Jeff D. Bude

Extracting the distribution of laser damage precursors on fused silica surfaces for 351 nm, 3 ns laser pulses at high fluences (20-150 J/cm 2 )

Surface laser damage limits the lifetime of optics for systems guiding high fluence pulses, particularly damage in silica optics used for inertial confinement fusion-class lasers (nanosecond-scale high energy pulses at 355 nm/3.5 eV). The density of damage precursors at low fluence has been measured using large beams (1-3 cm); higher fluences cannot be measured easily since the high density of resulting damage initiation sites results in clustering. We developed automated experiments and analysis that allow us to damage test thousands of sites with small beams (10-30 µm), and automatically image the test sites to determine if laser damage occurred. We developed an analysis method that provides a rigorous connection between these small beam damage test results of damage probability versus laser pulse energy and the large beam damage results of damage precursor densities versus fluence. We find that for uncoated and coated fused silica samples, the distribution of precursors nearly flattens at very high fluences, up to 150 J/cm2, providing important constraints on the physical distribution and nature of these precursors.

Model laser damage precursors for high quality optical materials

Surface damage is known to occur at fluences well below the intrinsic limit of the fused silica. A native surface precursor can absorb sub band-gap light and initiate a process which leads to catastrophic damage many micrometers deep with prominent fracture networks. Previously, the absorption front model of damage initiation has been proposed to explain how this nano-scale absorption can lead to macro-scale damage. However, model precursor systems designed to study initiation experimentally have not been able to clearly reproduce these damage events. In our study, we create artificial absorbers on fused silica substrates to investigate precursor properties critical for native surface damage initiation. Thin optically absorbing films of different materials were deposited on silica surfaces and then damage tested and characterized. We demonstrated that strong interfacial adhesion strength between absorbers and silica is crucial for the launch of an absorption front and subsequent damage initiation. Simulations using the absorption-front model are performed and agree qualitatively with experimental results.

Quasi-continuum photoluminescence: Unusual broad spectral and temporal characteristics found in defective surfaces of silica and other materials

We previously reported a novel photoluminescence (PL) with a distribution of fast decay times in fused silica surface flaws that is correlated with damage propensity by high fluence lasers. The source of the PL was not attributable to any known silica point defect. Due to its broad spectral and temporal features, we here give this PL the name quasi-continuum PL (QC-PL) and describe the features of QC-PL in more detail. The primary features of QC-PL include broad excitation and emission spectra, a broad distribution of PL lifetimes from 20 ps to 5 ns, continuous shifts in PL lifetime distributions with respect to emission wavelength, and a propensity to photo-bleach and photo-brighten. We found similar PL characteristics in surface flaws of other optical materials, including CaF2, DKDP, and quartz. Based on the commonality of the features in different optical materials and the proximity of QC-PL to surfaces, we suggest that these properties arise from interactions associated with high densities of defects,...

Laser damage mechanisms in conductive widegap semiconductor films

Laser damage mechanisms of two conductive wide-bandgap semiconductor films - indium tin oxide (ITO) and silicon doped GaN (Si:GaN) were studied via microscopy, spectroscopy, photoluminescence (PL), and elemental analysis. Nanosecond laser pulse exposures with a laser photon energy (1.03 eV, 1064 nm) smaller than the conductive films bandgaps were applied and radically different film damage morphologies were produced. The laser damaged ITO film exhibited deterministic features of thermal degradation. In contrast, laser damage in the Si:GaN film resulted in highly localized eruptions originating at interfaces. For ITO, thermally driven damage was related to free carrier absorption and, for GaN, carbon complexes were proposed as potential damage precursors or markers.

Metallic burn paper used for in situ characterization of laser beam properties.

In situ ablation of thin metal films on fused silica substrates by picosecond class lasers was investigated as a method of characterizing the beam at the sample plane. The technique involved plotting the areas enclosed by constant fluence contours identified in optical microscope images of the ablation sites versus the logs of the pulse energies. Inconel films on commercially available neutral density filters as well as magnetron sputtered gold films were used. It was also shown that this technique could be used to calibrate real-time beam profile diagnostics against the beam at the sample plane. The contours were shown to correspond to the boundary where part or all of the film was ablated.

Ultrafast photoluminescence as a diagnostic for laser damage initiation

Using high-sensitivity confocal time-resolved photoluminescence (CTP) techniques, we report an ultra-fast photoluminescence (40ps-5ns) from impurity-free surface flaws on fused silica, including polished, indented or fractured surfaces of fused silica, and from laser-heated evaporation pits. This fast photoluminescence (PL) is not associated with slower point defect PL in silica which has characteristic decay times longer than 5ns. Fast PL is excited by the single photon absorption of sub-band gap light, and is especially bright in fractures. Regions which exhibit fast PL are strongly absorptive well below the band gap, as evidenced by a propensity to damage with 3.5eV ns-scale laser pulses, making CTP a powerful non-destructive diagnostic for laser damage in silica. The use of CTP to provide insights into the nature of damage precursors and to help develop and evaluate new damage mitigation strategies will be presented.

