Power handling capabilities of broad-area high-power diode lasers are limited by the heat extraction capabilities of the device packaging. Traditional methods of heat extraction rely on conductive heat extraction from the diode chip and an emitting facet in contact with either quiescent or naturally convecting air. This leads to a thermal profile in the lasing direction of the cavity and a hot emitting facet. A hot facet accelerates material degradation, reducing the mean time to failure and limiting the safe operating power. Direct contact between the facet and a liquid coolant could enable higher levels of heat extraction compared to traditional cooling pathways. An innovative approach to cooling high-power, broad-area diode lasers via total immersion in liquid coolant is proposed and tested. In this study, we demonstrate that single emitters can operate with the emitting facet in direct contact with static coolant, with no negative change to device power or efficiency. Thermal analysis and models show that immersed diodes operate with improved thermal pathways, yielding lower total thermal resistance with the greatest improvement to thermal resistance at the facet-fluid interface.

Absorbing defects such as fractures and contaminants are a leading cause of surface damage in nanosecond pulsed lasers. Etching such defects has proven to be a powerful technique for increasing the laser damage threshold of fused silica, but to date no etching process has been reported for potassium dihydrogen phosphate (KH2PO4 or KDP) or its deuterated analog (DKDP). We show that physical dissolution in water is a viable strategy for etching DKDP surfaces but surface-redeposited byproducts can serve as laser damage precursors. We use a water-in-oil microemulsion to etch engineered surface fractures in DKDP. Etching widens surface fractures laterally and decreases their optical activity, as measured by photoluminescence. The removal of 1 mu m of the surface of a DKDP crystal increases the laser damage threshold (lambda = 355 nm, 7 ns) of the engineered surface fractures by 2-4 J/cm(2) (15-30%).

Polydivinylbenzene (PDVB) is a thermally stable, optically transparent, crosslinked polymer that until recently has been difficult to synthesize as a thin film. With the recent demonstration of initiated chemical vapor deposition (iCVD) of thin PDVB films, a renewed interest in the material properties of PDVB has developed. In particular, attention is now focused on its oxidation pathways and long-term stability under the desired application use conditions. Here, we report on the thermal and environmental stability of PDVB films and show that unreacted pendant vinyl groups drive polymer oxidation upon exposure to either air or light. We demonstrate that such vinyls can be effectively passivated by a simple ex-situ thermal annealing at ca. 300 degrees C in inert atmosphere that induces an 87% reduction of the PDVB oxidation rate in air and slows light (lambda=405nm) induced oxidation by 56%. While the thermal annealing is less effective at preventing oxidation under higher energy (lambda=365nm) UV light, we demonstrate that this aging pathway is based on the presence of reactive oxygen species rather than traditional photo-oxidation. Vinyl removal through ex-situ thermal annealing improves the chemical stability of iCVD PDVB to continuous air (over 500 days) or light (70 hours) exposure and offers a simple option to improve its environmental aging resistance which is important for long-term protective applications.

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.

We report two examples of initiated chemical vapor deposition (iCVD) polymers being developed for use in laser targets for high peak-power laser systems. First, we show that iCVD poly(divinylbenzene) is more photo-oxidatively stable than the plasma polymers currently used in laser targets. Thick layers (10–12μm) of this highly crosslinked polymer can be deposited with near-zero intrinsic film stress. Second, we show that iCVD epoxy polymers can be crosslinked after deposition to form thin adhesive layers for assembling precision laser targets. The bondlines can be made as thin as ~1μm, approximately a factor of 2 thinner than achievable using viscous resin-based adhesives. These bonds can withstand downstream coining and stamping processes.

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 (Young's 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.

