Laser damage of large fused silica optics initiates at imperfections. Possible initiation mechanisms are considered. We demonstrate that a model based on nanoparticle explosions is consistent with the observed initiation craters. Possible mechanisms for growth upon subsequent laser irradiation, including material modification and laser intensification, are discussed. Large aperture experiments indicate an exponential increase in damage size with number of laser shots. Physical processes associated with this growth and a qualitative explanation of self-accelerated growth is presented. Rapid growth necessitates damage growth mitigation techniques. Several possible mitigation techniques are mentioned, with special emphasis on CO₂ processing. Analysis of material evaporation, crack healing, and thermally induced stress are presented.

Multilayer polymer interference mirrors are based on hundreds of 10- to 200-nm thick, simultaneously produced layers of high and low index materials. The low absorptivity of the polymers used in these mirrors lead to very high reflectivity and excellent wavelength selectivity. Currently, the mirrors are being applied to reflective polarizers for displays, and cold and hot mirrors for solar control. New products are being developed for applications in laser protection, optical communication, and optical filtering applications. The design of filters based on multilayer polymers has additional degrees of freedom compared to PVD filters, including the ability to specify Brewster's angle from anywhere from 0 to over 90 degree(s), suppressing side-band reflections, and suppressing 2nd, 3rd, and 4th order reflections.

The LIGO project has completed the installation of large fused silica optical components in the vacuum systems of its observatories. Commissioning work on the Hanford 2 km interferometer has determined an upper limit to the optics losses, allowing comparison with design and pre-installation testing. Planning and development of sapphire optics for the next generation, advanced LIGO detector is now underway, including polishability, optical homogeneity, absorption, and birefringence. The advanced optics development also includes research aimed at lowering coating loss.

The need for higher bandwidth in telecommunication systems is driving the development of higher power amplifiers, both EDFAs and Raman systems. This means the optical components incorporated in these amplifiers and systems must be able to withstand high power operation. In the 1400-1600-nm wavelength region, greater than 1 W of optical power is required, and in the 980-nm pump region greater than 500 mW is required. After exploring the applications for high power components, this paper discusses short and long-term failure modes that have been discovered to date. These failure modes are mainly concerned with the quality of the fiber tip and subsequent anti-reflection (AR) coatings. Component designs to eliminate these failures will be presented along with proposed tests to assure they can withstand these, and ever increasing high optical power requirements for reliable, long-term operation.

Planar waveguide devices play an important role in DWDM networking applications in term of highly integration and cost effective production. This talk will review the application of UV laser to modify the silica based planar waveguide device, such as Array Waveguide Grating (AWG) or other planar filters for multiplex and Demultiplex and Mach- Zehnder interferometer (MZI) based devices. Irradiation of UV laser to change local index of refraction in waveguides provides a unique way to perfect and enhance the function of planar waveguide devices. Also, the DWDM devices fabrication using combination of waveguide fabrication and UV laser processing will be reviewed.

A summary is presented of some of the issues facing the developers of tunable filters for use in optical communication systems, including those exploiting acousto- optic, thermo-optic and electro-optic effects. The potential of electromechanically tuned variants is also assessed in relation to devices micromachined from silicon. Emphasis is given to electro-optically tuned devices, with experimental data presented for an example based on a nanophase polymer dispersed liquid crystal composite.

Photothermal deflection is widely used to study defects in optical coatings and role of these defects in laser damage. Because defects responsible for laser damage are probably submicrometer sized, both high spatial resolution and high sensitivity are required to detect defects as small as possible. In this work we theoretically and experimentally explore the capability of collinear photothermal deflection to give submicrometric resolution by reduction of the pump beam diameter to one micrometer. We have developed a microscope based on the photothermal deflection of a transmitted probe beam and well-suited for multiscale studies of defects in thin films. The pump and probe beams are collinear and focused through the same optics, which can be chosen in order to change the diameter at 1/e² of the pump beam on the sample surface from 100 micrometers to 1 micrometers . We present our first results obtained on specially prepared absorption targets and show that a lateral spatial resolution lower than 1 micrometers is reached.

