Impact of the electric field enhancement on the laser damage resistance of transmission gratings operating in the UV and nanosecond regime

The Megajoule Laser (LMJ) uses large transmission diffraction gratings composed of fused silica to focus high-energy laser beams at 351 nm. These gratings operate in the final stage of the laser beamline and must withstand high-energy nanosecond pulses. Increasing the laser-induced damage threshold (LIDT) of transmission gratings is a challenge that requires a detailed understanding of the damage process. Here we numerically and experimentally study the LIDT of silica-based transmission gratings as a function of the electric field intensities (EFI) occurring in the vicinity of the nanostructured surface of diffraction gratings. Damage tests are performed on different samples under varying polarizations and incident angles and compared with numerical EFI simulations. We show that the damage threshold is strongly correlated with the maximum EFI occurring inside the silica pillars of the grating. We evidence a linear relation between the LIDT and the EFI with parameters depending on the sample quality.