Optical damage performance of conductive widegap semiconductors: spatial, temporal, and lifetime modeling

The optical damage performance of electrically conductive gallium nitride (GaN) and indium tin oxide (ITO) films is addressed using large area, high power laser beam exposures at 1064 nm sub-bandgap wavelength. Analysis of the laser damage process assumes that onset of damage (threshold) is determined by the absorption and heating of a nanoscale region of a characteristic size reaching a critical temperature. This model is used to rationalize semi-quantitatively the pulse width scaling of the damage threshold from picosecond to nanosecond timescales, along with the pulse width dependence of the damage threshold probability derived by fitting large beam damage density data. Multi-shot exposures were used to address lifetime performance degradation described by an empirical expression based on the single exposure damage model. A damage threshold degradation of at least 50% was observed for both materials. Overall, the GaN films tested had 5-10 × higher optical damage thresholds than the ITO films tested for comparable transmission and electrical conductivity. The route to optically robust, large aperture transparent electrodes and power optoelectronics may thus involve use of next generation widegap semiconductors such as GaN.

Gigashot optical degradation in silica optics at 351 nm.

As applications of lasers demand higher average powers, higher repetition rates, and longer operation times, optics will need to perform well under unprecedented conditions. We investigate the optical degradation of fused silica surfaces at 351 nm for up to 10(9) pulses with pulse fluences up to 12 J/cm(2). The central result is that the transmission loss from defect generation is a function of the pulse intensity, I(p), and total integrated fluence, φ(T), and is influenced by oxygen partial pressure. In 10(-6) Torr vacuum, at low I(p), a transmission loss is observed that increases monotonically as a function of number of pulses. As the pulse intensity increases above 13 MW/cm(2), the observed transmission losses decrease, and are not measureable for 130 MW/cm(2). A physical model which supports the experimental data is presented to describe the suppression of transmission loss at high pulse intensity. Similar phenomena are observed in anti-reflective sol-gel coated optics. Absorption, not scattering, is the primary mechanism leading to transmission loss. In 2.5 Torr air, no transmission loss was detected under any pulse intensity used. We find that the absorption layer that leads to transmission loss is less than 1 nm in thickness, and results from a laser-activated chemical process involving photo-reduction of silica within a few monolayers of the surface. The competition between photo-reduction and photo-oxidation explains the measured data: transmission loss is reduced when either the light intensity or the O(2) concentration is high. We expect processes similar to these to occur in other optical materials for high average power applications.

Modeling of laser-induced damage and optic usage at the National Ignition Facility

Modeling of laser-induced optics damage has been introduced to benchmark existing optic usage at the National Ignition Facility (NIF) which includes the number of optics exchanged for damage repair. NIF has pioneered an optics recycle strategy to allow it to run the laser at capacity since fully commissioned in 2009 while keeping the cost of optics usage manageable. We will show how the damage model is being used to evaluate strategies to streamline our optics loop efficiency, as we strive to increase the laser shot rate without increasing operating costs.

Laser pre-exposure to mitigate damage on microparticle-contaminated fused silica surface in high power laser systems

Current fused silica surface processing, aimed at reducing known absorbing precursor concentration, has brought laboratory-tested ultraviolet laser-induced damage rates to nearly nil at fluences up to 10 J/cm2 . Yet this damage rate reduction has not been fully realized in large facility operation. A recently discovered source of damage in the facility is from particles ejected from damage of a neighboring optic under laser exposure and deposited onto the substrate surface. This state was observed to provide a means to couple energy from a subsequent laser pulse into the contaminated substrate and cause damage characterized by fracture. In this work, we explore the rate at which particles are removed from the surface and the rate at which particles lead to damage as a function of laser fluence and particle characteristics. This analysis allows for a derivation of an optimal pre-exposure fluence of a contaminated optic which maximizes particle removal probability while minimizing surface damage probability. For fluences up to 9.5 J/cm2 (351 nm, 5 ns square pulse), both particle removal and damage probabilities generally increased with particle size and laser fluence, with damage threshold around 6.5 J/cm2 . Two possible mechanisms that facilitate particle-induced damage on the substrate surface from laser-generated and deposited ejecta will be discussed, namely i) enhanced thermal contact from molten or partially molten ejecta and ii) fracture generated upon impact of solid ejecta with high kinetic energy.