Technological processing of bulk silica is needed at many points of the fabrication of optical components. In the Laser Integration Line (LIL) and Megajoule Laser (LMJ), with the usual but challenging optical constraints of keeping the wavefront quality on large optics the constraint of high flux laser resistance is added. This has led to many technological improvements of silica processing which have been transferred into the industrial tissue. Improved polishing and cleaning processes have been developed which avoid the contamination of surface with polishing agents and are now used for lenses, windows, and substrates preparation of mirrors and polarizers. But some components like the gratings which are to be used on LIL and LMJ need new processing steps which are typical of the semiconductor industry and whose effects are unknown in terms of laser induced damage threshold in silica surface and subsurface. After a summary of the specifications and the performances of these gratings at 1 and 3(omega) wavelength we will focus on the laser induced damage (LID) tests that were performed at different stages of the grating process and see how they impact on the LID threshold of the gratings.

Contamination of surfaces prior to or during deposition of dielectric multilayers has been a classical subject of technological studies, but post deposition contamination has not been reported as often. Previous results typically dealt with sealed optics and exposition to laser or UV conditions, or on-orbit optical systems: we report here observations on HfO₂/SiO₂ mirrors designed for the Megajoules project, with very high laser resistance. Performances in terms of laser damage thresholds at 1.06 micrometers have been measured periodically over periods of several months, on different sets of samples. Comparison of R/1 LIDT distributions are presented versus time and storage conditions. Degradation of LIDT is clearly observed. Recovery methods are investigated.

Laser energy deposition and redistribution in metal nanoparticles embedded in SiO₂ glass is studied by a kinetic model, which takes into account photon absorption, electron-electron and electron phonon interactions, as well as heat transfer to the glass matrix. The collision operators are usually written in an integral form. In this work, we transform those in differential operators with the use of Landau approximation. This approach allows to perform kinetic calculations beyond the nanosecond time scale. For a laser intensity relevant to high power lasers, the energy deposition on the electron population can lead to a significant Fermi smearing within very short times. An important part of the electron population is driven beyond a typical 10 eV energy, and consequently this can result in the creation of a plasma around the particle.

High damage threshold hafnia/silica HR mirrors were damage tested. The transient reflectivity of these mirrors was studied during laser irradiation and particularly during catastrophic damage by top layer chipping. The tests were performed in R/1 mode on 50 sites in order to correctly assess the statistical behavior of the damage threshold. During the ramp, we observed the formation of a first plasma coupled to a modification of the reflected pulse without formation of any pit or chipping damage. For further irradiations at higher fluences, the optical properties of the mirror were unchanged up to the creation of a catastrophic chipping damage coming with a more intense plasma. The statistical distributions of fluences obtained in the two cases were different. For the moment, there is no proven correlation between first plasma and chipping: plasma detection cannot be used as a nondestructive quality control.

As part of the LMJ (Laser Megajoule) program, CEA is building the LIL laser with full size optics and LMJ requirements. SAGEM has been selected as the supplier of large optical components and coatings with very high laser- induced damage threshold. Including spare parts, about 100 mirrors 610*430 mm² with LIDT-3ns>25 J/cm² have to be produced. Using a 5 m³ vacuum chamber and the 100 J/cm² mirror coating process developed at CEA-LETI, with Hafnium and SiO₂ materials, we are now typically in a serial production phase. To date, about thirty mirrors have been delivered. This paper focuses on the acceptance tests performed after coating, at SAGEM then CEA: LIDT measurement and Raster-Scan on samples; reflectance mapping on CEA automatic photometer; reflected wavefront deformation with &nullset; 800 mm/1ω CEA interferometer.

In previous years different multilayer systems were investigated with respect to their optical performance at the wavelength 193 nm of the ArF excimer laser. For most applications in this spectral range, fluoride coatings have been qualified and are widely established. Even though the applicability of oxide coatings is strongly restricted by the significant intrinsic absorptance of the Al₂O₃ component, these materials are utilized for special technology fields. In contrast to the dielectric deposition materials, the potentiality of protected and enhanced metal layer systems has not been investigated in detail as a basis for high reflectivity mirrors at the wavelength of 193 nm. The major advantage of metal layers is their relatively high reflectivity which can be achieved at low film thickness. Thus metal layers can be employed as a substitution of the first HL-pairs in a high reflecting dielectric stack, and the number of layers can be drastically reduced. Furthermore, the upper residual fluoride stack shields the metal coating from the E-field, improving the power handling capability of the metal layer. The present investigations are focused on the determination of the absorptance in dependence on the different number of fluoride HL-pairs in metal/dielectric layer systems.

