Surface treatments of dielectric laser accelerators for increased laser-induced damage threshold

This paper describes two surface treatments for silicon dual-pillar dielectric laser accelerators (DLAs), which increase the laser-induced damage threshold (LIDT) and therefore the maximum achievable acceleration gradient. Hydrogen annealing reduces the surface roughness of the silicon pillars and mesa, making the LIDT less dependent on surface perturbations incurred during fabrication, as well as causing damage to occur at predictable locations. Hydrogen annealing decreases the range of the measured LIDT from 5.3 - 10 mJ/cm(2) to 9.5 - 10.2 mJ/cm(2) with 1960 nm 300 fs pulses. Additionally, 90 nm of low-stress silicon nitride film coating on the annealed DLA improves the damage fluence by 27%. Smoother interfaces, possibly combined. with charge transfer from the pillars to the nitride coating, decrease the range of the LIDT and increase the damage fluence. The surface-treated DLAs accelerate 97 keV electrons with an acceleration gradient of 178 +/- 1.8 MeV/m, an increase of 11% over the previous maximal gradient achieved with dual-pillar DLAs. (C) 2020 Optical Society of America