Laser-Matter Coupling Mechanisms Governing Nanoscale Damage on Particulate-Contaminated Optical Surfaces

The recent fusion ignition breakthrough at Lawrence Livermore National Laboratory’s National Ignition Facility has generated a renewed interest in improving high damage threshold optical materials.While decades of improvements in optical material processing for high power laser systems has led to unprecedented levels of laser damage tolerance, surface contamination on surfaces generated during laser operation, optic assembly or handling can still lead to damage initiation and local failure of the optic. Laser micromachining of optics for damage repair and laser-induced damage cross-contamination can also introduce nanoparticulate debris, compounding the issue further.In addition to local damage initiated at the site of the debris and leading to failure, <i>non-local</i> mechanisms associated with contamination have been recognized wherein nano- to microscale particles on the entrance surface of optics can lead to Fresnel diffraction of incident light and damage on the exit surface.The dynamics of the laser-particulate interaction involve high gradients of pressure, temperature and corresponding changes to thermodynamic material properties, plasma formation and aerodynamic effects that drive nanoscale morphological changes to the optical surface. In addition, the formed plasma can interact with the laser pulse while the melted material layer on the particle is ejected and thus be the source of secondary nanoscale contamination/surface pitting.<br/>In this talk, I will present a study of the interaction of microscale, metallic and glass particles bound to optical surfaces with nanosecond and picosecond laser pulses at 1064- and 355-nm that results in nanoparticle ejection and nanoscale modification of the optical surfaces. Our in situ experimental platform allows direct measurements of the particle velocity, plasma formation, and the kinetics of the ejected nanoscale material. Large aperture damage tests were also performed to assess damage probabilities and probe the stochastic nature of particle-induced damage events. We use FDTD, Fourier beam propagation and ray tracing to understand the effect of particle shape and particle-induced nano-pitting on beam propagation. By varying the combination of particle and substrate materials, we are able to gain important insights to the governing mechanisms of laser-particle interactions which could lead to improvements in inertial confinement fusion laser system designs. This work was performed under contract DE-AC52-07NA27344.