Dynamics of transient absorption in bulk DKDP crystals following laser energy deposition.

The transient changes in the optical properties of bulk DKDP material arising from its exposure to high temperatures and pressures associated with localized laser energy deposition are investigated. Two methods for initiation of laser-induced breakdown are used, intrinsic, involving relatively large energy deposition brought about by focusing of the laser beam to high intensities, and extrinsic, arising from more localized deposition due to the presence of pre-existing absorbing damage initiating defects. Each method leads to a very different volume of material being affected, which provides for different material thermal relaxation times to help better understand the processes involved.

Fracture-induced subbandgap absorption as a precursor to optical damage on fused silica surfaces.

The optical damage threshold of indentation-induced flaws on fused silica surfaces was explored. Mechanical flaws were characterized by laser damage testing, as well as by optical, secondary electron, and photoluminescence microscopy. Localized polishing, chemical leaching, and the control of indentation morphology were used to isolate the structural features that limit optical damage. A thin defect layer on fracture surfaces, including those smaller than the wavelength of visible light, was found to be the dominant source of laser damage initiation during illumination with 355 nm, 3 ns laser pulses. Little evidence was found that either displaced or densified material or fluence intensification plays a significant role in optical damage at fluences >35 J/cm(2). Elimination of the defect layer was shown to increase the overall damage performance of fused silica optics.

Dynamics of material modifications following laser-breakdown in bulk fused silica.

We report on the material response during the cooling phase in bulk fused silica following localized energy deposition via laser-induced breakdown.We use a time-resolved microscope system to acquire images of the region of energy deposition at delay times covering the entire timeline of events. In addition, this system is configured to perform pump-and-probe damage testing measurements to investigate the evolution of the transient absorption of the modified material. The main features of a damage site are established at approximately 30 ns after the pump pulse, i.e. cracks reach their final size within this time frame. The results reveal that the cracks and melted core exhibit a transient absorption up until about 300 ns and 200 micros delay times, respectively, and suggest that the melted region returns to solid phase at approximately 70 ms delay.

Radiation produced by femtosecond laser-plasma interaction during dielectric breakdown.

Optical breakdown by femtosecond and nanosecond laser pulses in transparent dielectrics produces an ionized region of dense plasma confined within the bulk of the material. This ionized region is responsible for broadband radiation that accompanies the breakdown process. Spectroscopic measurements of the accompanying light have been used to show that, depending on the laser parameters, the spectra may originate from plasma-induced second-harmonic generation, supercontinuum generation, or thermal emission by the plasma. By monitoring the emission from the ionized region, one can ascertain the predominant breakdown mechanism and the morphology of the damage region.

Localized dynamics during laser-induced damage in optical materials.

Laser-induced damage in wide band-gap optical materials is the result of material modifications arising from extreme conditions occurring during this process. The material absorbs energy from the laser pulse and produces an ionized region that gives rise to broadband emission. By performing a time-resolved investigation of this emission, we demonstrate both that it is blackbody in nature and that it provides the first direct measurement of the localized temperature of the material during and following laser damage initiation for various optical materials. For excitation using nanosecond laser pulses, the plasma, when confined in the bulk, is in thermal equilibrium with the lattice. These results allow for a detailed characterization of temperature, pressure, and electron densities occurring during laser-induced damage.

Influence of pulse duration on ultrashort laser pulse ablation of biological tissues.

Ablation characteristics of ultrashort laser pulses were investigated for pulse durations in the range of 130 fs-10 ps. Tissue samples used in the study were dental hard tissue (dentin) and water. We observed differences in ablation crater morphology for craters generated with pulse durations in the 130 fs-1 ps and the 5 ps-10 ps range. For the water experiment, the surface ablation and subsequent propagation of stress waves were monitored using Mach-Zehnder interferometry. For 130 fs-1 ps, energy is deposited on the surface while for longer pulses the beam penetrates into the sample. Both studies indicate that a transition occurs between 1 and 5 ps.

A life cycle model of public policy issues in health care: the importance of strategic issues management.

Public policy affects health and social services organizations. Senior management has a responsibility to prevent inappropriate demands of stakeholders from predominating and to influence the outcome of public policy to the benefit of their organization through the strategic issues management process. This article presents a public policy issue life cycle model, life-cycle stages and suggested strategies, paths issues can take in the life cycle, and factors that affect issue paths. An understanding of these dynamics can aid senior managers in shaping and changing public policy issues and lessening external environment threats to their organization.

Rear-Surface Laser Damage on 355-nm Silica Optics Owing to Fresnel Diffraction on Front-Surface Contamination Particles.

