[en] Laser damage of large fused silica optics initiates at imperfections. Possible initiation mechanisms are considered. We demonstrate that a model based on nanoparticle explosions is consistent with the observed initiation craters. Possible mechanisms for growth upon subsequent laser irradiation, including material modification and laser intensification, are discussed. Large aperture experiments indicate an exponential increase in damage size with number of laser shots. Physical processes associated with this growth and a qualitative explanation of self-accelerated growth is presented. Rapid growth necessitates damage growth mitigation techniques. Several possible mitigation techniques are mentioned, with special emphasis on CO₂ processing. Analysis of material evaporation, crack healing, and thermally induced stress are presented

[en] Cracks can affect laser damage susceptibility in three ways. These are field intensification due to interference, enhanced absorption due to trapped material in the cracks, and increased mechanical weakness. Enhanced absorption is the most important effect

[en] Laser damage of large optics initiates at material imperfections. Absorbers of very small, nanoscale size are possible initiators. We will analyze experimental implications of assuming that the damage is initiated by a size distribution of nanoabsorbers. We will demonstrate that the model predicts damage fluence pulselength scaling consistent with experiment. The size distribution of nanoabsorbers is related to the resulting damage site density and to the shape of the damage probability curve (S-curve). Conditioning of KDP crystals can be explained within the same model. The relative efficiency of various conditioning strategies is discussed

[en] We have resolved the expansion of intensely irradiated atomic clusters on a femtosecond time scale. These data show evidence for resonant heating, similar to resonance absorption, in spherical cluster plasmas

[en] Laser modulated scattering (LMS) is introduced as a non-destructive evaluation tool for defect inspection and characterization of optical surfaces and thin film coatings. This technique is a scatter sensitive version of the well-known photothermal microscopy (PTM) technique. It allows simultaneous measurement of the DC and AC scattering signals of a probe laser beam from an optical surface. By comparison between the DC and AC scattering signals, one can differentiate absorptive defects from non-absorptive ones. This paper describes the principle of the LMS technique and the experimental setup, and illustrates examples on using LMS as a tool for nondestructive evaluation of high quality optics

[en] We simulate experiments performed with the Falcon laser at Lawrence Livermore National Laboratory to generate strong, cylindrically diverging blast waves of relevance to astrophysics. In particular, we are interested in producing and modeling radiative shocks. We compare numerical simulations with the data and with an analytic approximation to blast-wave propagation with a radiative-loss term included. Our goal is to develop a laboratory setting for studying radiative shocks of relevance to supernova remnants, gamma-ray burst afterglows, and other high-energy astrophysics phenomena. We will show that a good degree of agreement exists between the experimental data and the numerical simulations, demonstrating that it is indeed possible to generate radiative shocks in the laboratory using tabletop femtosecond lasers. In addition, we show how we can determine the energy-loss rate from the blast-wave evolution. This analytic method is independent of the exact mechanism of radiative cooling and is scalable to both the laboratory and astrophysical radiative blast waves. (c) 2000 The American Astronomical Society

[en] For the aggressive fluence requirements of the NIF laser, some level of laser-induced damage to the large (40 x 40 cm) 351 nm final optics is inevitable. Planning and utilization of NIF therefore requires reliable prediction of the functional degradation of the final optics. Laser damage tests are typically carried out with Gaussian beams on relatively small test areas. The tests yield a damage probability vs energy fluence relation. These damage probabilities are shown to depend on both the beam fluence distribution and the size of area tested. Thus, some analysis is necessary in order to use these test results to determine expected damage levels for large aperture optics. The authors present a statistical approach which interprets the damage probability in terms of an underlying intrinsic surface density of damaging defects. This allows extrapolation of test results to different sized areas and different beam shapes (NIF has a flattop beam). The defect density is found to vary as a power of the fluence (Weibull distribution)

[en] The goal of this project was to develop, through experiments and modeling, a better understanding of the physics issues and machining techniques related to short-pulse laser materials processing. Although we have successfully demonstrated many types of cuts in a wide range of materials, our general short-pulse machining scientific knowledge and our ability to model the complex physical processes involved are limited. During this past year we made good progress in addressing some of these issues, but there remain many unanswered questions. Section 2 begins with a theoretical look at short-pulse laser ablation of material using a 1-D radiation-hydrodynamic code which includes a self-consistent description of laser absorption and reflection from an expanding plasma. In Section 3 we present measurements of scaling relationships, hole drilling progression, electric field and polarization effects, and a detailed look at the interesting structures formed during hole drilling of metals under various conditions. Section 4 describes the consequences of the presence of a prepulse before the main drilling pulse. In Section 5 we take a brief look at the plasma plume: how it can be useful, and how we can avoid it. Finally, Section 6 contains a couple of examples of machining non-metals. The laser system used for practically all the experimental results presented here was a short-pulse laser based on Ti:sapphire, which produced 150-fs pulses (minimum) centered at 825 nm, of energy up to 5 mJ at 1 kHz, or 5 W average power

[en] The development of microsatellites requires the development of engines to modify their orbit. It is natural to use solar energy to drive such engines. For an unlimited energy source the optimal thruster must use a minimal amount of expendable material to minimize launch costs. This requires the ejected material to have the maximal velocity and, hence, the ejected atoms must be as light as possible and be ejected by as high an energy density source as possible. Such a propulsion can be induced by pulses from an ultra-short laser. The ultra-short laser provides the high-energy concentration and high-ejected velocity. We suggest a microthruster system comprised of an inflatable solar concentrator, a solar panel, and a diode-pumped fiber laser. We will describe the system design and give weight estimates.

[en] We present theoretical and experimental evidence that nonionizing prepulses with intensities as low as 10⁸--10⁹ W/cm² can substantially alter high intensity laser-solid interactions. We show that prepulse-heating and vaporization of the target can lead to a preformed plasma once the vapor is ionized by the rising edge of the high-intensity pulse. Our results indicate that peak prepulse intensity is not the only important parameter to consider in determining preformed plasma thresholds, and that a more comprehensive analysis of the prepulse duration and the target material is required