ABSTRACT
Monolayers containing subnanometer striations of silica and hafnia to form composite materials at varying ratios are explored as a method to develop high-index dielectric layers with increased laser-induced-damage thresholds (LIDTs). These layers can then be used in multilayer dielectric coatings for short-pulse, high-peak-power laser applications, particularly in regions of the highest electric-field intensity. Fabrication is achieved by means of exposure to two different evaporant vapor plumes, where local exposure to each plume is controlled via shielding to prevent simultaneous exposure. The LIDT of the resulting layers has been evaluated at 1053â nm with 600-fs pulses. The results indicate that such hafnia/silica layers exhibit LIDTs similar to silica for a refractive index of ≤1.65. These results suggest that the use of these layers in locations subjected to high electric-field intensity within multilayer dielectric coatings may significantly improve the LIDT, with this deposition process providing particular benefit for scaling to large-aperture, high-fluence components.
ABSTRACT
Contamination of pulse compression gratings during the manufacturing process is known to give rise to reduced laser damage performance and represents an issue that has not yet been adequately resolved. The present work demonstrates that the currently used etching methods introduce carbon contamination inside the etched region extending to a 50- to 80-nm layer below the surface. This study was executed using custom samples prepared in both, a laboratory setting and by established commercial vendors, showing results that are very similar. The laser-induced-damage performance of the etched and unetched regions in the grating-like samples suggest that contaminants introduced by etching process are contributing to the reduction of the laser-induced damage threshold.
ABSTRACT
The physical mechanisms and ensuing material modification associated with laser-induced damage in multilayer dielectric high reflectors is investigated for pulses between 0.6 and 100 ps. We explore low-loss multilayer dielectric SiO2/HfO2 mirrors which are commonly employed in petawatt-class laser systems. The spatial features of damage sites are precisely characterized, enabling the direct correlation of the observed damage morphology to the location of energy deposition and the corresponding standing-wave electric-field intensities within the layer structure. The results suggest that there are three discrete damage-initiation morphologies arising from distinctly different mechanisms: the first prevailing at laser pulse lengths shorter than about 2.3 ps, while the other two are observed for longer pulses. Modeling of the thermomechanical response of the material to localized laser-energy deposition was performed for each type of damage morphology to better understand the underlying mechanisms of energy deposition and subsequent material response.
ABSTRACT
A low-temperature chemical cleaning approach has been developed to improve the performance of multilayer dielectric pulse-compressor gratings for use in the OMEGA EP laser system. X-ray photoelectron spectroscopy results guided the selection of targeted cleaning steps to strip specific families of manufacturing residues without damaging the grating's fragile 3D profile. Grating coupons that were cleaned using the optimized method consistently met OMEGA EP requirements on diffraction efficiency and 1054 nm laser-damage resistance at 10 ps. The disappearance of laser-conditioning effects for the highest-damage-threshold samples suggests a transition from a contamination-driven laser-damage mechanism to defect-driven damage for well-cleaned components.
