Tayyab I. Suratwala

Nd-doped phosphate glasses for high-energy/high-peak-power lasers

Abstract The composition and properties of neodymium-doped (Nd-doped) phosphate glasses used for simultaneous high-energy (10 3 –10 6 J) and high-peak-power (10 12 –10 15 W) laser applications such as fusion energy research, are reviewed. The most common base glasses are meta-phosphates (O/P ∼3) with the approximate composition: 60P 2 O 5 –10Al 2 O 3 –30M 2 O/MO; K/Ba or K/Mg are typical modifiers. The spectroscopy of Nd 3+ in these glasses is well understood and laser properties can be accurately determined from measured spectroscopic properties. The major mechanisms for Nd 3+ non-radiative relaxation are reviewed and empirical expressions are presented that predict these effects in phosphate glasses. Optical and thermal–mechanical properties have been measured on a number of laser glasses and can be correlated with composition. Sub-critical crack growth rates in stress regions I, II and III have been reported for the first time in phosphate laser glasses. The mechanism for Pt inclusion formation and dissolution has been studied leading to damage resistant (Pt-inclusion-free) laser glasses.

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.

Surface chemistry and trimethylsilyl functionalization of Stöber silica sols

Abstract Various silica sols, with different surface chemistries, were reacted in solvent dispersions with hexamethyldisilazane (HMDS) or ethoxytrimethylsilane (ETMS) to produce hydrophobic, trimethylsilyl (TMS) functionalized sols. 1H and 29Si nuclear magnetic resonance were used to quantify the surface species and the TMS surface coverage. The amount of TMS surface coverage, which ranged from 5% to 33%, was a strong function of the starting silica-surface chemistry and the HMDS reaction time. Sols with a greater hydrogen-bonded silanol surface (as opposed to an ethoxy surface or isolated silanol surface) resulted in greater TMS coverage. HMDS reacts with both the solvent (ethanol) and the silica surface. Reaction rate measurements suggested that the silica surface reacts with HMDS at short times (minutes) and then with ETMS, which is a product of the HMDS/ethanol reaction, at long times (days). High TMS coverage is required for sol stability in non-polar solvents; the colloid size was found to increase in decane for sols with poor TMS coverage. In addition, coatings made from TMS sols showed an 80× slower remaining ethoxy-surface hydrolysis rate upon exposure to humidity than untreated sols. These TMS sol films will be utilized as anti-reflection coatings on moisture sensitive optics (e.g., potassium dihydrogen phosphate (KDP) crystals) used in high-peak-power laser systems.

The distribution of subsurface damage in fused silica

Managing subsurface damage during the shaping process and removing subsurface damage during the polishing process is essential in the production of low damage density optical components, such as those required for use on high peak power lasers. Removal of subsurface damage, during the polishing process, requires polishing to a depth which is greater than the depth of the residual cracks present following the shaping process. To successfully manage, and ultimately remove subsurface damage, understanding the distribution and character of fractures in the subsurface region introduced during fabrication process is important. We have characterized the depth and morphology of subsurface fractures present following fixed abrasive and loose abrasive grinding processes. At shallow depths lateral cracks and an overlapping series of trailing indentation fractures were found to be present. At greater depths, subsurface damage consists of a series of trailing indentation fractures. The area density of trailing fractures changes as a function of depth, however the length and shape of individual cracks remain nearly constant for a given grinding process. We have developed and applied a model to interpret the depth and crack length distributions of subsurface surface damage in terms of key variables including abrasive size and load.

Effects of OH content, water vapor pressure, and temperature on sub-critical crack growth in phosphate glass

Abstract The effects of temperature and water vapor pressure on the rate of sub-critical crack growth in meta-phosphate laser glasses containing different OH concentrations (128 and 773 ppmw) are reported. The crack velocity was measured using the double-cleavage-drilled-compression method. When plotted as a function of stress intensity, the samples have the classic region I, II and III crack growth properties similar to that reported for silicate glasses. The glass containing the larger OH content has a 10-fold greater crack velocity in region I; crack velocities is region II are the nearly the same for both glasses. The crack velocities are analyzed using a chemical kinetic and mass-transport limited reaction rate model. At temperatures >150°C and water vapor pressures >10 mmHg, crack tip blunting is observed and the glass containing the larger OH content is more prone to blunting.

Continuous melting of phosphate laser glasses

Continuous melting of phosphate laser glass is being used for the first time to prepare meter-scale amplifier optics for megajoule lasers; a description of the melting process is given. Two key factors in the successful melting of laser glasses are the elimination of damage-causing Pt-inclusions and dehydroxylation of the glass to concentrations less than 100 ppmw OH. Oxidizing conditions using 100% O2 or O2 +C l2 mixtures (at one atmosphere) can be used to dissolve Pt inclusions and the eAects of diAerent gases on the dissolution of Pt-inclusions show the trend O2a Cl2 > O2 N2. The removal of hydroxyl groups is achieved by reactive (O2 +C l2) or non-reactive (O2) gas bubbling; model calculations are used to simulate this process. ” 2000 Published by Elsevier Science B.V. All rights reserved.

