Paul R. Ehrmann

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.

Optical loss and Nd3+ non-radiative relaxation by Cu, Fe and several rare earth impurities in phosphate laser glasses

Abstract Extinction coefficients (at 1053 nm) and Nd 3+ fluorescence quenching rates are reported for Cu, Fe, Dy, Pr, Sm and Ce at doping concentrations up to 1000 ppmw in two meta-phosphate laser glasses melted under oxidizing conditions (1 atmosphere O 2 ). The extinction coefficient and quenching rate for Cu are 2.7(±0.1)×10 −3 cm −1 /ppmw and 10.4±0.2 Hz/ppmw, respectively. The extinction coefficient and quenching rate for Fe are concentration dependent below 300 ppmw due to an observed change in Fe 2+ /Fe 3+ distribution; an empirically derived expression is used to describe this effect. The extinction coefficient and quenching rates for Dy, Pr and Sm, are nearly the same: 1.6, 1.2 and 1.3(±0.05)×10 −5 cm −1 /ppmw and 0.89, 0.72 and 0.63±0.04 Hz/ppmw, respectively, while those for Ce are less: 0.84(±0.03)×10 −5 cm −1 /ppmw and 0.061±0.03 Hz/ppmw. The quenching results are explained using the Forster–Dexter theory for dipolar energy transfer.

Phosphate laser glass for NIF: production status, slab selection, and recent technical advances

The National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory is a stadium-sized high-energy (1.8 megajoule) / high-peak power (500 terawatt) laser system, which will utilize over 3000 meter-size Nd-doped metaphosphate glasses as its gain media. The current production status, the selection criteria of individual slabs for specific beam line locations, and some recent technical advances are reviewed. The glass blanks are manufactured by a novel continuous glass melting process, and the finished slabs are then prepared by epoxy bonding a Cu-doped phosphate glass edge cladding and by advanced finishing techniques. To date, nearly 3400 slab equivalents have been melted, 2600 have been rough-cut to blanks, 1200 have been finished, and 144 have been installed in NIF. A set of selection rules, which are designed to optimize laser performance (e.g., maintain gain balance between beam lines and minimize beam walkoff) and to maximize glass lifetime with respect to Pt damage site growth, have been established for assigning individual slabs to specific beam line locations. Recent technical advances for amplifier slab production, which include: 1) minimizing surface pitting (hazing) after final finishing; 2) minimizing humidity-induced surface degradation (weathering) upon storage and use; and 3) preventing mounting-induced surface fractures upon installation, have contributed in improving the laser glass quality.

Change of self-focusing behavior of phosphate glass resulting from exposure to ultraviolet nanosecond laser pulses

The self-focusing characteristic of 355 nm, 3.3 ns pulses propagating through phosphate glass samples is found to significantly change during repeated exposure. The results indicate this change is related to the formation of color centers in the material as well as the generation of a transient defect population during exposure to the laser pulses. A model is used to fit the experimental data and obtain an estimated range of values for the modified linear and nonlinear indices of refraction.

Dynamics of defects in Ce^3+ doped silica affecting its performance as protective filter in ultraviolet high-power lasers

We investigate defects forming in Ce³⁺-doped fused silica samples following exposure to nanosecond ultraviolet laser pulses and their relaxation as a function of time and exposure to low intensity light at different wavelengths. A subset of these defects are responsible for inducing absorption in the visible and near infrared spectral range, which is of critical importance for the use of this material as ultraviolet light absorbing filter in high power laser systems. The dependence of the induced absorption as a function of laser fluence and methods to most efficiently mitigate this effect are presented. Experiments simulating the operation of the material as a UV protection filter for high power laser systems were performed in order to determine limitations and practical operational conditions.

