This PDF file contains the front matter associated with SPIE Proceedings Volume 7504, including the Title Page, Copyright information, Table of Contents, the International Program Committee, Symposium Welcome, and the Summary of Meeting listing.

A film of hafnium oxide, doped with 5 atomic % nitrogen, was prepared by dual-ion-beam-assisted deposition. The properties were compared to a pure hafnium oxide film. The damage threshold is lower for the nitrogen-doped film. However, the multiple-pulse damage threshold for a 1 kHz train of 800 fs pulses shows no drop relative to the singlepulse value. These results are discussed within the context of a multiple-damage model, based on midgap trapping states.

The single pulse femtosecond laser induced damage threshold (LIDT) of hafnia and silica films is not affected by the ambient gas pressure. In vacuum, the multiple pulse LIDT drops to ~10% (~10%) of its atmospheric value for hafnia (silica). The water vapor content of the ambient gas was found to control the change in the LIDT. The LIDT of bulk fused silica surfaces did now show any dependence on the ambient gas pressure. Hydrocarbons (toluene) did not change the multiple pulse LIDT for Hafnia films

AlN films deposited on sapphire substrates were damaged by single UV nanosecond (at 248 nm) and IR femtosecond (at 775 nm) laser pulses in air at normal pressure. The films had high (27-35 atomic %) concentration of oxygen introduced into thin surface layer (5-10 nm thickness). We measured damage threshold and studied morphology of the damage sites with atomic force and Nomarski optical microscopes with the objective to determine a correlation between damage processes and oxygen content. The damage produced by nanosecond pulses was accompanied by significant thermal effects with evident signatures of melting, chemical modification of the film surface, and specific redistribution of micro-defect rings around the damage spots. The nanosecond-damage threshold exhibited pronounced increase with increase of the oxygen content. In contrast to that, the femtosecond pulses produced damage without any signs of thermal, thermo-mechanical or chemical effects. No correlation between femtosecond-damage threshold and oxygen content as well as presence of defects within the laser-damage spot was found. We discuss the influence of the oxygen contamination on film properties and related mechanisms responsible for the specific damage effects and morphology of the damage sites observed in the experiments.

Replacing growing damage sites with benign, laser damage resistant features in multilayer dielectric films may enable large mirrors to be operated at significantly higher fluences. Laser damage resistant features have been created in high reflecting coatings on glass substrates using femtosecond laser machining. These prototype features have been damage tested to over 40 J/cm² (1064nm, 3ns pulselength) and have been shown not to damage upon repeated irradiation at 40J/cm². Further work to optimize feature shape and laser machining parameters is ongoing.

As optical coatings are deployed in more extreme environments and applications, mechanical and environmental robustness must be taken into account when designing the film(s). Even minor degradation of the film structure from these outside factors can affect final fluence handling capabilities in operation, and limit the life of the coating. We present the results of a study of maximum thermal handling capabilities of Broad-Band IR Anti-Reflective coatings in the mid-IR (3 to 5 micron) regime. We prepared a family of coated optics utilizing different coating material sets on different substrate materials, and exposed them to a range of increasing temperatures. We examined the damage morphologies under dark field, bright field, and Nomarski microscopy.

We discuss the physical and optical properties of Sc₂O₃ single layers deposited by the dual ion beam sputtering technique at oxygen partial pressures ranging from 1.7×10^-5 to 5.1×10^-5 Torr. The films are amorphous with crystallite size ~10 nm and have surface roughness RMS values of 1.2±0.3 nm. The refractive index at 1 μm is 1.95. Absorption loss is shown to be sensitive to the oxygen partial pressure during growth. Multiple-pulse damage experiments suggest that the scandia film deposited at the higher oxygen partial pressure accumulates laser-induced trap defects more slowly than the scandia film deposited in a lower oxygen partial pressure atmosphere.

Experimental investigation of the laser conditioning efficiency by nanosecond pulses at 266 and 355 nm in high reflectivity mirrors used in optical parametric oscillators (OPOs) is present in this report. The high reflection coatings were deposited on the fused silica substrates. The materials used for e-beam coating deposition were ZrO₂ and SiO₂. Laser conditioning was investigated as function of number of pulses, wavelength and conditioning protocol. Ramped-fluence pre-exposure was used as a method to explore optimal improvement to the damage performance at 266 and 355 nm. No conditioning effect was observed using nanosecond pulses at 266 nm, but the mirror conditioning with 355 nm pulses increased the laser-induced damage threshold (LIDT) by 2.5-3 times. The experimental results support the concept that the laser conditioning effect observed in high quality optical thin films is associated with intrinsic electronic defects in the films.

High reflectance mirrors, using fluoride coating materials, have been irradiated for extended time periods by a 193 nm kilohertz repetitive laser source. This irradiation promoted a spectral shift in the reflectance band towards shorter wavelength. In efforts to determine the mechanism for the observed spectral shifts, various models were investigated by employing such techniques as: spectrophotometry, surface profile interferometry, coating design simulation and x-ray diffraction. The result of the investigation indicates that layers near the top surface of the coating structure underwent densification which resulted in the observed spectral shift.

Millijoule-energy femtosecond laser pulses at high repetition rates constitute major workhorses for nonlinear optics and ultrafast science. The evolution of dielectric multilayer mirror technology within last 15 years allows one to demonstrate low-loss and dispersion-controlled optics. Unfortunately, dispersive optics has lower laser damage threshold in comparison to standard quarter-wave stacks. Nevertheless only multilayer optics is able to support high energy laser pulses together with capability to impart almost any spectral or phase feature on the reflected light. There exist only a limited number of materials suitable for deposition of UV-Vis-IR mirrors. The highest damage threshold was demonstrated with alternating HfO2/SiO2 layers deposited by electron beam without ion-assistance. Utilizing performance of this materials pair we present a novel design approach that allows us to increase the damage threshold of dispersive UV-Vis-IR optics. It is based on the reduction of the electric field inside the multilayer stack.