High fluence laser damage precursors and their mitigation in fused silica

The use of any optical material is limited at high fluences by laser-induced damage to optical surfaces. In many optical materials, the damage results from a series of sources which initiate at a large range of fluences and intensities. Much progress has been made recently eliminating silica surface damage due to fracture-related precursors at relatively low fluences (i.e., less than 10 J/cm(2), when damaged by 355 nm, 5 ns pulses). At higher fluence, most materials are limited by other classes of damage precursors which exhibit a strong threshold behavior and high areal density (>10(5) cm(-2)); we refer to these collectively as high fluence precursors. Here, we show that a variety of nominally transparent materials in trace quantities can act as surface damage precursors. We show that by minimizing the presence of precipitates during chemical processing, we can reduce damage density in silica at high fluence by more than 100 times while shifting the fluence onset of observable damage by about 7 J/cm(2). A better understanding of the complex chemistry and physics of cleaning, rinsing, and drying will likely lead to even further improvements in the damage performance of silica and potentially other optical materials.

Laser Damage Precursors in Fused Silica

There is a longstanding, and largely unexplained, correlation between the laser damage susceptibility of optical components and both the surface quality of the optics, and the presence of near surface fractures in an optic. In the present work, a combination of acid leaching, acid etching, and confocal time resolved photoluminescence (CTP) microscopy has been used to study laser damage initiation at indentation sites. The combination of localized polishing and variations in indentation loads allows one to isolate and characterize the laser damage susceptibility of densified, plastically flowed and fractured fused silica. The present results suggest that: 1) laser damage initiation and growth are strongly correlated with fracture surfaces, while densified and plastically flowed material is relatively benign, and 2) fracture events result in the formation of an electronically defect rich surface layer which promotes energy transfer from the optical beam to the glass matrix.

NIF Pockels cell and frequency conversion crystals

The National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory is a stadium-sized facility containing a 192-beam, 1.8-Megajoule, 500-Terawatt, ultraviolet laser system together with a 10-meter diameter target chamber with room for nearly 100 experimental diagnostics. Each beam line requires three different large-aperture optics made from single crystal potassium dihydrogen phosphate (KDP). KDP is used in the plasma electrode pockels cell (PEPC) and frequency doubling crystals, while deuterated KDP (DKDP) crystals are used for frequency tripling. Methods for reproducible growth of single crystals of KDP that meet all material requirements have been developed that enable us to meet the optics demands of the NIF. Once material properties are met, fabrication of high aspect ratio single crystal optics (42 × 42 × 1 cm) to meet laser performance specifications is the next challenge. More than 20% of the required final crystal optics have been fabricated and meet the stringent requirements of the NIF system. This manuscript summarizes the challenges and successes in the production of these large single-crystal optics.

Enhanced Delamination of Ultrathin Free-Standing Polymer Films via Self-Limiting Surface Modification

Free-standing polymer thin films are typically fabricated using a sacrificial underlayer (between the film and its deposition substrate) or overlayer (on top of the film to assist peeling) in order to facilitate removal of the thin film from its deposition substrate. We show the direct delamination of extraordinarily thin (as thin as 8 nm) films of poly(vinyl formal) (PVF), polystyrene, and poly(methyl methacrylate). Large (up to 13 cm diameter) films of PVF could be captured on wire supports to produce free-standing films. By modifying the substrate to lower the interfacial energy resisting film-substrate separation, the conditions for spontaneous delamination are satisfied even for very thin films. The substrate modification is based on the electrostatic adsorption of a cationic polyelectrolyte. Eliminating the use of sacrificial materials and instead relying on naturally self-limiting adsorption makes this method suitable for large areas. We have observed delamination of films with aspect ratios (ratio of lateral dimension between supports to thickness) of 10(7) and have captured dry, free-standing films with aspect ratios >10(6). Films with an aspect ratio of 10(5) can bear loads up to 10(6) times the mass of the film itself. The presence of the adsorbed layer can be observed using X-ray photoelectron spectroscopy, and this layer is persistent through multiple uses. In the system studied, elimination of sacrificial materials leads to an enhancement in the failure strength of the free-standing thin film. The robustness, persistence, and the self-optimizing nature distinguish this method from various fabrication methods utilizing sacrificial materials and make it a potentially scalable process for the fabrication of ultrathin free-standing or transferrable films for filtration, MEMS, or tissue engineering applications.