Absorptance losses in MgF₂, CaF₂, and BaF₂ during 193-nm (DUV) and 157-nm (VUV) irradiation were investigated by employing a high-resolution laser calorimetric technique which allows the determination of both single and two-photon absorption coefficients. A strong wavelength dependence of the DUV and VUV absorption characteristics was observed: while effective two-photon absorption takes place at 193 nm, only minor effects were found at 157 nm. Due to the strong nonlinear behaviour the absorptance at 193 nm exceeds that at 157 nm above a critical energy density. A first explanation for this absorption behaviour is given implying the energetic band structure of CaF₂. Furthermore, different single and two-photon absorption coefficients were determined for different CaF₂ samples, indicating a two-photon two-step absorption mechanism. In addition, laser-induced aging was found in MgF₂ at 193 nm, but not at all at 157 nm. The laser-induced fluorescence (LIF) spectra of alkaline- earth fluorides revealed a different emission behaviour for 193-nm and 157-nm excitation. Time-resolved as well as energy dependent measurements of intensities gave evidence for recombination fluorescence arising from self-trapped excitons as well as from specific impurities.

SiC surface was polished accurately with KrF laser irradiation in the presence of the HF and H₂O₂ mixed solutions. The SiC is very hard; moreover, the material is resistant to chemicals. Then we tried to polish the softened SiC chemically on the soft mat. The SiC surface was pressurized at 50 g/cm² on the fluorocarbon polishing mat. Next the HF and H₂O₂ mixed solutions are infiltrated into the thin gap between the sample and the fluorocarbon, and KrF laser irradiated through the fluorocarbon turntable. By this irradiation, the SiC surface was oxidized and produced SiO₂ and CO₂. Then CO₂ is diffused in an atmosphere, and only SiO₂ solidified on the sample surface. The moment the SiO₂ was formed, it dissolves in HF solution. After the etching, the polishing progresses by the friction with the fluorocarbon. The surface roughness was obtained 80 nm with 60 minute polishing with KrF laser irradiation (400 mJ/cm², 20 pps) in 15% HF/H₂O₂ ambience.

Laser-induced damage of thin films coated on optical components used for UV and DUV lasers are serious problems. Gradient refractive index antireflection films are produced by the combination of evaporation and leaching process on fused silica substrate. It has a high laser-induced damage threshold of 7.4-8.7 J/cm² at 266 nm. The reflectance is about 0.35%.

This paper discusses the development of sol-gel thin films at AWE with respect to the formation of high reflectivity optical coatings. The use of such mirrors allows the separation of 1053 nm [1ω], 527 nm [2ω] and 351 nm [3ω] light from high power Neodymium laser systems before entering a target chamber and focusing onto a plasma physics target. The coating technique to achieve successful mirrors is discussed along with the analysis of the multilayers. Issues of scaling up from 50-mm diameter components to the coating of 150-mm square optics are reported along with the simultaneous achievement of high reflectivity, uniformity, wavefront preservation and high laser induced damage thresholds (LIDT).

By using a first-order time-dependent perturbation theory and including the effect of energy drift from electron transport on the intraband transitions of electrons due to phonons, the kinetic Fokker-Planck equation for conduction electrons in semiconductors is systematically derived in the presence of a laser pulse. A contribution from the anti- diffusion current in the equation is found as a result of the coupling between the spontaneous-phonon emission and the energy drift of electrons due to joule heating by absorbing power from a laser. Analytical expressions for source terms of the equation are given simultaneously up to second-order in perturbation theory, including the stimulated interband optical transitions of electrons from single-photon laser- field absorption, impact ionization due to the Coulomb interaction between electrons and holes, and nonradiative recombination due to the phonon-mediated interaction. Some possible damage mechanisms in semiconductors including optical, electrical, and structural damage are explored. The energy spectra of the electron distribution function are studied and used to analyze the transient behavior of both the conduction electron density and the average kinetic energy of electrons (proportional to the electron temperature). The dynamical effects of anti-diffusion recombination, thermal diffusion and lattice temperature are all shown and explained. A new kink-like feature is observed around the edge of the conduction band in the electron distribution function due to anti-diffusion.

Periodically-poled lithium niobate (PPLN) is a well- established nonlinear material for down converting the output from a solid state laser, the pump wavelength, to generate an emission close to 4 micrometers . A simple OPO configuration was used with a resonated signal beam passing through a heated PPLN crystal, to avoid photo-refraction. A Q-switched Nd:YAG pump laser was used for generating short- duration, high-intensity one micron pulses. Direct modulation of the diode sources enable the emission to be controlled. The pump beam had good beam quality, with an M² of about 1.8 in each axis, and modulated average power in excess of 6 W. Damage to PPLN crystals used in the solid state laser has been observed. Irreversible damage of the nonlinear medium has been recorded: initially to the output face of PPLN crystals, but latterly to the bulk structure. These observations were made over a range of pumping conditions and OPO configurations. Estimates of the damage threshold have been surprisingly low, of the order of 1.5 J/cm².

