David Alessi

Picosecond laser damage performance assessment of multilayer dielectric gratings in vacuum

Precise assessment of the high fluence performance of pulse compressor gratings is necessary to determine the safe operational limits of short-pulse high energy lasers. We have measured the picosecond laser damage behavior of multilayer dielectric (MLD) diffraction gratings used in the compression of chirped pulses on the Advanced Radiographic Capability (ARC) kilojoule petawatt laser system at the Lawrence Livermore National Laboratory (LLNL). We present optical damage density measurements of MLD gratings using the raster scan method in order to estimate operational performance. We also report results of R-on-1 tests performed with varying pulse duration (1-30 ps) in air, and clean vacuum. Measurements were also performed in vacuum with controlled exposure to organic contamination to simulate the grating use environment. Results show sparse defects with lower damage resistance which were not detected by small-area damage test methods.

Characterization of laser-induced damage by picosecond pulses on multi-layer dielectric coatings for petawatt-class lasers

We investigate the optical damage performance of multi-layer dielectric (MLD) coatings suitable for use in high energy, large-aperture petawatt-class lasers. We employ small-area damage test methodologies to evaluate the damage resistance of various coatings as a function of deposition methods and coating materials under simulated use conditions. In addition, we demonstrate that damage initiation by raster scanning at lower fluences and growth threshold testing are required to estimate large-aperture optics’ performance.

Role of defects in laser-induced modifications of silica coatings and fused silica using picosecond pulses at 1053 nm: II Scaling laws and the density of precursors

We investigate the role of defects in laser-induced damage of fused silica and of silica coatings produced by e-beam and PIAD processes which are used in damage resistant, multi-layer dielectric, reflective optics. We perform experiments using 1053 nm, 1-60 ps laser pulses with varying beam size, number of shots, and pulse widths in order to understand the characteristics of defects leading to laser-induced damage. This pulse width range spans a transition in mechanisms from intrinsic material ablation for short pulses to defect-dominated damage for longer pulses. We show that for pulse widths as short as 10 ps, laser-induced damage properties of fused silica and silica films are dominated by isolated absorbers. The density of these precursors and their fluence dependence of damage initiation suggest a single photon process for initial energy absorption in these precursors. Higher density precursors that initiate close to the ablation threshold at shorter pulse widths are also observed in fused silica, whose fluence and pulse width scaling suggest a multiphoton initiation process. We also show that these initiated damage sites grow with subsequent laser pulses. We show that scaling laws obtained in more conventional ways depend on the beam size and on the definition of damage for ps pulses. For this reason, coupling scaling laws with the density of precursors are critical to understanding the damage limitations of optics in the ps regime.

Optical damage performance measurements of multilayer dielectric gratings for high energy short pulse lasers

We investigate the laser damage resistance of multilayer dielectric (MLD) diffraction gratings used in the pulse compressors for high energy, high peak power laser systems such as the Advanced Radiographic Capability (ARC) Petawatt laser on the National Ignition Facility (NIF). Our study includes measurements of damage threshold and damage density (ρ(Φ)) with picosecond laser pulses at 1053 nm under relevant operational conditions. Initial results indicate that sparse defects present on the optic surface from the manufacturing processes are responsible for damage initiation at laser fluences below the damage threshold indicated by the standard R-on-1 test methods, as is the case for laser damage with nanosecond pulse durations. As such, this study supports the development of damage density measurements for more accurate predictions on the damage performance of large area optics.

The role of defects in laser-induced modifications of silica coatings and fused silica using picosecond pulses at 1053 nm: I Damage morphology

Laser-induced damage with ps pulse widths straddles the transition from intrinsic, multi-photon ionization and avalanche ionization-based ablation with fs pulses to defect-dominated, thermal-based damage with ns pulses. We investigated the morphology of damage for fused silica and silica coatings between 1 ps and 60 ps at 1053 nm. Using calibrated laser-induced damage experiments, in situ imaging, and high-resolution optical microscopy, atomic force microscopy, and scanning electron microscopy, we show that defects play an important role in laser-induced damage down to 1 ps. Three types of damage are observed: ablation craters, ultra-high density pits, and smooth, circular depressions with central pits. For 10 ps and longer, the smooth, circular depressions limit the damage performance of fused silica and silica coatings. The observed high-density pits and material removal down to 3 ps indicate that variations in surface properties limit the laser-induced damage onset to a greater extent than expected below 60 ps. Below 3 ps, damage craters are smoother although there is still evidence as seen by AFM of inhomogeneous laser-induced damage response very near the damage onset. These results show that modeling the damage onset only as a function of pulse width does not capture the convoluted processes leading to laser induced damage with ps pulses. It is necessary to account for the effects of defects on the processes leading to laser-induced damage. The effects of isolated defects or inhomogeneities are most pronounced above 3 ps but are still discernible and possibly important down to the shortest pulse width investigated here.

Apparatus and techniques for measuring laser damage resistance of large-area, multilayer dielectric mirrors for use with high energy, picosecond lasers

We present techniques for measuring the damage performance of a variety of optical components with ps laser pulses, introduce a novel beam diagnostic technique, and explore the sensitivity of damage resistance to laser spot size for the case of high-reflectivity, multilayer dielectric (MLD) mirrors.

Laser-induced modifications of HfO2 coatings using picosecond pulses at 1053 nm: Using polarization to isolate surface defects

For pulse lengths between 1 and 60 ps, laser-induced modifications of optical materials undergo a transition from mechanisms intrinsic to the materials to defect-dominated mechanisms. Elucidating the location, size, and identity of these defects will greatly help efforts to reduce, mitigate, or eliminate these defects. We recently detailed the role of defects in the ps laser-modifications of silica coatings. We now discuss the similar role of defects in HfO2 1/2-wave coatings and also include the environmental effects on the damage process. By switching between S and P polarizations, we distinguish the effects of defects at the surface from those throughout the material. We find that defects very near the surface are dependent on the environment, leading to worse damage in vacuum than in air. Air suppresses or lessens the effects of these defects, suggesting a photo-chemical component in the mechanism of laser damage in HfO2 coatings.For pulse lengths between 1 and 60 ps, laser-induced modifications of optical materials undergo a transition from mechanisms intrinsic to the materials to defect-dominated mechanisms. Elucidating the location, size, and identity of these defects will greatly help efforts to reduce, mitigate, or eliminate these defects. We recently detailed the role of defects in the ps laser-modifications of silica coatings. We now discuss the similar role of defects in HfO2 1/2-wave coatings and also include the environmental effects on the damage process. By switching between S and P polarizations, we distinguish the effects of defects at the surface from those throughout the material. We find that defects very near the surface are dependent on the environment, leading to worse damage in vacuum than in air. Air suppresses or lessens the effects of these defects, suggesting a photo-chemical component in the mechanism of laser damage in HfO2 coatings.

Increasing Laser Contrast by Relativistic Self-Guiding and its Application to Laser-Based Proton Acceleration

Laser-produced energetic protons via target normal sheath acceleration are deteriorated by amplified spontaneous emission (ASE). Here we test a new method of reducing ASE and increasing proton energy by relativistic self-guiding in the plasma.

High dynamic range temporal contrast measurement and characterization of oscillators for seeding high energy petawatt laser systems

We have measured the temporal contrast and performance of oscillators to determine their feasibility for future ultra-high-contrast experiments on the Advanced Radiographic Capability at the National Ignition Facility.