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We compare the 355 nm, 45o AOI p-pol 8 n-s laser damage performance of standing-wave hafnia single layers fabricated using argon and xenon as working gas. A suite of metrology tools has been employed to understand the structural, chemical and paramagnetic defect states in the two films. The resultant films from the xenon deposition process are highly dense and have high 3w laser damage performance.

International audience; The standardization and comparison of laser-damage protocols and results are essential prerequisites for development and quality control of large optical components used in high-power laser facilities. To this end, the laser-induced–damage thresholds of two different coatings were measured in a round-robin experiment involving five well-equipped damage testing facilities. Investigations were conducted at the wavelength of 1 μm in the sub picosecond pulse duration range with different configurations in terms of polarization, angle of incidence, and environment (air versus vacuum). In this temporal regime, the damage threshold is known to be deterministic, i.e., the continuous probability distribution transitions from 0 to 1 over a very narrow fluence range. This in turn implies that the damage threshold can be measured very precisely. These characteristics enable direct comparison of damage-threshold measurements between different facilities, with the difference in the measured values indicating systematic errors or other parameters that were not previously appreciated. The results of this work illustrate the challenges associated with accurately determining the damage threshold in the short-pulse regime. Specifically, the results of this round-robin damage-testing effort exhibited significant differences between facilities. The factors to be taken into account when comparing the results obtained with different test facilities are discussed: temporal and spatial profiles, environment, damage detection, sample homogeneity, and nonlinear beam propagation.

The Raman tensor of the dominant A1 modes of the nonlinear optical crystalline material potassium dihydrogen phosphate and its 70% deuterated analog have been ascertained. Challenges in determining the A1 mode tensor element values based on previous reports have been resolved using a specially designed experimental setup that makes use of spherical crystal samples. This novel experimental design enabled the determination of measurement artifacts, including polarization rotation of the pump and/or scattered light propagating through the sample and the contribution of additional overlapping phonon modes, which have hindered previous efforts. Results confirmed that the polarization tensor is diagonal, and matrix elements were determined with high accuracy.

It is well known that dielectric coatings used in high energy laser systems for beam steering are susceptible to laser damage. The laser damage ensued in high refractive index materials, such as hafnia, is responsible for limiting the laser operation fluence and lifetime. Although hafnia is an ideal high refractive index material used in dielectric coatings for a broad range of laser wavelengths, defects developed during the deposition process leads to laser-induced damage. In order to increase the resistance to laser damage and improve laser performance, it is imperative to understand the underlying physics of laser damage in high index coating materials. Earlier work observed a substantial difference in laser damage thresholds for hafnia coatings produced by different deposition methods, yet the underlying mechanisms for the observed difference remains elusive. In this work we investigated the responses of single layer hafnia films produced by two deposition processes, electron beam (e-beam) evaporation and ion beam sputtering (IBS) methods upon UV ns-laser exposure. The films underwent laser damage testing using a 1-on-1 laser damage testing protocol with a beam size of 650 µm (1/e2) at 355 nm and 8 ns pulse duration. Both S and P polarizations were tested at a 45° angle of incidence. Chemical, structural and morphological characterizations of the films both pre- and post-laser damage were performed using Rutherford backscattering spectroscopy, glancing incidence X-Ray diffraction, and optical and scanning/transmission electron microscopy. We found that films deposited from the e-beam process had a higher damage onset threshold (4.4 +/- 0.1 J/cm2) than those deposited by IBS method (2.1 +/- 0.2 J/cm2). Furthermore, a polarization-dependent damage threshold onset was observed for the e-beam evaporated coatings but was not observed in IBS films. Although the typical size of the damage in general is larger for the e-beam produced films, the morphology shows similar foamy appearance in both films. The density of the damage sites, on the other hand, was much greater in the IBS produced films than that by the e-beam method. The observed difference can be attributed to their resulting structural/textural differences inherited in each method: porous in the e-beam films and dense with isolated nanobubbles in the IBS films, which can lead to a large difference in laser-defect coupling. The underlying physical mechanism will be discussed in detail. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. IM Release# LLNL-ABS-809117

Laser-induced damage in coating materials with a high index of refraction, such as hafnia, limits the performance of high power and high energy laser systems. Understanding the underlying physics responsible for laser damage holds the key for developing damage-resistant optical films. Previous studies have reported a substantial difference in laser damage onset for hafnia films produced by different deposition methods, yet the underlying mechanisms for the observed difference remain elusive. We combined laser damage testing with analytical characterizations and theoretical simulations to investigate the response of hafnia films produced by electron (e-) beam evaporation vs ion beam sputtering (IBS) methods upon UV ns-laser exposure. We found that e-beam produced hafnia films were overall more damage resistant; in addition, we observed a polarization anisotropy associated with the onset of damage in the e-beam films, while this effect was absent in the latter films. The observed differences can be attributed to the stark contrast in the pressure inside the pores inherent in both films. The high pressure inside the IBS-induced nanobubbles has been shown to reduce the threshold for laser-induced plasma breakdown leading to film damage. The polarization effects in the e-beam coatings can be related to the asymmetric electric field intensification induced by the columnar void structure. Our findings provide a fundamental basis for developing strategies to produce laser damage-resistant coatings for UV pulsed laser applications.