In a series of studies related to laser-induced damage of optical materials and deposition of plastics, we discovered a broadly emitting photoluminescence with fast lifetimes that we termed quasicontinuum photoluminescence (QC-PL). Here, we suggest that a high density of optically active defects leads to QC-PL, where interactions between defects affect the temporal and spectral characteristics of both excitation and emission. We develop a model that predicts the temporal characteristics of QC-PL, based on energy transfer interactions between high densities of defects. Our model does not explain all spectral broadening and redshifts found in QC-PL, since we do not model spectral changes in defects due to proximity to other defects. However, we do provide an example of a well-defined system that exhibits the QC-PL characteristics of a distribution in shortened lifetimes and broadened, redshifted energy levels: an organic chromophore (fluorescein) that has been dried rapidly on a fused silica surface. Recently, we showed that regions of fused silica exposed to up to 1 billion high-fluence laser shots at 351 rm nm at subdamage fluences exhibit significant transmission losses at the surface. Here, we find that these laser-exposed regions also exhibit QC-PL. Increases in the density of induced defects on these laser-exposed surfaces, as measured by the local transmission loss, lead to decreases in the observed lifetime and redshifts in the spectrum of the QC-PL, consistent with our explanation for QC-PL. We have found QC-PL in an increasing variety of situations and materials, and we believe it is a phenomenon commonly found on surfaces and nanostructured materials.

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.

Many transition metal complexes exhibit infrared or visible optical absorption arising from d-d transitions that are the key to functionality in technological applications and biological processes. The observed spectral characteristics of the absorption spectra depend on several underlying physical parameters whose relative contributions are still not fully understood. Although conventional arguments based on ligand-field theory can be invoked to rationalize the peak absorption energy, they cannot describe the detailed features of the observed spectral profile such as the spectral width and shape, or unexpected correlations between the oscillator strength and absorption peak position. Here, we combine experimental observations with first-principles simulations to investigate origins of the absorption spectral profile in model systems of aqueous Cu(2+) ions with Cl(-), Br(-), NO2(-) and CH3CO2(-) ligands. The ligand identity and concentration, fine structure in the electronic d-orbitals of Cu(2+), complex geometry, and solvation environment are all found to play key roles in determining the spectral profile. Moreover, similar physiochemical origins of these factors lead to interesting and unexpected correlations in spectral features. The results provide important insights into the underlying mechanisms of the observed spectral features and offer a framework for advancing the ability of theoretical models to predict and interpret the behavior of such systems.

Plasma polymers are often limited by their susceptibility to spontaneous and photo‐oxidation. We show that the unusual photoluminescence (PL) behavior of a plasma polymer of trans‐2‐butene is correlated with its PL strength. These photo‐processes occur under blue light illumination (λ = 405 nm), distinguishing them from traditional ultraviolet degradation of polymers. These photo‐active defects are likely formed during the plasma deposition process, and we show that a polymer synthesized using initiated (i)CVD, a non‐plasma method, has 1000× lower PL signal and enhanced photo‐stability. Non‐plasma methods, such as iCVD, may therefore be a route to overcoming material aging issues that limit the adoption of plasma polymers.

Amorphous hydrogenated carbon (a-C:H), a polymer-like network typically synthesized by plasma chemical vapor deposition, has long been known to exhibit optical absorption of visible light (lambda > 400 nm). Here, we report that this absorption is accompanied by rapid photo-oxidation (within minutes) that behaves in most respects like classic polymer photo-oxidation with the exception that it occurs under visible light illumination rather than ultraviolet illumination. Lower plasma power during deposition produces a-C:H that is less crosslinked, is less absorptive, has longer photoluminescence lifetimes, and has a slower photo-oxidative degradation than a-C:H deposited at higher plasma power. The optical gap of the material deposited here is similar to 2.5 eV, and we correspondingly observe photo-oxidation at photon energies of 2.7 and 3.1 eV. A reduced photo-oxidative response is observed at sub-gap energies, suggesting that defect states or absorption tails enable absorption at lower energies. The photo-oxidation depends on both the total accumulated dose as well as the intensity of the illumination, suggesting either a transport limitation of 02 in the a-C:H or a complex reaction pathway. Under typical laboratory lighting conditions, plasma CVD a-C:H continues to photo-oxidize for more than 20 weeks, demonstrating that any characterization of the long-term behavior of a-C:H under use conditions requires control of not only atmospheric conditions but also ambient lighting environment. (C) 2015 Elsevier Ltd. All rights reserved.

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.

Extinction coefficients (at 1053 nm) and Nd3+ 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 O2). 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 Fe2+/Fe3+ 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 Förster–Dexter theory for dipolar energy transfer.