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 mum) 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(2), a surface contaminated with particles can damage below 11.5 J/cm(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.

Screening of substance abuse in public welfare and child protective service clients: a comparative study of rapid assessment instruments vs. the SASSI.

In light of new welfare reforms, it is essential that public welfare workers move individuals from welfare to work in an expedient fashion. Client alcohol and/or drug use/abuse is one barrier that may inhibit this process. The following study (N = 93) administered both the Hudson Index of Alcohol Involvement, the Hudson Index of Drug Involvement (rapid assessment instruments) and the Miller Substance Abuse Subtly Screening Inventory (SASSI) to public welfare and Child Protection system clients. Results indicated that the two rapid assessment measures strongly correlated with the advanced, commonly used SASSI measure. It is suggested that public welfare workers should institute these rapid assessment instruments at intake to screen clients for alcohol and drug use/abuse. When indicated, the client could then be referred to the appropriate agency for further assessment and treatment, to better prepare them for employment as mandated by new welfare reforms. The rapid assessment instruments were found to be shorter than the SASSI, easily administered, and able to detect alcohol and/or drug use/abuse effectively and efficiently in these populations.

High-intensity third-harmonic generation in beta barium borate through second-order and third-order susceptibilities.

The azimuthal dependence of third-order and cascaded second-order nonlinear coupling was used to measure the relative contributions of each to direct third-harmonic generation in beta-barium borate. This permitted the measurement of the values of tensor elements chi(3)(10) , chi(3)(11) , and chi(3)(16) relative to the known chi(2)(ij) . Finally, conversion efficiencies to 3 omega of up to 6% were achieved with a femtosecond chirped-pulse amplification laser with 200 GW/cm(2) in collimated beams.

Designing fully continuous phase screens for tailoring focal-plane irradiance profiles.

An iterative algorithm for constructing fully continuous phase screens for tailoring far-field intensity profiles is presented. The algorithm is robust, stable, and, if run properly, maintains the continuous nature of the phase throughout the iterative process. The iterative procedure is applied to generate continuous phase screens to produce a 12th-power super-Gaussian far-field intensity profile.

High-power narrow-band pulses with wavelengths tunable about 1.053 µm from a synchronously pumped optical parametric oscillator.

We have constructed an optical parametric oscillator to generate 75-ps near-transform-limited pulses with wavelengths tunable about 1.053 µm for use in pump-probe studies of self-focusing. The singly resonant oscillator uses a Brewster-cut LiB(3)O(5) crystal that is oriented for type-II phase matching and synchronously pumped by the amplified and frequency-tripled pulse trains from a mode-locked and Q-switched Nd:YLP laser. An intracavity Pockels cell is used to switch out single 0.5-MW pulses at rates of 1 to 10 Hz. The design, construction, and performance of the oscillator are discussed. Measured performance is compared with design predictions and with detailed numerical simulations.

Accurate description of ultrawide-angle beam propagation in homogeneous media by Lanczos orthogonalization.

We demonstrate accurate modeling of ultrawide-angle beam propagation in homogeneous media by Lanczos orthogonalization for acceptable longitudinal step sizes. Accuracy is determined by comparison with solutions to the Helmholtz equation obtained by a band-limited Fourier series method.

Accurate solution of the Helmholtz equation by Lanczos orthogonalization for media with loss or gain.

The numerical scheme for solving the Helmholtz equation, based on the Lanczos orthogonalization scheme, is generalized so that it can be applied to media with space-dependent absorption or gain profiles.

Spatial coherence of laboratory soft-x-ray lasers.

Employing a time-dependent model with counterpropagating beams and saturable gain, we calculate the degree of coherence for a range of x-ray lasers. From these results we infer a scaling law that can be used to estimate the degree of coherence of a general class of x-ray lasers.

A proposed cost effectiveness method for use in policy formulation in human service organizations.

This article builds on a previous publication and presents additional information on how cost effectiveness analysis may be used by policy and decision-makers in human service organizations (HSO's). It presents a brief history of the use of cost effectiveness analysis to demonstrate its efficacy in assisting policy makers in a variety of fields. It then comparatively describes two of the best known cost effective methods--cost benefit analysis (CBA) and cost effective analysis (CEA), and argues that the latter is the more appropriate method to be used to effect policy decisions in HSO's. The discussion centres around refining the limitations of the CEA method and proposes a simple model for use in this regard. Recommendations and implications are directed toward managers, administrators and decision-makers of HSO's. Finally, the article is attempting to fill a distinct void in the literature in this field and also suggests the use of more meaningful policy formulation and analyses methods in HSO's.