The stability of thin film coatings for applications especially in the UV spectral range is oftentimes a limiting factor in the further development of radiation sources and beam delivery systems. Particularly, functional coatings on laser and conversions crystals as well as resonator mirrors show an insufficient lifetime due to laser-induced degradation. Previous investigations in the power handling capability of UV coatings mostly concentrate on the properties of pure oxide materials and particle mitigation. Recent innovations in ion beam sputtering technology enabled efficient deposition of mixture coatings of different oxide materials. In combination with an advanced thickness monitoring equipment, the described IBS deposition systems are capable of employing designs with sub-layers of a few nm thickness. In the present investigation, the stability of classical designs using pure oxide materials is compared with gradient index design concepts based on mixture materials. Reflecting and transmitting thin film coatings employing classical and gradient index approaches manufactured under comparable conditions are characterized in respect to their power handling capability. The results are analyzed before the background of theoretical expectations regarding contributions from field enhancement and absorptance effects.

Sandia's Large Optics Coating Operation provides laser damage resistant optical coatings on meter-class optics required for the ZBacklighter Terawatt and Petawatt lasers. Deposition is by electron beam evaporation in a 2.3 m × 2.3 m × 1.8 m temperature controlled vacuum chamber. Ion assisted deposition (IAD) is optional. Coating types range from antireflection (AR) to high reflection (HR) at S and P polarizations for angle of incidence (AOI) from 0° to 47°. This paper reports progress in meeting challenges in design and deposition of these high laser induced damage threshold (LIDT) coatings. Numerous LIDT tests (NIF-MEL protocol, 3.5 ns laser pulses at 1064 nm and 532 nm) on the coatings confirm that they are robust against laser damage. Typical LIDTs are: at 1064 nm, 45° AOI, Ppol, 79 J/cm² (IAD 32 layer HR coating) and 73 J/cm² (non-IAD 32 layer HR coating); at 1064 nm, 32° AOI, 82 J/cm² (Ppol) and 55 J/cm² (Spol ) (non-IAD 32 layer HR coating); and at 532 nm, Ppol, 16 J/cm² (25° AOI) and 19 J/cm² (45° AOI) (IAD 50 layer HR coating). The demands of meeting challenging spectral, AOI and LIDT performances are highlighted by an HR coating required to provide R > 99.6% reflectivity in Ppol and Spol over AOIs from 24° to 47° within ~ 1% bandwidth at both 527 nm and 1054 nm. Another issue is coating surface roughness. For IAD of HR coatings, elevating the chamber temperature to ~ 120 °C and turning the ion beam off during the pause in deposition between layers reduce the coating surface roughness compared to runs at lower temperatures with the ion beam on continuously. Atomic force microscopy and optical profilometry confirm the reduced surface roughness for these IAD coatings, and tests show that their LIDTs remain high.

In this study, fused silica substrates were etched to different depths, ranging from about 200 nm to 600 nm. And then single-layer, anti-reflection (AR) and high reflection (HR) coatings were deposited to the etched substrates using E-beam evaporation. It was found that 355nm laser induced damage thresholds (LIDTs) of single-layer and AR coatings on the etched substrates were much higher than that on un-etched substrates, and the existing of the top 200 nm of the substrate was the most important factor influencing the damage behaviors of single-layer and AR coatings. Step Profiler was employed in measuring the depth of damage sites in the coatings deposited on the un-etched substrates. It showed that laser damage was initiated from the coating-substrate interface for single-layer and AR coatings, while from the inside of coating stack for HR samples. On the basis of above results, we concluded that subsurface defects in the redeposition layer of substrate could be a serious factor that lowered the 355nm LIDTs of single-layer and AR coatings.

This paper presents a study of the laser damage threshold (LDT) of ion beam sputtered (IBS) Ta₂O₅/SiO₂ AR coated and uncoated samples ranging from low grade plate glass through neutral density glass to laser grade fused silica substrates. We find that the damage threshold varies by just over a factor of 3 from 9 to 33 J/cm² for 1064nm pulsed laser light with 20ns pulse length and 20 Hz repetition rate. The damage threshold of the uncoated samples ranges from 80 J/cm² to greater than 200 J/cm². We have estimated the single pulse temperature increase in the uncoated neutral density glasses to be as much as 400-500°C at the damage fluence. This is unexpected as multiple pulse exposure would rapidly bring the glass up to melting temperatures. A possible explanation is saturation of the absorption at high fluence levels resulting in lower temperature than estimated. We conclude that the absorption level in the IBS coating is too low to give a significant contribution to the LDT of the AR coated samples.

Advantages of utilizing ultralow refractive index layers in optical coatings include broadband and wide angle spectral performance. Ultralow refractive index films can be fabricated by chemical method and physical vapor deposition. In the chemical method, ultralow refractive indices are controlled by sol-gel derived nanostructures. In the physical method, ultralow refractive indices are obtained by self-shadowing nature in oblique angle deposition. Techniques for ultralow refractive index silica formation and characterizations are reviewed. Chemical, mechanical and optical properties of the ultralow refractive index silica are presented. Optical coatings consisting of the ultralow refractive index silica layers are discussed in a spectral regime from IR down to DUV.

