact accelerators and free-electron lasers. However, existing methods face challenges in producing attosecond bunches that simultaneously achieve high energy, high density, and a quasi-monoenergetic spectrum. Here, we introduce what is believed to be a novel scheme for generating high-quality relativistic attosecond electron bunches based on a short-wavelength laser-driven transverse injection mechanism. In this approach, an intense short-wavelength laser detaches electrons from an off-axis nanoscale target, forming a dense electron layer. By optimizing transverse injection into the laser focus center, a spectral compression mechanism is triggered as electrons cross the laser axis, enabling efficient direct laser acceleration. The resulting electron bunch exhibits outstanding characteristics with a central energy of hundreds of MeV, a narrow energy spread less than 1 %, an ultrashort duration of 1-10 as, and an extremely high density of ∼1024 cm-3. Such high-quality relativistic attosecond electron sources hold significant potential for advancing ultrafast science.