We compare the distribution of hafnia chemistries as a function of sun and planet position in an ion beam sputtering system. Hafnia film chemistries were investigated both without and with planetary rotation. In the former case, the film thickness, stoichiometries and entrapped argon varied drastically as a function of sun position, with one sun position exhibiting high entrapped argon content. With full planetary rotation used during deposition, the film stoichiometry is nearly ideal with 6% entrapped argon content. It is observed that the center of the planets is an exception, with a slightly metallic stoichiometry and high entrapped argon. Interestingly, all hafnia optical films produced in this study exhibit an inverse relationship between oxygen content and entrapped argon.

The typical measurement error reported for laser damage tests is the fluence uncertainty due to inaccuracies in measuring the laser beam energy and its diameter. However, the inherent uncertainty of the testing protocol should also be included in the reported laser damage threshold error bars. Underestimating measurement errors can lead to false conclusions about the impact of process changes on laser damage resistance. In this study, four different laser damage precursor fluence distributions were created from randomly generated numbers and then evaluated using the ISO and raster scan laser damage test protocols to determine a laser damage threshold. Measurement errors are determined for flat top test beams for multiple cases. To add real world relevance, the impact of Gaussian test beams with beam pointing instability was modeled for the lowest accuracy laser damage precursor distribution. The impact of damage test area compared to optic dimension is also examined. The measurement error for the raster scan test ranged from 8% to 24% depending on the test beam spatial profile (flat top or Gaussian) and beam pointing stability. ISO measurement errors ranged from 4% to 250% for a simulated 10 J/cm2 test and was much more sensitive to the laser damage precursor distribution as well as the spatial profile and pointing of the test beam. Both testing protocols poorly predicted the laser damage resistance of large areas with Gaussian precursor laser damage distributions.

We investigate the interaction of 355-nm and 1064-nm nanosecond laser pulses with nominally spherical metallic particles dispersed on the input surface of transparent substrates or high-reflectivity (HR) multilayer dielectric coatings, respectively. The objective is to elucidate the interaction mechanisms associated with contaminant-induced degradation and damage of transparent and reflective optical elements for high-power laser systems. The experiments involve time-resolved imaging capturing the dynamics of the interaction pathway, which includes plasma formation, particle ejection, and secondary contamination by droplets originating from the liquefied layer of the particle. The results suggest that HR coatings are more susceptible to secondary contamination by liquid droplets produced by the particles because of the different geometry of excitation and the location of plasma initiation. Modeling results focus on better understanding the melting of the particle surface, leading to ejections of liquid droplets and the pressure applied to the substrate, leading to mechanical damage.

Laser drilling and cutting of materials is well established commercially, although its throughput and efficiency limit applications. This work describes a novel approach to improve laser drilling rates and reduce laser system energy demands by using a gated continuous wave (CW) laser to create a shallow melt pool and a UV ps-pulsed laser to impulsively expel the melt efficiency and effectively. Here, we provide a broad parametric study of this approach applied to common metals, describing the role of fluence, power, spot size, pulse-length, sample thickness, and material properties. One to two order-of-magnitude increases in the average removal rate and efficiency over the CW laser or pulsed-laser alone are demonstrated for samples of Al and stainless steel for samples as thick as 3 mm and for holes with aspect ratios greater than 10:1. Similar enhancements were also seen with carbon fiber composites. The efficiency of this approach exceeds published values for the drilling of these materials in terms of energy to remove a given volume of material. Multi-laser material removal rates, high-speed imaging of ejecta, and multi-physics hydrodynamic simulations of the melt ejection process are used to help clarify the physics of melt ejection leading to these enhancements. Our study suggests that these high-impulse multi-laser enhancements are due to both laser-induced surface wave instabilities and cavitation of the melt for shallow holes and melt cavitation and ejection for deeper channels.

The propagation of 355-nm, nanosecond pulses in absorbing glasses is investigated for the specific case examples of the broadband absorbing glass SuperGrey and the Ce3+-doped silica glass. The study involves different laser irradiation conditions and material characterization methods to capture the transient material behaviors leading to laser-induced damage. Two damage-initiation mechanisms were identified: (1) melting of the surface as a result of increased temperature; and (2) self-focusing caused by a transient change in the index of refraction. Population of excited states greatly affects both mechanisms by increasing the transient absorption cross section via excited-state absorption and introducing a change of the refractive index to support the formation of graded-index lensing and self-focusing of the beam inside the material. The governing damage-initiation mechanism depends on the thermodynamic properties of the host glass, the electronic structure characteristics of the doped ion, and the laser-spot size.

