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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.

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.

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).

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.

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.

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.

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.

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.

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.

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.

Laser-induced growth of optical damage often determines the useful lifetime of an optic in a high power laser system. We have extended our previous work on growth of laser damage in fused silica with simultaneous 351 nm and 1053 nm laser irradiation by measuring the threshold for growth with various ratios of 351 nm and 1053 nm fluence. Previously we reported that when growth occurs, the growth rate is determined by the total fluence. We now find that the threshold for growth is dependent on both the magnitude of the 351 nm fluence as well as the ratio of the 351 nm fluence to the 1053 nm fluence. Furthermore, the data suggests that under certain conditions the 1053 nm fluence does not contribute to the growth.

Intense laser pulses can cause superheating of the near-surface volume of materials. This mechanism is widely used in applications such as laser micromachining, laser ablation, or laser assisted thin film deposition. The relaxation of the near solid density superheated material is not well understood, however. In this work, we investigate the relaxation dynamics of the superheated material formed in several dielectrics with widely differing physical properties. The results suggest that the relaxation process involves a number of distinct phases, which include the delayed explosive ejection of microscale particles starting after the pressure of the superheated material is reduced to about 4 GPa and for a time duration on the order of 1 μs. The appearance of a subset of collected ejected particles in fused silica is similar to that of micro-tektites and provides information about the state of the superheated material at the time of ejection. These results advance our understanding of a key aspect of the laser–material interaction pathway and can lead to optimization of associated applications ranging from material processing to laser surgery.

The interaction of damage initiating defect precursors in KDP and DKDP crystals with laser pulses is investigated as a function of laser parameters to obtain experimental results that contain information about the type and nature of the defects. Specifically, the focus is to understand (a) the interaction of the precursors with sub-damage laser pulses leading to improvement to the damage performance (laser conditioning) and (b) the synergetic effects during multi-wavelength irradiation. Our results expose complex behaviors of the defect precursors associated with damage initiation and conditioning at different wavelengths that provide a major step towards revealing the underlying physics.

Laser annealing of defects to increase bulk laser-induced damage resistance of KDP/DKDP crystals is investigated for a wide range of laser parameters to reveal both the conditions necessary to optimize the effect and the underlying physical mechanisms.

We present experimental results aiming to reveal the relationship between damage initiating defect populations in KDP and DKDP crystals under irradiation at different wavelengths. Our results indicate that there is more than one type of defects leading to damage initiation, each defect acting as damage initiators over a different wavelength range. Results showing disparities in the morphology of damage sites from exposure at different wavelengths provides additional evidence for the presence of multiple types of defects responsible for damage initiation.

The damage performance of potassium dihydrogen phosphate (KDP, and its deuterated analog DKDP) crystals under multi-wavelength, simultaneous exposure to the harmonics of a nanosecond Nd:YAG laser system is of particular interest because it approximates the conditions taking place during frequency conversion. In this work, damage initiation under simultaneous exposure to two pulses at different wavelengths is investigated as a function of fluence in both KDP and DKDP. We have developed a novel damage testing instrumentation which allows us to measure the damage pinpoint density as a function of laser parameters, including wavelength, fluence, and pulse-length. This new method enables us to carefully quantify the damage effects of both, single-wavelength and dual-wavelength pulsed irradiation. In the latter case, we measure the laser-induced damage behavior when the fluence at one wavelength is varied while the fluence of the second wavelength is kept constant. Our results suggest that the behavior of laser-induced damage initiation under simultaneous multi-wavelength irradiation is complex and crystal dependent.

We investigate the laser-induced damage behavior of KDP and DKDP crystals under simultaneous multi-wavelength irradiation as a function of laser fluences to approximate conditions during frequency conversion. Our results reveal fundamental behaviors of damage initiation

Time-resolved photoinduced absorption measurements were performed at 77 K and room temperature on thin films of tris[9,9-dihexyl-2-(phenyl-4'-(-pyridin-2"-yl)fluorene]iridium(III) [Ir(DPPF) 3 ] doped into a blend of poly(N-vinylcarbazole) (PVK) with 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD). We observe that in the PVK-PBD host blend, charge trapping (CT) plays an important role in the excited-state dynamics, in addition to exciplex formation and intensity-dependent decay of primary excitations. We develop a physical model which includes all interactions and which is in excellent agreement with the data. We find that 35% of the initial photoexcitation channels into CT states and that exciplexes are formed at a rate of 1/10 ps - 1 . For the Ir(DPPF) 3 doped host composite, we write the rate equations for all population densities (which include the above excited-state species) and include energy-transfer rates from the host to the guest molecules. In both 0.2% and 2% Ir(DPPF) 3 :(PVK-PBD) blends, Forster energy-transfer rates drop to half their low-temperature values at room temperature. We attribute this difference to a limited availability of guest molecules ready for energy transfer following charge trapping and insufficient spectral overlap due to shifts in the highest occupied and lowest unoccupied molecular orbital levels of the guest upon hole trapping. We conclude that the overall host-guest energy transfer is almost complete at room temperature in the 2% phosphorescent blend, with a large contribution (35%) from CT states which exhibit emission at all probe wavelengths.

Summary form only given. Measurements of the nonlinear absorption spectra and the dispersion of the nonlinear refraction are of interest in modeling nonlinear optical materials and determination of structure/properties relations. In the past, most of these experiments were performed at a single wavelength. Tuning over a broad spectral region can provide with a comprehensive information but, often times, one has to perform calibration of the experiment at each wavelength. An alternative is to use a white-light continuum produced by focusing short pulsed lasers into suitable media (e.g. sapphire) and using a pump-probe geometry where the probe is the continuum and the pump is an intense pulse that induces the nonlinearity. We have developed a femtosecond continuum spectrometer to measure nondegenerate spectra from 300 nm in the UV to 1.7 /spl mu/m in the IR. The pump beam is the output of a regeneratively amplified fiber ring oscillator at 775 nm or the output of a subsequent optical parametric generator/amplifier producing wavelengths from 500 nm to 1650 nm. The continuum is produced by focusing /spl sim/2 /spl mu/J of different frequencies from these two sources. Dual diode arrays for the IR and visible are used to measure the spectral changes in the transmittance against a reference beam. The spectra can be obtained as a function of time delay between pump and probe pulses. This method is applied for measuring nonlinear absorption spectra of organic materials.

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.

The light self-diffraction in the case of the high modulated phase gratings induced in semiconductor materials is studied by solving numerically the set of the coupled wave equations, which were used previously for gratings produced by linear phenomena. We have extended this formalism to include the case of the absorbing materials and the nonuniformity of the interfering laser beams, which induce the gratings. Finally, we compare this description with our experimental data obtained in the self-diffraction on the phase gratings induced in crystalline silicon wafers by Nd:YAG laser beams. The experimental results are fitting better our theoretical description.

The bulk damage performance of potassium dihydrogen phosphate crystals under simultaneous exposure to 1064, 532, and 355nm nanosecond-laser pulses is investigated in order to probe the mechanisms governing damage initiation during frequency conversion. The results provide insight into the dependence on laser parameters of the defect-assisted excitation process responsible for coupling of the laser energy into the lattice under exposure to high power laser light. In addition, it is suggested that the damage performance can be directly related to and predicted from the damage behavior of the crystal at each wavelength separately.

Laser-induced initiation of bulk damage sites in KDxH2−xPO4 crystals is investigated under simultaneous exposure to 532 and 355nm nanosecond laser pulses in order to simulate the operational conditions during harmonic conversion as well as probe the damage mechanisms. The results demonstrate synergetic damage effects under the dual-wavelength excitation. Furthermore, the damage performance is directly related to and can be predicted from the damage performance at each wavelength separately. The measured relative effective absorption coefficients at the two wavelengths are found to depend on the laser fluence.