Ultrafast laser-induced variations of electron energy bands of transparent solids significantly influence ionization and conduction-band electron absorption driving the initial stage of laser-induced damage (LID). The mechanisms of the variations are attributed to changing electron functions from bonding to anti-bonding configuration via laser-induced ionization; laser-driven electron oscillations in quasi-momentum space; and direct distortion of the inter-atomic potential by electric field of laser radiation. The ionization results in the band-structure modification via accumulation of broken chemical bonds between atoms and provides significant contribution to the overall modification only when enough excited electrons are accumulated in the conduction band. The oscillations are associated with modification of electron energy by pondermotive potential of the oscillations. The direct action of radiation's electric field leads to specific high-frequency Franz-Keldysh effect (FKE) spreading the allowed electron states into the bands of forbidden energy. Those processes determine the effective band gap that is a laser-driven energy gap between the modified electron energy bands. Among those mechanisms, the latter two provide reversible band-structure modification that takes place from the beginning of the ionization and are, therefore, of special interest due to their strong influence on the initial stage of the ionization. The pondermotive potential results either in monotonous increase or oscillatory variations of the effective band gap that has been taken into account in some ionization models. The classical FKE provides decrease of the band gap. We analyzing the competition between those two opposite trends of the effective-band-gap variations and discuss applications of those effects for considerations of the laser-induced damage and its threshold in transparent solids.

In the last decade, investigations in fs-laser damage mechanisms have proven that the interaction of ultra-short laser pulses with dielectric matter is driven by electronic processes. This fact leads to a deterministic damage behavior and a well defined dependency of the laser-induced damage threshold (LIDT) on the absorption gap of the dielectric optical material. These facts have been considered in detail for dielectric thin films. Additionally, numerous investigations in filamentation, supercontinuum generation (SCG) and optical breakdown in bulk material have been published. The phenomena are also traced back to the interaction of ultra-short laser pulses with dielectric material. In contrast to thin films, these effects are based on a large interaction length of the beam with the dielectria. Consequently, additional nonlinear effects can contribute to the damage process of bulk materials. In the presented work, nonlinear absorption and the LIDT of fused silica are investigated. The influence of the propagation distance of the laser beam in the solid was in the focus of interest for an experiment on a series of fused silica samples of identical material and different thickness. The results of this study show that nonlinear absorption and laser-induced damage strongly vary in dependence of the sample thickness. The variations in the damage threshold range over more than one order of magnitude.

The influence of Na stabilized F and M centers on the DUV absorption behavior of CaF₂ is comparatively studied for nanosecond and femtosecond laser pulses by in-situ transmission and laser induced fluorescence measurements. For 193 nm nanosecond pulses the steady state transmission of ArF laser pulses through CaF₂ is measured in dependence on the incident fluence H ≤ 10 mJ cm^-2 pulse^-1. The related absorption coefficients α^st(H) are proportional to H and rationalized by effective 1- and 2-photon absorption coefficients α^eff and β^eff, respectively. The α^eff and β^eff values increase with the Na content of the CaF₂ samples as identified by the fluorescence of Na related M_Na centers at 740 nm. This relation is simulated by a complex rate equation model describing the ArF laser induced M_Na generation and annealing. M_Na generation starts with intrinsic 2-photon absorption in CaF₂ yielding self-trapped excitons (STE). These pairs of F and H centers can separate upon thermal activation and the F centers combine with F_Na to form M_Na centers. M_Na annealing occurs by its photo dissociation into a pair of F and F_Na centers. Comparative transmission measurements with DUV femtosecond pulses are done using the fourth harmonic of a Ti:Safs- laser at 197 nm. The resulting β_eff values virtually show no dependence on the MNa center concentration. Furthermore, the absolute β_eff values are lower by a factor of three compared to those obtained for nanosecond pulses. This is explained by additional two-step absorption for nanosecond pulses after formation of self-trapped excitons (STE).

Single crystal calcium fluoride (CaF₂) is an important lens material in deep-ultraviolet optics, where it is exposed to high radiation densities. The known rapid damage process in CaF₂ upon ArF laser irradiation cannot account for irreversible damage after long irradiation times. We use density functional methods to calculate the properties of laser-induced point defects and to investigate defect stabilization mechanisms on a microscopic level. The mobility of the point defects plays a major role in the defect stabilization mechanisms. Besides stabilization by impurities, we find that the agglomeration of F-centers plays a significant role in long-term laser damage of CaF₂. We present calculations on the stability of defect structures and the diffusion properties of the point defects.

The design of high-energy laser windows critically depends on the availability of appropriate numbers for the allowable tensile stress. Relying on a "modulus of rupture" in conjunction with a "safety factor" usually results in overestimating the required thickness, which degrades the optical performance. The primary purpose of this paper is to clarify issues relating to Weibull's theory of brittle fracture and make use of the theory to assess the results of equibiaxial flexure testing that was carried out on laser-window material candidates. Specifically, we describe the failure-probability distribution in terms of the characteristic strength σ_C--i.e., the effective strength of a uniformly stressed 1-cm² area---and the shape parameter m, which reflects the dispersion of surface-flaw sizes. A statistical analysis of flexural strength data thus amounts to obtaining the parameters σ_C and m, which is best done by directly fitting estimated cumulative failure probabilities to the appropriate expression derived from Weibull's theory. In this light, we demonstrate that (a) at the 1% failure-probability level, fusion-cast CaF₂ and OxyFluoride Glass perform poorly compared to CVD-ZnSe; (b) available data for fused SiO₂ and sapphire confirm the area-scaling principle, thus validating Weibull's theory; and (c) compressive coatings enhance the characteristic strength but degrade the shape parameter, which mitigates their benefit. In Appendix, it is shown that four-point bending data for fusion-cast CaF₂ do not obey a simple two-parameter model but are indicative of a bimodal surface-flaw population.