The results of detailed experiments and high fidelity modeling of melt pool dynamics, droplet ejections and hole drilling produced by periodic modulation of laser intensity are presented. Ultra-high speed imaging revealed that melt pool oscillations can drive large removal of material when excited at the natural oscillation frequency. The physics of capillary surface wave excitation is discussed and simulation is provided to elucidate the experimental results. The removal rates and drill through times as a function of driving frequency is investigated. The resonant removal mechanism is driven by both recoil momentum and thermocapillary force but the key observation is the latter effect does not require evaporation of material, which can significantly enhance the efficiency for laser drilling process. We compared the drilling of holes through a 2 mm-thick Al plate at modulation frequencies up to 20 kHz. At the optimal frequency of 8 kHz, near the resonant response of the melt pool, the drilling efficiency is greater than 10x with aspect ratio of 12:1, and without the collateral damage that is observed in unmodulated CW drilling.

Optimum material selection for high fluence interference coatings is wavelength dependent (increased bandgap with decreased wavelength) whereas the deposition process depends on pulse length (increased densification with decreased pulse length).

High intensity and high energy laser facilities place increasing demands on optical components, requiring large surface area optics with exacting specifications. Petawatt lasers are high energy, short-pulse laser systems generally based on chirped-pulse amplification, where an initial low energy short pulse is stretched, amplified, and then recompressed to produce fs to ps high-power laser pulses. In such petawatt lasers, the highest demands are placed on the final optics, including gratings which compress the pulses and mirrors which direct and focus the final high-power beams. The limiting factor in these optical components is generally laser-induced damage. Designing and fabricating these optical components to meet reflection, dispersion, and other requirements while meeting laser-induced damage requirements is the primary challenge discussed in this tutorial. We will introduce the reader to the technical challenges and tradeoffs required to produce mirrors for petawatt lasers and discuss current research directions.

Understanding the origin of laser damage-prone precursors in high index materials such as hafnia holds the key to the development of laser damage-resistant multilayer dielectric coated optics for high power and energy laser systems. In this study, we investigate the source of sub-stoichiometry, a potent laser damage precursor, in hafnia films produced by an ion beam sputtering (IBS) deposition method and the effect of such defects on the film performance upon ns ultraviolet (UV) laser (8 ns, 355 nm) exposure. Chemical analysis of data obtained via Rutherford backscattering spectroscopy (RBS) suggests that hafnia films deposited at two different planetary locations from the same deposition run exhibit anisotropic and location-dependent stoichiometries. While the oxygen-to-hafnium ratio is at the stoichiometric value of 2 for the hafnia film at the edge location, the ratio is significantly deviated and is 1.7 for that deposited at the planetary center. The sub-stoichiometric hafnia films display a much lower 1-on-1 damage onset at 1.6 ± 0.2 J/cm2 compared to 2.3 ± 0.2 J/cm2 in a stoichiometric film. The low damage performance films also have an over three times higher damage density at fluences above initiation. Coupled with Monte Carlo simulations, we reveal that sub-stoichiometry is primarily attributed to preferential removal of oxygen during film deposition by the bombardment of energetic reflected argon neutrals. The resulting oxygen deficiencies create the sub-bandgap states which facilitate the strong laser energy coupling and reduce the resistance to laser-induced damage in the hafnia single layer films.

The thin film damage competition series at the Boulder Damage Symposium provides an opportunity to observe general trends in laser damage behavior between different coating types (high reflector, anti-reflector, Polarizer, and Fabry-Perot filter), wavelength ranges (193 – 1064 nm), and pulse length ranges (40 fs – 18 ns). Additionally, the impact of deposition process, coating material, cleaning process, and layer count can be studied within a single year or more broadly across the history of this competition. Although there are instances where participants attempted to isolate a single variable to better understand it’s impact on laser resistance, this series of competitions isolates the variable of the damage testing service and protocol for a wide variety of participants. In total 275 samples from 58 different participants have been tested at four different laser damage testing facilities over the last ten years. Hafnia was clearly the best high refractive index material except for UV applications; although a wide range of high refractive index materials performed well. The best deposition process varied significantly between the different competitions, so it was much more strongly dependent on the coating type, wavelength, and pulse duration. For 1064 nm coatings with nanosecond scale pulse lengths, e-beam coatings tended to be the best performers. For short pulse length NIR mirrors and nanosecond pulse length UV mirrors, densified coating processes which all involved sputtering of the target material were the best performers. For UV AR coatings and excimer mirrors, both tested at nanosecond pulse lengths, they tended to favor very low energetic deposition methods yielding soft coatings such as sol gel dip coating for the AR and resistive heating of fluorides for the excimer mirrors. Finally cleaning method and layer count have had a less obvious correlation with laser resistance over the history of this thin film damage competition.

