Design of a plasmonic metasurface laser accelerator with a tapered phase velocity for subrelativistic particles

A metallic metasurface-based laser-driven particle accelerator for subrelativistic particles is proposed and studied theoretically. The metasurface consists of a nonperiodic array of nanoslits which focuses the field of the driving laser, utilizing the phenomenon of extraordinary plasmonic transmission, to maximize the acceleration gradient. In order to account for the actual change in the particles' velocity during their propagation through the structure, the separation between successive slits is not constant but rather optimized according to the expected trajectory of the particles. The metasurface laser accelerator (MLA) is designed for an ultrafast driving laser source operating at 2 μm wavelength. An approximate analytical model verified by particle tracking simulations predicts a net average acceleration with a normalized acceleration gradient of 1.34 times the incident laser field. Compared to other laser-driven accelerator designs, the MLA provides substantially higher efficiency, due to the field enhancement associated with nanoantennas, and relaxed fabrication challenges (especially for subrelativistic particles). It is found that the output particle beam is microbunched, suggesting the possibility of using a short MLA structure as a prebuncher to improve the initial capture efficiency in a subsequent longer MLA device. The impact of space-charge effects is also studied, and the loaded gradient and optimal bunch charge are estimated.