[en] Identification and spatial registration of laser-induced damage relative to incident fluence profiles is often required to characterize the damage properties of laser optics near damage threshold. Of particular interest in inertial confinement laser systems are large aperture beam damage tests (>1cm²) where the number of initiated damage sites for φ>14J/cm² can approach 10⁵-10⁶, requiring automatic microscopy counting to locate and register individual damage sites. However, as was shown for the case of bacteria counting in biology decades ago, random overlapping or 'clumping' prevents accurate counting of Poisson-distributed objects at high densities, and must be accounted for if the underlying statistics are to be understood. In this work we analyze the effect of random clumping on damage initiation density estimates at fluences above damage threshold. The parameter ψ aρ = ρ/ρ₀, where a = 1/ρ₀ is the mean damage site area and ρ is the mean number density, is used to characterize the onset of clumping, and approximations based on a simple model are used to derive an expression for clumped damage density vs. fluence and damage site size. The influence of the uncorrected ρ vs. φ curve on damage initiation probability predictions is also discussed

[en] In the present work, in order to explain the electrochemical Li insertion and release phenomena in PPP-700 (polyparaphenylene, HTT=700 °C, in Ar) in-situ Raman scattering experiments have been carried out, using an experimental Li cell with propylene carbonate (PC) as an electrolyte.

[en] Using confocal Raman and scanning probe microscopies, we show that the electrical properties of lateral epitaxial overgrown GaN films vary at the submicron scale. Wing regions, which are located directly above the SiO_x stripes, contain carrier densities ∼10²⁰ cm^-3, but possess a Fermi level deep in the band gap. This cannot be explained by having a high density of free electrons in the conduction band, but is consistent with high levels of compensation and impurity band transport. In the coalescence region, stripes of different electrical properties are evident, indicating the incorporation of impurities and defects being dictated by the growth dynamics. Copyright 2001 American Institute of Physics

[en] The microstructural thermal response of fused silica subjected to laser-induced breakdown was investigated. Rapid thermal annealing of laser modified material at the surface was achieved using a CO₂ laser and the relaxation response of photoluminescence, infrared (IR) reflectance, electron and white light microscope images were recorded. Subsequent nanosecond-pulsed laser damage threshold measurements revealed thermally driven kinetics which were dominated by absorbing defect annealing at heat-treatment temperatures (T_HT) below ∼1200 K and material toughening at higher T_HT. A decrease in the peak photoluminescence lifetime with T_HT revealed two types of defects which were correlated with non-bridging IR vibrational modes. Near the glass transition temperature, a weakening of the laser modified material was observed and explained in terms of a residual compressive stress relaxation. A nonlinear absorption model was used to predict optical breakdown threshold and compared with critical fracture predictions based on crack tip annealing. Combined with a qualitative stress relaxation analysis, our model agrees well with the experimental data and yields insight to the rate-limiting contributions driving the onset of laser-induced breakdown in defective silica. (paper)

[en] Chemical vapour deposition (CVD) is used for the production of fused silica optics in high-power laser applications. However, relatively little is known about the ultraviolet laser damage threshold of CVD films and how they relate to intrinsic defects produced during deposition. We present here a study relating structural and electronic defects in CVD films to 355 nm pulsed-laser damage threshold as a function of post-deposition annealing temperature (T_HT). Plasma-enhanced CVD based on SiH₄/N₂O under oxygen-rich conditions was used to deposit 1.5, 3.1 and 6.4 µm thick films on etched SiO₂ substrates. Rapid annealing was performed using a scanned CO₂ laser beam up to T_HT ∼ 2100 K. The films were then characterized using x-ray photoemission spectroscopy, Fourier transform infrared spectroscopy (FTIR) and photoluminescence spectroscopy. A gradual transition in the damage threshold of annealed films was observed for T_HT values up to 1600 K, correlating with a decrease in non-bridging silanol and oxygen deficient centres. An additional sharp transition in damage threshold also occurs at ∼1850 K indicating substrate annealing. Based on our results, a mechanism for damage-related defect annealing is proposed, and the potential of using high-T_HT CVD SiO₂ to mitigate optical damage is also discussed. (paper)