Influence of BK7 substrate solarization on the performance on hafnia and silica multilayer mirrors

Transport mirrors within the National Ignition Facility, a 192-beam 4-MJ fusion laser at 1053 nm, will be epxosed to backscattered light from plasmas created from fusion targets and backlighters. This backscattered light covers the UV and visible spectrum from 351 - 600 nm. The transport mirror BK7 substrates will be intentionally solarized to absorb >95% of the backscattered light to prevent damage to the metallic mechanical support hardware. Solarization has minimal impact on the 351- 1053-nm laser-induced damage threshold or the reflected wavefront of the multilayer hafnia silica coating. Radiation sources of various energies were examined for BK7 darkening efficiency within the UV and visible region with 1.1 MeV gamma rays from a Cobalt 60 source ultimately being selected. Finally, bleaching rates were measured at elevated temperatures to generate a model for predicting the lifetime at ambient conditions (20°C), before solarized BK7 substrates exceed 5% transmission in the UV and visible region. Over a 30-mm thickness, BK7 glass will bleach in 10 years to 5% transmission at 600 nm, the most transmissive wavelengths over the 351 - 600 nm regions.

Estimation of excited-state absorption and photobleaching in Fe 2+ -doped lithium sodium silicate glass under exposure to high-power nanosecond laser pulses

Fe-doped lithium sodium silicate glasses codoped with Sn and C to promote the Fe²⁺ redox state are investigated under simultaneous excitation at the first and third harmonics of a nanosecond Nd:YAG laser. The aim is to evaluate critical parameters associated with the potential use of this material as an optical filter that transmits the third harmonic but blocks the fundamental frequency. Estimations of the excited-state absorption coefficient and photobleaching (reduction of absorption at the fundamental) are provided. The results provide insight on the design and expected operational parameters of this type of Fe-doped materials.

Ultralow stress, thermally stable cross-linked polymer films of polydivinylbenzene (PDVB)

Although closely related to polystyrene, poly(divinylbenzene) (PDVB) has found limited utility due to the difficulties associated with its synthesis. As a highly cross-linked polymer, PDVB is infusible and insoluble and thus nearly impossible to shape into films by either melt or solvent-based processes. Here, we report the initiated chemical vapor deposition (iCVD) of nearly stress-free, highly transparent, free-standing films of PDVB up to 25 μm thick. Films initially grow under tensile intrinsic stress but become more compressive with thickness and eventually converge to zero-stress values once they reach ≥10 μm in thickness. Upon initial heating, the evaporative loss of unreacted monomer left in the polymer matrix induces between 35 and 45 MPa of tensile stress in the films. Afterward, subsequent heating cycles induce reversible stress and film expansion behaviors. We estimate the degree of cross-linking to be 44%, resulting in high thermal stability (up to 300 °C) and mechanical stiffness (Youngs modulus of 5.2 GPa). The low stress combined with high cross-linking makes iCVD PDVB an excellent candidate for protective coatings in harsh environments.

Acoustic activation of water-in-oil microemulsions for controlled salt dissolution

HYPOTHESIS The dynamic nature of the oil-water interface allows for sequestration of material within the dispersed domains of a microemulsion. Microstructural changes should therefore change the dissolution rate of a solid surface in a microemulsion. We hypothesize that microstructural changes due to formulation and cavitation in an acoustic field will enable control over solid dissolution rates. EXPERIMENTS Water-in-oil microemulsions were formulated using cyclohexane, water, Triton X-100, and hexanol. The microstructure and solvation properties of Winsor Type IV formulations were characterized. Dissolution rates of KH2PO4 (KDP), were measured. A kinetic analysis isolated the effect of the microstructure, and rate enhancements due to cavitation effects on the microstructure were characterized by measuring dissolution rates in an ultrasonic field. FINDINGS Dispersed aqueous domains of 2-6 nm radius dissolve a solid block of KDP at 0-10 nm/min. Dissolution rate is governed not by the domain-surface collision frequency but rather by a dissolution probability per domain-surface encounter. Higher probabilities are correlated with larger domains. Rapid and reversible dissolution rate increases of up to 270× were observed under ultrasonic conditions, with <20% of the increase due to bulk heating effects. The rest is attributed to cavitation-induced changes to the domain microstructure, providing a simple method for remotely activating and de-activating dissolution.