The need for optics that can sustain higher laser fluences and intensities grows as new technological advancements allow laser systems to operate at increased in peak power. This has motivated a substantial effort in recent decades to study laser-induced damage mechanisms and their mitigations. One well known laser-induced damage mechanism is filamentation in fused silica glass, due to Kerr self-focusing of the light [1]. The study of filamentation has been an ongoing effort for the last few decades [2] as it turned out to be a major limitation to laser systems at high peak intensities. Past studies have led to a set of simplified rules that allows for the operation of laser system below the onset point of filamentation to occur, namely what is known as the “IL rule” (intensity times the length before filamenting equals some empirical constant) and the Bespalov-Talanov (BT) perturbation growth theory [3-8]. The necessity to increase the laser beam intensities and optimize the throughput, closer to the point where the optical propagation length in the material is comparable to the predicted filamentation distance, requires revisiting and improving our understanding of the current rule set.

Hafnia is a high refractive index material used in the manufacturing of dielectric coatings for next generation lasers. The formation of defects during deposition is the major barrier to realizing high laser-damage resistant coatings for future high energy density laser applications. Understanding the precursors responsible for laser-induced damage in hafnia is therefore critical. In this work, we investigate the mechanism of laser-induced damage in 90-nm thick hafnia films produced by an oxygen assisted dual ion beam sputtering (IBS) process. Under pulsed, nanosecond ultraviolet laser exposure (355 nm, 8 ns), the laser-induced damage onset is found to be strongly dependent on the amount of argon and excessive oxygen entrapped in the nanobubbles within the hafnia films. The presence of nanobubbles is revealed and confirmed by small angle X-ray scattering and scanning/transmission electron microscopy coupled with high-angle annular dark-field. The damage onset is stable initially but decreases as the energy of oxygen goes beyond 100 eV. The damage initiation is ascribed to a laser-induced plasma generation within the nanobubbles through multiphoton ionization. The results reveal that nanobubbles formed in the IBS produced coatings are a potent precursor. Although nanobubbles are commonly present in IBS films, their negative impact on laser damage resistance of hafnia films has not been previously recognized. Our findings provide a fundamental basis for the development of potential mitigation strategies required for the realization of laser damage resistant hafnia films.

There are 830 transport mirrors with a combined surface area of approximately 255 m2 of precision multilayer coatings deposited on 50 metric tons of BK7 glass in the high fluence transport section of the National Ignition Facility (NIF). With peak fluences over 20 J/cm2 at 1053 nm, less than five percent of these mirrors are exchanged annually due to laser damage since full system operations began in 2009. Multiple technologies have been implemented to achieve these low exchange rates. The coatings are complex dichroics designed to reflect the fundamental wavelength (1053 nm) and an alignment beam (374 nm) while suppressing target backscatter wavelengths (351 nm and 400-700 nm) from backward propagation up the beamlines. Each optic is off-line laser conditioned to nominally 50% over the average fluence and nominally 90% of the peak fluence allowing the final laser conditioning to occur on-line during NIF operations. Although the transport section of NIF is sealed in a clean argon environment, air knives were installed on upward facing transport mirrors to blow off particulates that could accumulate and initiate laser damage. Beam dumps were installed in between the final optics assembly and the final transport mirrors to capture ghost reflections from the anti-reflection coated surfaces on the transmissive optics used for polarization rotation, frequency conversion, and focusing the 192 laser beams on target. Spot blockers, normally used for the final optics, are sometimes used to project a shadow over transport mirror laser damage in an effort to arrest laser damage growth and extend transport mirror lifetime. Post analysis of laser-damaged mirrors indicates that the dominant causes of laser damage are from surface particulates and the 351-nm wavelength target backscatter.

This year’s competition is a continuation of the 2020 thin film damage competition conducted with nanosecond (ns) laser pulses and aimed to survey state-of-the-art visible high reflectors in the femtosecond (fs) pulse length regime. The requirements for the coatings were a minimum reflection of 99.5% at 0 degrees incidence angle at 532 nm. The fs-laser source available for testing in 2021 delivered pulses at 515 nm; as a result, to accommodate both ns- and fs-damage tests on the same samples, the latter tests were conducted at an incidence angle of 25 degrees to re-center the reflection spectral band of the coatings, i.e., a blue shift from 532 nm to 515 nm. The choice of coating materials, design, and deposition method were left to the participants. Laser damage testing of samples received in 2020 and new sample submissions in 2021 was per- formed at a single testing facility using the S-on-1 standardized test protocol with a 200-fs pulse length laser system operating at 5 kHz in a single-longitudinal mode. A double-blind test assured sample and submitter anonymity. The damage performance results (LIDT), sample rankings, details of the deposition processes, coating materials and substrate cleaning methods are shared. These experiments enabled not only direct comparison among the participants but also the performance contrast in two different pulse length regimes. We found that different deposition methods and coating materials yield comparably high laser resistance in the fs-pulse regime. Namely, hafnia/silica, hafnia/alumina/silica or tantala/alumina/silica multilayer coatings were the most damage resistant under exposure to fs-laser pulses within the coating deposition groups. In contrast, hafnia/silica coatings by e-beam were identified as best performers in the ns-pulse regime.