Fluoride-based wet chemical etching of fused silica optical components is useful to open up surface fractures for diagnostic purposes, to create surface topology, and as a possible mitigation technique to remove damaged material. To optimize the usefulness of etching, it is important to understand how the morphology of etched features changes as a function of the amount of material removed. In this study, we present two geometric etch models that describe the surface topology evolution as a function of the amount etched. The first model, referred to as the finite-difference etch model, represents the surface as an array of points in space where at each time-step the points move normal to the local surface. The second model, referred to as the surface area-volume model, more globally describes the surface evolution relating the volume of material removed to the exposed surface area. These etch models predict growth and coalescence of surface fractures such as those observed on scratches and ground surfaces. For typical surface fractures, simulations show that the transverse growth of the cracks at long etch times scales with the square root of etch time or the net material removed in agreement with experiment. The finite-difference etch model has also been applied to more complex structures such as the etching of a CO₂ laser-mitigated laser damage site. The results indicate that etching has little effect on the initial morphology of this site implying little change in downstream scatter and modulation characteristics upon exposure to subsequent high fluence laser light. In the second part of the study, the geometric etch model is expanded to include fluid dynamics and mass transport. This later model serves as a foundation for understanding related processes such as the possibility of redeposition of etch reaction products during the etching, rinsing or drying processes.

Removal of laser-induced damage sites provides a possible mitigation pathway to improve damage resistance of coated multilayer dielectric mirrors. In an effort to determine the optimal mitigation geometry which will not generate secondary damage precursors, the electric field distribution within the coating layers for a variety of mitigation shapes under different irradiation angles has been estimated using the finite difference time domain (FDTD) method. The coating consists of twenty-four alternating layers of hafnia and silica with a quarter-wave reflector design. A conical geometrical shape with different cone angles is investigated in the present study. Beam incident angles range from 0° to 60° at 5° increments. We find that light intensification (square of electric field, |E|²) within the multilayers depends strongly on the beam incident direction and the cone angle. By comparing the field intensification for each cone angle under all angles of incidence, we find that a 30° conical pit generates the least field intensification within the multilayer film. Our results suggest that conical pits with shallow cone angles (≤ 30°) can be used as potential optimal mitigation structures.

Many investigations in laser induced damage thresholds (LIDT) have shown a very deterministic behavior for the ultra short pulse regime. From the current understanding, femtosecond laser damage is driven by electronic photon-matter interaction. This process can be theoretically described by photo-, and avalanche-ionization and the respective relaxation processes. On the basis of a wavelength dependency of the laser-induced damage threshold of titania, a dominant influence of multi-photon absorption (MPI) on the damage behavior could be demonstrated. This particular characteristic could be observed in a stepwise increase of the LIDT in the transition ranges of the orders of multi-photon absorption. This paper presents an analysis of the wavelength dependence of femtosecond LIDT with different theoretic models and a comparison of simulated data with the measured wavelength dependence of titania. Both, the wavelength position and the quantitative change of the laser power resistance in the range of the n to the n+1 photon absorption are calculated in a theoretical analysis. Additionally, the influence of different microstructures of titania on the quantized MPIcharacteristic is investigated.

Digital holography (DH) technique allows obtaining quantitative amplitude- and phase-contrast images. However, there are only several attempts to use advantages of digital holography in time resolved manner. Herewith we demonstrate that combination of conventional off-axis DH and ultrashort probing laser pulses results in versatile tool suitable for ultra-fast phenomena studies. We demonstrate its applications in spatial-temporal characterization of ionizing pre-damage propagation of femtosecond pulses in transparent material as well as characterization of post-damage ablation processes in metals. Phase-and amplitude-contrast images are obtained in single-shot mode with the best to our knowledge temporal resolution of ≈ 30 fs.

In this contribution we will summarize the fundamental mechanisms that lead to subpicosecond laser damage in dielectric films, discuss the resulting scaling laws of single pulse (1-on-1) damage with respect to pulse duration and bandgap, of the multiple pulse (S-on-1) damage threshold as a function of pulse number, and compare these findings to recent experimental results.

A new instrument dedicated to laser damage measurement in subpicosecond scale has been developed at the Fresnel Institute (3 ps to 100 fs, 1030 nm). The objective of this work is to realize a comparative study of the behavior of hafnia thin films prepared by different techniques (Reactive Low Voltage Ion Plating, Electron Beam Deposition, Dual Ion Beam Sputtering) under subpicosecond pulse irradiation in the near infra red. Laser-induced damage thresholds are measured for one-on-one procedures. Laser damage setup and first results at 1 ps are presented and initiation mechanisms are studied thanks to damage morphologies and optical properties characterization. Results show a dependence of damage threshold with deposition techniques and so with microstructure of the film.

A Petawatt facility called PETAL (PETawatt Aquitaine Laser) is under development near the LIL (Ligne d'Integration Laser) at CEA Cesta, France. PETAL facility uses chirped pulse amplification (CPA) technique [1]. This system needs large pulse compression gratings exhibiting damage threshold of more than 4 J/cm² beam normal at 1.053μm and for 500fs pulses. In this paper, we study an alternative design to the classic Multidielectric (MLD) gratings [2] called "mixed metal-multidielectric grating" (MMLD). In this design, the dielectric mirror stack of the MLD grating is replaced by a gold reflective layer covered with a few pairs of HfO₂/SiO₂ [3]. The number of pairs must be high enough to ensure a sufficient reflection coefficient in order to prevent damage of the gold layer. On the top of the stack, a silica layer is coated to receive the grating. After some considerations on the grating design and optimization, a comparison between MLD and MMLD mirrors is also carried out. We finally detail the measured diffraction efficiencies obtained on MMLD gratings.

In order to determine the current status of thin film laser resistance within the private, academic, and government sectors, a damage competition was started at the 2008 Boulder Damage Symposium. This damage competition allows a direct comparison of the current state of the art of high laser resistance coatings since they are tested using the same damage test setup and the same protocol. In 2009 a high reflector coating was selected at a wavelength of 786 nm at normal incidence at a pulse length of 180 femtoseconds. A double blind test assured sample and submitter anonymity so only a summary of the results are presented here. In addition to the laser resistance results, details of deposition processes, coating materials and layer count, and spectral results will also be shared.

As a consequence of the ongoing interest for deployment of laser systems into space, suitable optical components have to be developed and must be extensively space qualified to ensure reliable, continuous, and autonomous operation. The exposure to space environment can adversely affect the longevity of optics, mainly coatings, and lead to system degradation. An increased operational risk is due to the air-vacuum effect, which can strongly reduce the laser damage resistance of optical coatings. For this purpose, a vacuum laser damage test bench has been developed and is operated at DLR. In extensive test campaigns, all damage-prone optics of the ALADIN laser system (being the laser source of the upcoming ESA ADM Aeolus mission) were tested under operative conditions at the fundamental and at the harmonic wavelengths of Nd:YAG. Further operational risks are due directly to operation under high vacuum. In the past, several space-based laser missions have suffered from anomalous performance loss or even failure after short operation times. This degradation is due to selective contamination of laser-exposed optical surfaces fed by outgassing constituents. These volatile components are omnipresent in vacuum vessels. Various organic and inorganic species were tested at our facilities for their criticality on deposit built-up. Finally, active optical components like Q-switch crystals or frequency converter crystals can also suffer from bulk absorption induced by high-energy radiation (gray tracking) and dehydration. To analyze these effects, an ultrahigh vacuum phase matching unit was set up to test various combinations of SHG and THG frequency converters.

Chirped pulse amplification [CPA] has been implemented on laser facilities to produce high irradiance conditions in a vacuum chamber for the study of novel plasma physics processes. When such focussed laser beams interact with solid targets, the material is disrupted and leads to ejecta as solids, liquids, vapours and radiation. These target by-products can degrade and damage optical or diagnostic surfaces in the interaction chamber. This paper describes the effects of such emissions on surfaces by the use of metrology, microscopy, image processing, fragment capture in foams and autoradiography. Target plume divergence is discussed so that precautions for future experiments can be evaluated.

The impact of molecular contamination on the lifetime of fused silica UV optics used in high power laser facility is studied. In our particular case corresponding to Laser MégaJoule (LMJ) beams, the irradiation conditions are a fluence higher than 10 J/cm² at a wavelength of 351 nm (3 ω) for 3 ns pulse duration and a single shot/day frequency. A confine environment, a long period of exposition and proximity of the optical components with outgassing materials are critical parameters for the optics contamination. Consequently, experiments were performed in the UV section of the Ligne d'Intégration Laser (LIL), actual prototype of the LMJ. Moreover, the optics storage conditions were studied. Indeed, to ensure an efficient replacement of the optics on the laser bundle, many optical components are fabricated long before. They are stored during months in polypropylene frames put in containers. Then, we intentionally contaminated silica samples with one sort of polypropylene. We evidenced an important increase of laser induced damage density on samples contaminated by both bundle and storage environments. Surface analyses have been used to identify the potential causes of this effect. Various hypotheses of damage mechanisms are proposed.

Recent works have shown that for low contaminants level, damage density is independent of the amount of contaminants. In these conditions, sub-surface defects (cracks), generated along the optical process, is considered as the main source of damage. Hence, efforts have been made to improve SSD measurement in order to improve its suppression during industrial process. We have developed a method to measure SSD depth which is detailed in this presentation. This method is based on successive acid etching steps. The principle is to establish contamination level (ICP-AES measurement) as a function of etched thickness of SiO₂. The experimental setup has been specially designed to minimize contaminations, reduce hydrofluoric acid quantities and to ease the etch rate determination. SSD depth is given by the asymptotic impurities. This method has been applied to a grinded fused silica intentionally doped in barium tracer. Results have been successfully compared to other characterization techniques such as MRF dimpling or empirical law correlating SSD and surface roughness.

There is a longstanding, and largely unexplained, correlation between the laser damage susceptibility of optical components and both the surface quality of the optics, and the presence of near surface fractures in an optic. In the present work, a combination of acid leaching, acid etching, and confocal time resolved photoluminescence (CTP) microscopy has been used to study laser damage initiation at indentation sites. The combination of localized polishing and variations in indentation loads allows one to isolate and characterize the laser damage susceptibility of densified, plastically flowed and fractured fused silica. The present results suggest that: 1) laser damage initiation and growth are strongly correlated with fracture surfaces, while densified and plastically flowed material is relatively benign, and 2) fracture events result in the formation of an electronically defect rich surface layer which promotes energy transfer from the optical beam to the glass matrix.

Subsurface cracks in fused silica optics are known to be damage initiators under laser irradiation. Each step of optic production, from sawing to polishing, creates its own type of cracks. An efficient optic manufacturing process requires that each production step removes cracks from the previous step. The extent of cracks has to be measured for each production step. We review and compare different subsurface damage (SSD) characterization techniques applied to ground and fine ground fused silica samples.

We investigate the residual stress field created near mitigated sites and its influence on the efficiency on the CO2 laser mitigation of damage growth process. A numerical model of CO2 laser interaction with fused silica is developed that take into account laser energy absorption, heat transfer, thermally-induced stress and birefringence. Specific photoelastic methods are developed to characterize the residual stress near mitigated sites in fused silica samples. The stress distribution and quantitative values of stress levels are obtained for sites treated with the CO2 laser in various conditions of energy deposition (beam size, pulse duration, incident power). The results obtained also show that the presence of birefringence/residual stress around the mitigated sites has a critical effect on their laser damage resistance.

