Minimization of electric field intensity in pillars of MLD gratings through the design of the planar dielectric multilayers

Multilayer dielectric gratings (MLDG) are key optical components of Petawatt-class laser that are used to compress short pulses of high intensities. Laser-induced damage can occur on the top area of the components, typically arising in the pillars periodically etched. This phenomenon limits the power yielded by high power laser facilities such as PETAL (PETwatt Aquitaine Laser) laser facility. PETAL is expected to delivery pulses with a wavelength around 1053 nm, an energy around 3 kJ and a pulse duration between 0.5 and 10 ps. Coupled with LMJ (Laser MegaJoule), PETAL aims to study materials in extreme conditions to reproduce the environment in the heart of stars or planets, fusion by inertial confinement, particularly rapid ignition and shock ignition, and nuclear physics for medical proton therapy. In this study, we present a process to improve the laser-induced damage threshold of PETAL pulse-compression gratings in sub-picosecond regime by reducing the electric field intensity in the pillars. PETAL gratings have specific parameters of operation: Transverse Electric polarization, under vacuum, a period equal to 1780 lines per mm and diffraction efficiency higher than 95% for the -1st order. Theoretical designs are calculated with a code developed at the Fresnel Institute. The code solves Fresnel equations by using the differential method, Fast Fourier Factorization (FFF) and S matrix propagation algorithm. As a result, we obtain the distribution of the electric field and diffraction efficiency of any given diffraction order. First, starting with a given MLD mirror, we calculate an etching profile that maximizes the diffraction efficiency at the -1st order by taking into account the manufacturing constraints of future suppliers. Then, we optimize the mirror stack without changing the etching profile. We modify only the first top layers under the grooves. We obtained theoretical designs with the same etching profile and identical diffraction efficiency, associated with different electric field intensity values and expected different laser induced damage thresholds.