Creation of small laser-induced damages without large star structure is discussed. The method for producing laser- induced damages by the specific temporal radiation is reported. Particularly, the laser system for production images inside transparent material by generating succession of plenty pairs of pulses is described.

Methods for predicting the refractive indices of materials are evaluated with respect to the design criteria for composite optical limiters. The efficiency of a nonlinear Christiansen optical limiter is dependent upon refractive index of the individual phases of the composite device and their comparative response characteristics. The overall performance of the structure-property relationships for the linear refractive indices is generally excellent within the training sets; however, the accuracy of an individual case sample is challenged by the low energy case requirements. Outside the general class of materials for which these methods were designed, significant degradation occurs as a result in incomplete parameterizations or compounding errors of imprecise predictions. Accurate predictions of linear and nonlinear refractive indices by TDHF methods are more problematic, yet these techniques demonstrate sensitivity to chemical bonding environments.

A variation to an existing laser damage model has been developed. The principle modification in this paper is the inclusion of the temperature dependent absorption coefficient. The absorption coefficient was specifically calculated for silicon irradiated by lasers at 1.06 and 1.32 microns. At these wavelengths, the incident laser photons are at and below the indirect absorption band gap of silicon. Four temperature dependent absorption processes of silicon are considered. These include one photon free carrier absorption, one photon/one phonon indirect absorption, one photon/three phonon indirect absorption and two photon/one phonon indirect absorption. From these four processes, simple absorption coefficients are derived for both 1.06 and 1.32 micrometers radiation. The resulting predictions of laser damage for 1.06 and 1.32 micrometers laser radiation on silicon substrates are provided.

This paper presents an explicit derivation of relative damage threshold for entrance, exit and total internal reflection (TIR) surfaces where the TIR surface is either coated or contaminated. The derivation follows the same arguments as presented in earlier works which dealt with an uncoated or clean TIR surface.

This paper presents the results of a recent experiment to measure the relative laser induced damage thresholds on total internal reflection (TIR) surfaces. The TIR surfaces were realized by testing the reflective surface of an undoped YAG prism at 1.06 micrometers and 12-ns pulse length. Several prisms were tested and the data from each identically processed surface combined to provide a statistically significant sample set. The lowest fluence at which damage was observed using post exposure microscopy was taken as the damage threshold for each polarization. The experimentally observed ratio of the S and P polarization damage thresholds at the TIR surface of 0.584+/- 0.081. This ratio is consistent with a mechanism of damage which is proportional to the third power of the intensity.

This paper introduces the role of search length in laser damage threshold measurements. It is shown that the search length, the number of interrogations, is an important parameter in the determination of threshold. Order statistics are introduced to be able to scale from one search length to another. This scaling is applied to experimental data and shown to account for differences between measurements made on similar samples by different organizations.

Pits formation during the laser induced damage with GaAs surface either for picosecond or for femtosecond laser pulses, shows that defects always play an important role in damage morphology of GaAs surface, as previously these pits formation have also been seen for micro and nanosecond laser pulses. The nature of these pits gives an important information about the morphological features of damaged GaAs surface in picosecond and femtosecond regime. In this paper we report a comparative study of laser induced damage morphology of GaAs surface based on the nature of these pits formation.

Polymeric Optical Waveguides have attracted a lot of attention with a view to their flexibility for fabrication and their cost effectiveness. In the search for new materials recently the study of Styrene Acrylonitrile (SAN) thin film optical waveguides has been reported. SAN exhibits a considerable transmission (80-90%) in the visible and infrared regions, which is important for integrated optical devices. But the propagation loss is a major drawback of SAN based waveguides. Here we report 4-layer waveguides with Glass plus SAN plus Polystyrene plus air, structure. With the help of this multilayer structure we were able to reduce propagation loss by 30% in comparison to single layer SAN based waveguides. We also present the other basic properties like index profile, guide thickness, birefringence.

The existence of glass surface layers has a great influence upon the optical properties of the thin films coated on glass substrate, particularly upon laser-induced damage threshold. A new and simple method is proposed to analyze the glass surface layers based on p-polarized light reflectance. