Diffraction gratings were discovered during the 18th century, and they are now widely used in spectrometry analysis, with outstanding achievements spanning from the probing of single molecules in biological samples to the analysis of solar systems in astronomy. The fabrication of high-quality diffraction gratings requires precise control of the period at a nanometer scale. The discovery of lasers in the 1960s gave birth to optical beam lithography in the 1970s. This technology revolutionized the fabrication of diffraction gratings by offering highly precise control of the grating period over very large scales. It is surprising to see that a few years after, the unique spectral properties of diffraction gratings revolutionized, in turn, the field of high-energy lasers. We review in this paper the physics of diffraction gratings and detail the interest in them for pulse compression of high-power laser systems.

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Diffraction gratings: from principles to applications in high-intensity lasers

LMJ/PETAL laser facility: Overview and opportunities for laboratory astrophysics

Laser-induced femtosecond damage thresholds of Au and Ag coated pulse compression gratings were measured using 800 nm laser pulses ranging in duration from 30 to 200 fs. These gratings differ from conventional metal-on-photoresist pulse compression gratings in that the gratings patterns are generated by etching the fused silica substrate directly. After etching, the metal overcoating was optimized based on diffraction efficiency and damage threshold considerations. The experiment on these gratings was performed under vacuum for single-shot damage. Single-shot damage threshold, where there is a 0% probability of damage, was determined to be within a 400–800 mJ/cm2 range. The damage threshold exhibited no clear dependence on pulse width, but showed clear dependence on gold overcoat surface morphology. This was confirmed by electromagnetic field modeling using the finite element method, which showed that non-conformal coating morphology gives rise to significant local field enhancement near groove edges, lowering the diffraction efficiency and increasing Joule heating. Large-scale gratings with conformal coating have been installed successfully in the 500 TW Scarlet laser system.

Femtosecond laser damage threshold of pulse compression gratings for petawatt scale laser systems

We report on manufacturing and testing results of high efficiency mixed metal dielectric gratings (MMLD) for high power pulse compression applications. The gratings with 1780 l/mm are etched in the top low index layer of a Au-(SiO2/HfO2)4-SiO2 mirror stack. Various grating profiles manufactured in order to modify the near electric field distribution are damage tested on a facility operating at 1.053 µm, 500 fs pulse duration. We evidence that damage threshold is governed by the value of the maximum electric field intensity inside the grating pillar. Moreover thresholds close to 3 J/cm2 beam normal are obtained with this new MMLD grating being thus an interesting alternative to gold and pure dielectric gratings for pulse compression applications.

https://hal-cea.archives-ouvertes.fr/cea-01217072/document

Mixed metal dielectric gratings for pulse compression

The laser Megajoule (LMJ), developed by the French Commissariat a l'Energie Atomique et aux Energies Alternatives (CEA), will be a cornerstone of the French Simulation Program, which combines improvement of physics models, high performance numerical simulation, and experimental validation. The LMJ facility is under construction at CEA CESTA near Bordeaux and will provide the experimental capabilities to study High-Energy Density Physics (HEDP). One of its goals is to obtain ignition and burn of DT-filled capsules imploded, through indirect drive scheme, inside rugby-shape hohlraum. The PETAL project consists in the addition of one short-pulse (ps) ultra-high-power, high-energy beam (kJ) to the LMJ facility. PETAL will offer a combination of a very high intensity multi-petawatt beam, synchronized with the nanosecond beams of the LMJ. This combination will expand the LMJ experimental field on HEDP. This paper presents an update of LMJ & PETAL status, together with the development of the overall program including targets, plasma diagnostics and simulation tools.

https://iopscience.iop.org/article/10.1088/1742-6596/688/1/012067/pdf

The Laser Mega-Joule : LMJ & PETAL status and Program Overview

This paper studies gratings engraved in multilayer dielectric stacks for ultra high intensity laser compressors application. We design various grating profiles with high reflected efficiencies for 1780 l/mm multilayer dielectric gratings (MLD). Each grating is defined to exhibit a different electric field maximum value in the pillars of the grating. A damage testing facility operating at 1.053 μm, 500 fs pulse duration is used to damage test the parts manufactured from these designs. It is evidenced that for fixed incident angle and materials the damage of the grating is directly related to the electric field intensity maximum in the material, which depends on the groove profile. Laser induced damage thresholds of 5 J/ cm2 are experimentally reached with very high and uniform efficiencies.

https://hal-cea.archives-ouvertes.fr/cea-01053352/document

M. Balas

Papers

Effect of electric field on laser induced damage threshold of multilayer dielectric gratings