A comprehensive study of laser-induced damage associated with particulate damage on optical surfaces is presented. Contaminant-driven damage on silica windows and multilayer dielectrics is observed to range from shallow pitting to more classical fracture-type damage, depending on particle-substrate material combination, as well as laser pulse characteristics. Ejection dynamics is studied in terms of plasma emission spectroscopy and pump-probe shadowgraphy. Our data is used to assess the momentum coupling between incident energy and the ejected plasma, which dominates the laser-particle-substrate interaction. Beam propagation analysis is also presented to characterize the impact of contaminant-driven surface pitting on optical performance.

The ablation of magnetron sputtered metal films on fused silica substrates by a 1053 nm, picosecond class laser was studied as part of a demonstration of its use for in-situ characterization of the laser spot under conditions commonly used at the sample plane for laser machining and damage studies. Film thicknesses were 60 and 120 nm. Depth profiles and SEM images of the ablation sites revealed several striking and unexpected features distinct from those typically observed for ablation of bulk metals. Very sharp thresholds were observed for both partial and complete ablation of the films. Partial film ablation was largely independent of laser fluence with a surface smoothness comparable to that of the unablated surface. Clear evidence of material displacement was seen at the boundary for complete film ablation. These features were common to a number of different metal films including Inconel on commercial neutral density filters, stainless steel, and aluminum. We will present data showing the morphology of the ablation sites on these films as well as a model of the possible physical mechanisms producing the unique features observed.

This study investigates the physical processes involved in picosecond pulse (20-28 ps FWHM) laser ablation of Al 6061, 316L stainless steel, and undoped crystalline Si (〈100〉) over a range of laser wavelength (355 nm and 1064 nm) and fluence (0.1-40 J/cm2). Experimental measurements of material ablation rate show enhanced removal at the 355 nm wavelength, primarily due to laser-plasma interaction (LPI) within the ablative plume that approaches an order of magnitude increase over the measured removal at 1064 nm. A transition in the ablation rate at 355 nm is identified around ∼10 J/cm2 above which the removal efficiency increases by a factor of two to three. Multi-physics radiation hydrodynamic simulations, considering LPI effects and utilizing a novel mixed-phase equation of state model, show that the transition in ablation efficiency is due to the onset of melt ejection through cavitation, where laser-driven shock heating sets the depth of melt penetration and the ensuing release wave from the ablation surface drives cavitation through the imposition of tensile strain within the melt. High-speed pump-probe imaging of the ejecta and ejecta collection studies, as well as scanning electron microscopy of the ablation craters, support the proposed cavitation mechanism in the higher fluence range. The ablation process is critically influenced by LPI effects and the thermophysical properties of the material.

The National Ignition Facility (NIF) at Lawrence Livermore National Laboratory, is the first of its kind megajoule-class laser facility with 192 beams capable of delivering over 1.8 MJ and 500TW of 351 nm light for high accuracy laser-matter interaction experiments. It has been commissioned and operated since 2009 to support a wide range of missions including the study of inertial confinement fusion, high energy density physics, material science, and laboratory astrophysics. In the first section of this paper we discuss the current status of laser performance obtained during the 408 target experiments completed in 2017. The performance spanned a wide range of laser energies, powers and pulse durations as requested for these target experiments. A special emphasis is given on energy delivery and cone power accuracy in the UV, as these are key parameters for successful experiments. In the second section of the paper, the results obtained during the 2017 performance quad campaign are briefly described. During this campaign a series of laser-only shots were taken to perform tests at elevated energies on a single NIF quad. These tests were designed to assess laser performance limits and operational costs against predictive models. This campaign culminated with the delivery of ~54 kJ of UV on a single quad of NIF, and 14 kJ on a single beam aperture, which are both to our knowledge the largest energies achieved to date for a neodymium-glass, frequency tripled architecture.

The thin film damage competition series at the Boulder Damage Symposium provides an opportunity to observe general trends in laser damage behavior between different coating types (high reflector, antireflector, polarizer, and Fabry–Perot filter), wavelength ranges (193 to 1064 nm), and pulse length ranges (40 fs to 18 ns). Additionally, the impact of deposition process, coating material, cleaning process, and layer count can be studied within a single year or more broadly across the history of this competition. Although there are instances where participants attempted to isolate a single variable to better understand its impact on laser resistance, this series of competitions isolates the variable of the damage testing service and protocol for a wide variety of participants to enable the observation of general trends. In total, 275 samples from 58 different participants have been tested at four different laser damage testing facilities over the last 10 years. Hafnia was clearly the best high refractive index material except for ultraviolet (UV) applications, although a wide range of high refractive index materials performed well. The best deposition process varied significantly between the different competitions. The best deposition process was dependent on the coating type, wavelength, and pulse duration. For 1064-nm coatings with nanosecond scale pulse lengths, e-beam coatings tended to be the best performers. For short-pulse length NIR mirrors and nanosecond pulse length UV mirrors, densified coating processes, which all involved sputtering of the target material, were the best performers. For UV antireflector (AR) coatings and excimer mirrors, both tested at nanosecond pulse lengths, they tended to favor very low energetic deposition methods yielding soft coatings, such as sol gel dip coating for the AR and resistive heating of fluorides for the excimer mirrors. Finally, cleaning method and layer count had a less obvious correlation with laser resistance over the history of this thin film damage competition series.

The material response following nanosecond, UV laser induced breakdown inside of the exit surface of fused silica is investigated using multimodal time resolved microscopy. The study spans up to about 75 ns delay from the onset of material modification during the laser pulse through the observation of material ejection. A number of distinct processes were identified, including: a) the onset of optical absorption in the material arising from the buildup of an electronic excitation, b) the expansion of the hot modified region (plasma) along the surface and inside the bulk, c) the formation of radial and circumferential cracks, d) the swelling of the affected region on the surface and, e) the onset of ejection of material clusters at about 30 ns delay and its progression to a well-defined jet by about 75 ns delay. Limited theoretical modeling is used to aid the interpretation of the data.

For pulse lengths between 1 and 60 ps, laser-induced modifications of optical materials undergo a transition from mechanisms intrinsic to the materials to defect-dominated mechanisms. Elucidating the location, size, and identity of these defects will greatly help efforts to reduce, mitigate, or eliminate these defects. We recently detailed the role of defects in the ps laser-modifications of silica coatings. We now discuss the similar role of defects in HfO2 1/2-wave coatings and also include the environmental effects on the damage process. By switching between S and P polarizations, we distinguish the effects of defects at the surface from those throughout the material. We find that defects very near the surface are dependent on the environment, leading to worse damage in vacuum than in air. Air suppresses or lessens the effects of these defects, suggesting a photo-chemical component in the mechanism of laser damage in HfO2 coatings.

We investigate the optical damage performance of multi-layer dielectric (MLD) coatings suitable for use in high energy, large-aperture petawatt-class lasers. We employ small-area damage test methodologies to evaluate the damage resistance of various coatings as a function of deposition methods and coating materials under simulated use conditions. In addition, we demonstrate that damage initiation by raster scanning at lower fluences and growth threshold testing are required to estimate large-aperture optics’ performance.

Laser-induced damage threshold (LIDT) testing was performed on commercially-available multilayer dielectric coatings to qualify for use in the High Repetition-Rate Advanced Petawatt Laser System (HAPLS) for Extreme Light Infrastructure Beamlines. Various tests were performed with uncompressed pulses (150 ps) from a 780 nm-centered Ti:Sapphire regenerative ampliflier, and the raster scan method was used to determine the best-performing coatings. Performance varied from 2–8 J/cm 2 across samples from 6 different manufacturers.

Laser induced damage (breakdown) initiated on the exit surface of transparent dielectric materials using nanosecond pulses creates a volume of superheated material reaching localized temperatures on the order of 1 eV and pressures on the order of 10 GPa or larger. This leads to material ejection and the formation of a crater. The volume of this superheated material depends largely on the laser parameters such as fluence and pulse duration. To elucidate the material behaviors involved, we examined the morphologies of the ejected superheated material particles and found distinctive morphologies. We hypothesize that these morphologies arise from the difference in the structure and physical properties (such as the dynamic viscosity and presence of instabilities) of the superheated material at the time of ejection of each individual particle. Some of the ejected particles are on the order of 1 µm in diameter and appear as “droplets”. Another subgroup appears to have stretched, foam-like structure that can be described as material globules interconnected via smaller in diameter columns. Such particles often contain nanometer size fibers attached on their surface. In other cases, only the globules have been preserved suggesting that they may be associated with a collapsed foam structure under the dynamic pressure as it traverses in air. These distinct features originate in the structure of the superheated material during volume boiling just prior to the ejection of the particles.

This broadband, low dispersion mirror damage competition is a continuation of last year's test with 150 ps pulse length results published in 2015 and 40 fs pulse length results in this study. This competition allows a direct laser resistance comparison between pulse durations because the samples were laser damage tested under identical conditions. The requirements of the coatings are a minimum reflection of 99.5% at 45 degrees incidence angle at "P" polarization with a Group Delay Dispersion (GDD) of < ± 100 fs2 over a spectral range of 773 nm ± 50 nm. The choice of coating materials, design, and deposition method were left to the participant. Laser damage testing was performed using the raster scan method with a 40 fs pulse length on a single testing facility to enable a direct comparison among the participants. GDD measurements were performed to validate specification compliance. A double blind test assured sample and submitter anonymity. In addition to the laser resistance results and GDD measurements, details of the deposition processes, cleaning method, coating materials and layer count are also shared.

Laser fluence and operational tempo of ICF systems operating in the UV are typically limited by the growth of laser- induced damage on their final optics (primarily silica optics). In the early 2000 time frame, studies of laser damage growth with relevant large area beams revealed that for some laser conditions damage sites located on the exit surface of a fused silica optic grew following an exponential growth rule: D(n) = D0 exp (n α(φ)), where D is final site diameter, D0 is the initial diameter of the site, φ is the laser fluence, α(φ) is the growth coefficient, and n is the number of exposures. In general α is a linear function of φ, with a threshold of φTH. In recent years, it has been found that that growth behavior is actually considerably more complex. For example, it was found that α is not a constant for a given fluence but follows a probability distribution with a mean equal to α(φ). This is complicated by observations that these distributions are actually functions of the pulse shape, damage site size, and initial morphology of damage initiation. In addition, there is not a fixed fluence threshold for damage sites growth, which is better described by a probability of growth which depends on site size, morphology and laser fluence. Here will review these findings and discuss implications for the operation of large laser systems.

Precise assessment of the high fluence performance of pulse compressor gratings is necessary to determine the safe operational limits of short-pulse high energy lasers. We have measured the picosecond laser damage behavior of multilayer dielectric (MLD) diffraction gratings used in the compression of chirped pulses on the Advanced Radiographic Capability (ARC) kilojoule petawatt laser system at the Lawrence Livermore National Laboratory (LLNL). We present optical damage density measurements of MLD gratings using the raster scan method in order to estimate operational performance. We also report results of R-on-1 tests performed with varying pulse duration (1-30 ps) in air, and clean vacuum. Measurements were also performed in vacuum with controlled exposure to organic contamination to simulate the grating use environment. Results show sparse defects with lower damage resistance which were not detected by small-area damage test methods.

We investigate the laser damage resistance of multilayer dielectric (MLD) diffraction gratings used in the pulse compressors for high energy, high peak power laser systems such as the Advanced Radiographic Capability (ARC) Petawatt laser on the National Ignition Facility (NIF). Our study includes measurements of damage threshold and damage density (ρ(Φ)) with picosecond laser pulses at 1053 nm under relevant operational conditions. Initial results indicate that sparse defects present on the optic surface from the manufacturing processes are responsible for damage initiation at laser fluences below the damage threshold indicated by the standard R-on-1 test methods, as is the case for laser damage with nanosecond pulse durations. As such, this study supports the development of damage density measurements for more accurate predictions on the damage performance of large area optics.

We present techniques for measuring the damage performance of a variety of optical components with ps laser pulses, introduce a novel beam diagnostic technique, and explore the sensitivity of damage resistance to laser spot size for the case of high-reflectivity, multilayer dielectric (MLD) mirrors.

We present an experimental technique along with the method of data analysis to give nondegenerate two-photon absorption (2PA) spectra. We use a femtosecond pump pulse and a white-light continuum (WLC) probe to rapidly generate the 2PA spectra of a variety of materials. In order to analyze data taken with this method, the spectral and temporal characteristics of the WLC must be known, along with the linear dispersion of the sample. This allows determination of the temporal walk-off of the pump and probe pulses as a function of frequency caused by group-velocity mismatch. Data correction can then be performed to obtain the nonlinear losses. We derive an analytical formula for the normalized nonlinear transmittance that is valid under quite general experimental parameters. We verify this on ZnS and use it for the determination of 2PA spectra of some organic compounds in solution. We also compare some of the data on organics with two-photon fluorescence data and find good agreement.

We report the two-photon-induced photoisomerization of 3-[1-(1,2-dimethyl-1H-indol-3yl)-ethylidene]-4-isopropylidene-dihydrofuran-2,5-dione (1), a photochromic compound with Imax ) 385 nm, using 775-nm femtosecond pulsed laser irradiation. The resulting photoisomer had Imax ) 582 nm. The kinetic rate constant for the two-photon-induced electrocyclic isomerization reaction was measured at two different intensities (two different powers), showing a quadratic dependence with respect to the pump intensity. Results of pump-probe solution phase experiments and guest/host polymer thin film interferometric imaging studies are reported. A two-photon absorption molecular cross section U2 ) 10.3 10 -48 cm 4 ‚s/photon was measured using Z-scan, further supporting a two-photon-induced isomerization process. Two-photon-induced interferometric recording in a fulgide-containing polymer film was demonstrated.