Localized damage repair and polishing of silica-based optics using mid- and far-IR CO₂ lasers has been shown to be an effective method for increasing optical damage threshold in the UV. However, it is known that CO₂ laser heating of silicate surfaces can lead to a level of residual stress capable of causing critical fracture either during or after laser treatment. Sufficient control of the surface temperature as a function of time and position is therefore required to limit this residual stress to an acceptable level to avoid critical fracture. In this work we present the results of 351 nm, 3ns Gaussian damage growth experiments within regions of varying residual stress caused by prior CO₂ laser exposures. Thermally stressed regions were non-destructively characterized using polarimetry and confocal Raman microscopy to measure the stress induced birefringence and fictive temperature respectively. For 1~40s square pulse CO₂ laser exposures created over 0.5-1.25kW/cm² with a 1-3mm 1/e² diameter beam (T_max~1500-3000K), the critical damage site size leading to fracture increases weakly with peak temperature, but shows a stronger dependence on cooling rate, as predicted by finite element hydrodynamics simulations. Confocal micro-Raman was used to probe structural changes to the glass over different thermal histories and indicated a maximum fictive temperature of 1900K for T_max≥2000K. The effect of cooling rate on fictive temperature caused by CO₂ laser heating are consistent with finite element calculations based on a Tool-Narayanaswamy relaxation model.

Small micrometer-sized roughness on optical surfaces, caused by laser damage and/or redeposition of laser ablated material, can cause local electric field intensification which may lead to damage initiation both on the optics and/or downstream. We examined the smoothing of etched periodic surface structures on SiO₂ substrate with 10.6μm CO₂ laser using atomic force microscopy. The characteristic surface tension driven mass flow of the glass under different laser parameters were simulated using computational fluid dynamics and correlated with experimental results. We found that during CO₂ laser polishing the estimate viscosity of the silica glass appears to be higher than typical literature values measured at a temperature similar to the laser heating conditions. This discrepancy can be explained by the observation that at high temperature, a significant portion of the hydroxyl content in the layer of heated silica glass can diffuse out resulting in a much stiffer glass.

Past work in the area of laser-induced damage growth has shown growth rates to be primarily dependent on the laser fluence and wavelength. More recent studies suggest that growth rate, similar to the damage initiation process, is affected by a number of additional parameters including pulse duration, pulse shape, site size, and internal structure. In this study, we focus on the effect of pulse duration on the growth rate of laser damage sites located on the exit surface of fused silica optics. Our results demonstrate, for the first time, a significant dependence of growth rate at 351 nm on pulse duration from 1 ns to 15 ns as τ^0.3 for sites in the 50-100 μm size range.

Over the past year we have been working on specialized MR fluids for polishing KDP crystals. KDP is an extremely difficult material to conventionally polish due to its water solubility, low hardness, and temperature sensitivity. Today, KDP crystals are finished using single-point diamond turning (SPDT) tools and nonaqueous lubricants/coolants. KDP optics fabricated using SPDT, however, are limited to surface corrections due to tool/method characteristics with surface quality driven by microroughness from machine pitch, speed, force, and diamond tool character. MRF polishing offers a means to circumvent many of these issues since it is deterministic which makes the technique practical for surface and transmitted wavefront correction, is low force, and is temperature independent. What is lacking is a usable nonaqueous MR fluid that is chemically and physically compatible with KDP which can be used for polishing and subsequently cleaned from the optical surface. In this study, we will present the fluid parameters important in the design and development of nonaqueous MR fluid formulations capable of polishing KDP and how these parameters affect MRF polishing. We will also discuss requirements peculiar to successful KDP polishing and how they affect optical figure/finish and laser damage performance at 1064 nm and 532 nm.

We have been developing tools for nonlinear spectroscopy aimed toward the ultimate goal of building a nonlinear spectrophotometer analogous to the ubiquitous linear spectrophotometer where a sample is placed in the instrument, a button is pushed, and the absorption spectrum is obtained sometime later. This paper describes our progress toward this goal, describing many difficulties and complications as well as opportunities. We also show spectroscopic data and analysis of a variety of materials that we have taken with preliminary nonlinear spectroscopic instrumentation we have already developed. One of the more interesting observations obtained along this research path is the realization that linear dispersion theory can also be applied to nonlinear systems when formulated properly such that Kramers-Kronig relations can be used to connect the dispersion of nonlinear refraction to the spectrum of nonlinear absorption. In some circumstances this can be more easily applied to nonlinear systems than to linear systems since the nonlinear absorption spectrum can be limited in wavelength. In addition, we have developed tools that can simultaneously give the spectrum of nonlinear absorption as well as the dispersion of the nonlinear refraction over an octave spectral range from 400nnm to 800 nm, the so called White-Light-Continuum Z-scan. Much of the research on nonlinear optical materials has been a collaborative effort requiring the skills and expertise of organic chemists and materials manufacturers. The goals of this part of the research are to determine predictive structure-property relation capabilities. The database needed for this research makes the nonlinear spectrophotometer a necessity.

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.

KDP is a non linear optical crystal used for frequency conversion in high power applications. In this paper we present results of optical non-destructive measurements on different fast growth KDP crystals in order to characterize the precursors responsible for laser-induced damage. We performed two types of analysis: the first one is temporally resolved photoluminescence excited by a pulsed Nd:YAG laser at 4.66 eV and 3.49 eV (266 nm and 355 nm). The second is spatially resolved photothermal deflection pumped by a CW Argon laser at 3.53 eV (351 nm). With these two complementary techniques we highlight the presence of large scale inhomogeneities and we discriminate nano-scale defects statistically distributed in the bulk of the crystals.

Laser-induced damage on the surface of optical components typically is manifested by the formation of microscopic craters that can ultimately degrade the optics performance characteristics. It is believed that the damage process is the result of the material exposure to high temperatures and pressures within a volume on the order of several cubic microns located just below the surface. The response of the material following initial localized energy deposition by the laser pulse, including the timeline of events and the individual processes involved during this timeline, is still largely unknown. In this work we introduce a time-resolved microscope system designed to enable a detailed investigation of the sequence of dynamic events involved during surface damage. To best capture individual aspects of the damage timeline, this system is employed in multiple imaging configurations (such as multi-view image acquisition at a single time point and multi-image acquisition at different time points of the same event) and offers sensitivity to phenomena at very early delay times. The capabilities of this system are demonstrated with preliminary results from the study of exitsurface damage in fused silica. The time-resolved images provide information on the material response immediately following laser energy deposition, the processes later involved during crater formation or growth, the material ejecta kinetics, and overall material motion and transformation. Such results offer insight into the mechanisms governing damage initiation and growth in the optical components of ICF class laser systems.

In situ spatial and temporal surface temperature profiles of CO2 laser-heated silica were obtained using a long wave infrared (LWIR) HgCdTe camera. Solutions to the linear diffusion equation with volumetric and surface heating are shown to describe the temperature evolution for a range of beam powers, over which the peak surface temperature scales linearly with power. These solutions were used with on-axis steady state and transient experimental temperatures to extract thermal diffusivity and conductivity for a variety of materials, including silica, spinel, sapphire, and lithium fluoride. Experimentally-derived thermal properties agreed well with reported values and, for silica, thermal conductivity and diffusivity are shown to be approximately independent of temperature between 300 and 2800K. While for silica our analysis based on a temperature independent thermal conductivity is shown to be accurate, for other materials studied this treatment yields effective thermal properties that represent reasonable approximations for laser heating. Implementation of a single-wavelength radiation measurement in the semi-transparent regime is generally discussed, and estimates of the apparent temperature deviation from the actual outer surface temperature are also presented. The experimental approach and the simple analysis presented yield surface temperature measurements that can be used to validate more complex physical models, help discriminate dominant heat transport mechanisms, and to predict temperature distribution and evolution during laser-based material processing.

Laser induced damage can take several forms. Perhaps the most common is thermal breakdown, caused by heating of the coating to a catastrophic failure induced by local melting, delamination, evaporation, or some other change. We demonstrate that micromachined dielectric membranes show strong differences in their hydroxyl signatures as measured by Fourier transform infrared spectroscopy. The changes correspond to regions of high fluence (3200 J/cm²) from a Nd:YAG laser. It is found that the absorption peaks associated with O-H stretching mode decrease after laser treatment, indicating a reduction in the number of film hydroxyl groups.

Phase-defects on optics used in high-power lasers can cause light intensification leading to laser-induced damage of downstream optics. We introduce Linescan Phase Differential Imaging (LPDI), a large-area dark-field imaging technique able to identify phase-defects in the bulk or surface of large-aperture optics with a 67 second scan-time. Potential phase-defects in the LPDI images are indentified by an image analysis code and measured with a Phase Shifting Diffraction Interferometer (PSDI). The PSDI data is used to calculate the defects potential for downstream damage using an empirical laser-damage model that incorporates a laser propagation code. A ray tracing model of LPDI was developed to enhance our understanding of its phase-defect detection mechanism and reveal limitations.

The rasterscan test procedure implemented in order to determine low laser damage density of large aperture UV fused silica optics was improved in terms of accuracy and repeatability. Tests have been carried on several facilities using several pulse durations and spatial distributions. We describe the equipment, test procedure and data analysis to perform this damage test with small beams (Gaussian beams, about 1mm @ 1/e, and top hat beams). Then, beam overlap and beam shape are the two key parameters which are taken into account in order to determine damage density. After data analysis and treatment, a repeatable metrology has been obtained. Moreover, the consideration of error bars on defects distributions permits to compare data between these installations. This allows us to look for reproducibility, a necessary condition in order to share results and to make reliable predictions of laser damage resistance. For that, a careful attention has been paid to beam analysis.

Determination of absolute Laser-Induced Damage Threshold (LIDT) value from experimentally obtained statistical data is still very important metrological problem in terms of accuracy and repeatability. In fact experimentally estimated LIDTs are always affected by many factors such as (temporal, energy, pointing and beam shape) stability of laser pulses used for damage testing as well as properties (homogeneity and limited size) of the sample to be tested. These problems are especially important in case of small aperture limited samples when testing with nanosecond pulses where the mechanism of damage is usually driven by defects. Several known experimental techniques or its modifications (for example raster scan) are typically applied for LIDT estimation in 1-on-1 mode namely Damage Frequency Method (DFM) and General Binary Search Technique (GBST). Almost all methods lead to the same value of LIDT under ideal experimental conditions and sufficiently large number of interrogated test sites. The goal of this numerical study is to analyze the performance (accuracy and repeatability) of above mentioned algorithms with respect to damage limiting surface defect density under non-ideal experimental conditions: energetic instabilities of laser radiation and aperture limited to max. 300 sites. Herewith we also introduce "moving average" measurement concept. The conclusions are drawn about the precision of all above mentioned methods.

It has been well known that laser-induced damage in optical thin films depends on the applied test laser beam diameter, if the damage initiation is dominated by inclusions and defects. This correlation has been subject of numerous discussions and has also been implemented in the corresponding ISO standard (ISO 11254-2). There, a minimum beam diameter is advised which further reduction affects the result significantly. Previous work indicates, depending on sample properties, a constant damage threshold towards large beam diameters. However, experimental data for this behavior is limited. This paper presents a detailed investigation within the ns-timescale on a series of four comparable sample sets of HfO₂ /SiO₂ high reflectors. The respective samples are designed and manufactured for ISO damage testing at 1ω, 2ω, 3ω, and 4ω irradiation of the Nd:YAG laser.

Laser-Induced Damage Threshold (LIDT) measurements are typically performed in order to characterize the optical resistance of laser components. However the sensitivity of online damage detection techniques is often limiting factor for the accuracy and reproducibility of so called S-on-1 and 1-on-1 measurements. In fact the sensitivity of damage detection has the biggest impact to precision of these experiments. In this paper we describe the idea of making improvements on scattered light registration based damage detection. It was learned, that scattered light intensity is linearly proportional to incident energy of laser pulses while material is not damaged. However in case of induced damage the linear proportionality becomes nonlinear: this feature is used in order to detect optically induced surface and bulk changes in material. According to the base of those theoretical considerations the adaptive scattering detector was proposed and made up. It was put into practice by projecting it on discrete element schematics. The improved sensitivity of laserinduced damage detection was reached. This technique helps to avoid degradation of damaged site and pollution of surrounding area due to laser ablation during the S-on-1 tests since it allows blocking the repetitive irradiation immediately when damage appears. Numerous tests were made, that shows, that adaptive scattering detector can precisely detect damages in their initiation state, independently from material ability of light scattering. Calibration of this detector can be automated, therefore the influence of human factor is minimized. This fact opens up the possibility to run whole damage threshold measurement procedure automatically.

To enable laser-based radiography of high energy density physics events on the Z-Accelerator^[4,5] at Sandia National Laboratories, a facility known as the Z-Backlighter has been developed. Two Nd:Phosphate glass lasers are used to create x-rays and/or proton beams capable of this radiographic diagnosis: Z-Beamlet (a multi-kilojoule laser operating at 527nm in a few nanoseconds) and Z-Petawatt (a several hundred joule laser operating at 1054nm in the subpicosecond regime) ^[1,2]. At the energy densities used in these systems, it is necessary to use high damage threshold optical materials, some of which are poorly characterized (especially for the sub-picosecond pulse). For example, Sandia has developed a meter-class dielectric coating capability for system optics. Damage testing can be performed by external facilities for nanosecond 532nm pulses, measuring high reflector coating damage thresholds >80J/cm² and antireflection coating damage thresholds >20J/cm^{2 [3]}. However, available external testing capabilities do not use femtosecond/picosecond scale laser pulses. To this end, we have constructed a sub-picoseond-laser-based optical damage test system. The damage tester system also allows for testing in a vacuum vessel, which is relevant since many optics in the Z-Backlighter system are used in vacuum. This paper will present the results of laser induced damage testing performed in both atmosphere and in vacuum, with 1054nm sub-picosecond laser pulses. Optical materials/coatings discussed are: bare fused silica and protected gold used for benchmarking; BK7; Zerodur; protected silver; and dielectric optical coatings (halfnia/silica layer pairs) produced by Sandia's in-house meter-class coating capability.

Light scattering from interface imperfections or defects is a topic of persistent interest. On the one hand, light scattering can be a critical issue that limits throughput and imaging quality of optical components even if high-end polishing and coating techniques are employed. This in particular holds for multilayer coatings and for short wavelengths of application. On the other hand, scattered light also carries valuable information about its origins, such as the structural properties of multilayer coatings. A large variety of instruments for scatter measurements in the visible, ultraviolet, infrared spectral ranges has been developed at the Fraunhofer IOF in Jena during the past years. In addition, software tools have been developed based on existing scattering theories which enable an advanced analysis of the observed scattering properties. For the rather complicated scattering of thin film coatings, a simplified model was developed recently which introduces two simple parameters to describe the roughness evolution from interface to interface and optical thickness deviations. An iterative solution of the inverse scattering problem yields information about alterations of the structural and optical properties inside multilayer coatings based on angle resolved scatter measurements. Experimental results will be presented for highly reflective metal fluoride coatings for 193 nm as well as for a Rugate filter after laser damage tests. It will be discussed how the procedure could be implemented in a laser damage test environment to enable the damage behavior to be characterized in-situ during irradiation. This can provide valuable information about the fundamental damage processes even prior to the ultimate damage events.

This short paper lays out the goals for a new series of investigations into the proper means of estimating the lifetime of an optic exposed to many, S, laser pulses. The problem of properly identifying a means to estimate component life as important, interesting and being needed is discussed. The fundamental issue in estimation of life is the extrapolation from small to large values of S, frequently this extrapolation is many orders of magnitude. The need for a universal fitting function to enable this extrapolation is the seminal point of these investigations. A key question is the universality of the form of the fitting function. This paper makes the case that such a fitting function requires experimental confirmation. The paper concludes with a call for colleagues to provide such life test data to help in the search of the universal fitting function or functions.

In recent years, technological developments in the area of extreme ultraviolet lithography (EUVL) have experienced great improvements. Currently, the application of EUV radiation apart from microlithography comes more and more into focus. Main goal of our research is to utilize the unique interaction between soft x-ray radiation and matter for probing, modifying, and structuring solid surfaces. In this contribution we present a setup capable of generating and focusing EUV radiation. It consists of a table-top laser-produced plasma source. In order to obtain a small focal spot resulting in high EUV fluence, a modified Schwarzschild objective consisting of two spherical mirrors with Mo/Si multilayer coatings is adapted to this source, simultaneously blocking unwanted out-of-band radiation. By demagnified (10x) imaging of the plasma an EUV spot of 5 μm diameter with a maximum energy density of ~7.4 J/cm² is generated (pulse length 8.8 ns). We present first applications of this integrated source and optics system, demonstrating its potential for high-resolution modification and structuring of solid surfaces. As an example, etch rates for PMMA, PC and PTFE depending on EUV fluences were determined, indicating a linear etch behavior for lower energy densities. In order to investigate damage tests on EUV sensors and optics, 1-on-1 damage tests were performed on grazing incidence gold mirrors, Mo/Si multilayer mirrors and mirror substrates. To our knowledge, this is the first time that such experiments using nanosecond EUV-pulses were carried out.