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This study investigates the physical processes involved in picosecond pulse (20-28 ps FWHM) laser ablation of Al 6061, 316L stainless steel, and undoped crystalline Si (〈100〉) over a range of laser wavelength (355 nm and 1064 nm) and fluence (0.1-40 J/cm2). Experimental measurements of material ablation rate show enhanced removal at the 355 nm wavelength, primarily due to laser-plasma interaction (LPI) within the ablative plume that approaches an order of magnitude increase over the measured removal at 1064 nm. A transition in the ablation rate at 355 nm is identified around ∼10 J/cm2 above which the removal efficiency increases by a factor of two to three. Multi-physics radiation hydrodynamic simulations, considering LPI effects and utilizing a novel mixed-phase equation of state model, show that the transition in ablation efficiency is due to the onset of melt ejection through cavitation, where laser-driven shock heating sets the depth of melt penetration and the ensuing release wave from the ablation surface drives cavitation through the imposition of tensile strain within the melt. High-speed pump-probe imaging of the ejecta and ejecta collection studies, as well as scanning electron microscopy of the ablation craters, support the proposed cavitation mechanism in the higher fluence range. The ablation process is critically influenced by LPI effects and the thermophysical properties of the material. [ABSTRACT FROM AUTHOR]

Laser-induced pinpoint bulk damage of deuterated potassium dihydrogen phosphate at 351 nm is shown to depend on the propagation direction relative to the crystallographic axes and on growth temperature in addition to the previously reported dependence on continuous filtration. Pulse-length scaling is also consistent with earlier reports. The leading hypothesis for the cause of pinpoint damage is absorbing nanoparticle impurities, and our results are consistent with but not conclusive for that model. Advances in technology have led to greatly improved damage resistance. OCIS codes: 140.3330, 140.346.

The second-harmonic signal in collagen, even in highly organized samples such as rat tail tendon fascicles, varies significantly with position. Previous studies suggest that this variability may be due to the parallel and antiparallel orientation of neighboring collagen fibrils. We applied high-resolution second-harmonic generation microscopy to confirm this hypothesis. Studies in which the focal spot diameter was varied from ~1 to ~6 [micro]m strongly suggest that regions in which collagen fibrils have the same orientation in rat tail tendon are likely to be less than ~1 [micro]m in diameter. These measurements required accurate determination of the focal spot size achieved by use of different microscope objectives; we developed a technique that uses second-harmonic generation in a quartz reference to measure the focal spot diameter directly. We also used the quartz reference to determine a lower limit ([d.sub.XXX] > 0.4 pm/V) for the magnitude of the second-order nonlinear susceptibility in collagen. OCIS codes: 190.1900, 190.4710, 180.5810, 170.5810, 170.0180, 170.7160.

Pressure waves generated by the surface ablation of water with ultrashort laser pulses in the 140 fs-10 ps duration range were studied with a Mach-Zehnder interferometry. Formation and propagation of spherical and, in ablation with longer pulses, combination of spherical and cylindrical pressure waves were observed. Measurements of the amplitude and dynamics of the pressure waves show that the conversion efficiency of laser energy to mechanical energy of the pressure waves is less than 1%, thus justifying the claim that ultrashort laser pulse ablation induces low-collateral damage and has a large potential in biomedical applications. [ABSTRACT FROM AUTHOR]

Pressure waves generated by the surface ablation of water with ultrashort laser pulses in the 140 fs-10 ps duration range are studied by using Mach-Zehnder interferometry. Laser ablation can affect the bulk of the material thermally and mechanically.

A ground-based laser system for space debris cleaning requires pulse power well above the critical power for self-focusing in the atmosphere. Self-focusing results in beam quality degradation and is detrimental for the system operation. We demonstrate that, for the relevant laser parameters, when the thickness of the atmosphere is much less than the focusing length (that is, of the orbit scale), the beam transit through the atmosphere produces the phase distortion only. The model thus developed is in very good agreement with numerical modeling. This implies that, by using phase mask or adaptive optics, it may be possible to eliminate almost completely the impact of self-focusing effects in the atmosphere on the laser beam propagation. [ABSTRACT FROM AUTHOR]

Light intensity modulations caused by opaque obstacles (e.g., dust) on silica lenses in high-power lasers often enhance the potential for laser-induced damage. To study this effect, particles (10-250 [micro] m) with various shapes were sputter deposited on the input surface and irradiated with a 3-ns laser beam at 355 nm. Although a clean silica surface damages at fluences above 15 J/[cm.sup.2], a surface contaminated with particles can damage below 11.5 J/[cm.sup.2]. A pattern that conforms to the shape of the input surface particle is printed on the output surface. Repetitive illumination resulted in catastrophic drilling of the optic. The damage pattern correlated with an interference image of the particle before irradiation. The image shows that the incident beam undergoes phase (and amplitude) modulations after it passes around the particle. We modeled the experiments by calculating the light intensity distribution behind an obscuration by use of Fresnel diffraction theory. The comparison between calculated light intensity distribution and the output surface damage pattern showed good agreement. The model was then used to predict the increased damage vulnerability that results from intensity modulations as a function of particle size, shape, and lens thickness. The predictions provide the basis for optics cleanliness specifications on the National Ignition Facility to reduce the likelihood of optical damage. [C] 2000 Optical Society of America OCIS code: 140.3380