We investigate the role of defects in laser-induced damage of fused silica and of silica coatings produced by e-beam and PIAD processes which are used in damage resistant, multi-layer dielectric, reflective optics. We perform experiments using 1053 nm, 1-60 ps laser pulses with varying beam size, number of shots, and pulse widths in order to understand the characteristics of defects leading to laser-induced damage. This pulse width range spans a transition in mechanisms from intrinsic material ablation for short pulses to defect-dominated damage for longer pulses. We show that for pulse widths as short as 10 ps, laser-induced damage properties of fused silica and silica films are dominated by isolated absorbers. The density of these precursors and their fluence dependence of damage initiation suggest a single photon process for initial energy absorption in these precursors. Higher density precursors that initiate close to the ablation threshold at shorter pulse widths are also observed in fused silica, whose fluence and pulse width scaling suggest a multiphoton initiation process. We also show that these initiated damage sites grow with subsequent laser pulses. We show that scaling laws obtained in more conventional ways depend on the beam size and on the definition of damage for ps pulses. For this reason, coupling scaling laws with the density of precursors are critical to understanding the damage limitations of optics in the ps regime.

Laser-induced damage with ps pulse widths straddles the transition from intrinsic, multi-photon ionization and avalanche ionization-based ablation with fs pulses to defect-dominated, thermal-based damage with ns pulses. We investigated the morphology of damage for fused silica and silica coatings between 1 ps and 60 ps at 1053 nm. Using calibrated laser-induced damage experiments, in situ imaging, and high-resolution optical microscopy, atomic force microscopy, and scanning electron microscopy, we show that defects play an important role in laser-induced damage down to 1 ps. Three types of damage are observed: ablation craters, ultra-high density pits, and smooth, circular depressions with central pits. For 10 ps and longer, the smooth, circular depressions limit the damage performance of fused silica and silica coatings. The observed high-density pits and material removal down to 3 ps indicate that variations in surface properties limit the laser-induced damage onset to a greater extent than expected below 60 ps. Below 3 ps, damage craters are smoother although there is still evidence as seen by AFM of inhomogeneous laser-induced damage response very near the damage onset. These results show that modeling the damage onset only as a function of pulse width does not capture the convoluted processes leading to laser induced damage with ps pulses. It is necessary to account for the effects of defects on the processes leading to laser-induced damage. The effects of isolated defects or inhomogeneities are most pronounced above 3 ps but are still discernible and possibly important down to the shortest pulse width investigated here.

The Advanced Radiographic Capability (ARC) laser system at the National Ignition Facility (NIF) is designed to ultimately provide eight beamlets with a pulse duration adjustable from 1 to 30 ps, and energies up to 1.5 kJ per beamlet. Currently, four beamlets have been commissioned. In the first set of 6 commissioning target experiments, the individual beamlets were fired onto gold foil targets with energy up to 1 kJ per beamlet at 20–30 ps pulse length. The x-ray energy distribution and pulse duration were measured, yielding energy conversion efficiencies of 4–9 × 10−4 for x-rays with energies greater than 70 keV. With greater than 3 J of such x-rays, ARC provides a high-precision x-ray backlighting capability for upcoming inertial confinement fusion and high-energy-density physics experiments on NIF.

We present a comprehensive statistical model which includes both the probability of growth and growth rate to describe the evolution of exit surface damage sites on fused silica optics over multiple laser shots spanning a wide range of fluences. We focus primarily on the parameterization of growth rate distributions versus site size and laser fluence using Weibull statistics and show how this model is consistent with established fracture mechanics concepts describing brittle materials. Key growth behaviors and prediction errors associated with the present model are also discussed.

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.

Subpicosecond transient absorption measurements were performed for several polymethine and squarylium dyes in ethanol solution and a polymeric host over the spectral range 400-1500 nm. A variety of nonlinear effects including saturable absorption, reverse saturable absorption, and gain were observed and analyzed. We observe strong excited-state absorption (ESA) in all dyes in the range 450-600 nm. We also report the first prediction and observation of additional ESA bands in the near-infrared range. The predictions were based on quantum chemical calculations and the ESA experiments were performed with femtosecond pump-continuum probe techniques. For polymethine dye 2-[2-[3-[(1,3-dihydro-3,3-dimethyl-1-phenyl-2H-indol-2-ylidene) ethylidene]-2-phenyl-1-cyclohexen-1-yl]ethenyl]-3,3-dimethyl-1-phenylindolium perchorate, an additional ESA band was detected near 1250 nm, and for squarylium dye 1,3-Bis-[(1,3-dihydro-1-butyl-3,3-dimetyl-2H-benzo[e]indol-2-ylidene)methyl]squ araine, two additional ESA bands were found around 870- and 1380-nm, respectively. To further study the nature of these transitions, the steady-state excitation anisotropy was also studied and compared with predictions. The relationship between ESA spectra of organic dyes and their molecular structure is discussed.

Efficient Pd-catalyzed Heck coupling methodology was employed to provide two new fluorene derivatives with phosphonate (2) and nitro (3) electron-withdrawing functionalities. Both derivatives exhibit two-photon absorption (2PA), as determined by nonlinear absorption measurements using a femtosecond pump/white light continuum probe “NLO spectrometer”. Both fluorene derivatives have high 2PA cross sections (650 and 1300 × 10-50 cm4 s photon-1 molecule-1 for compounds 2 and 3, respectively).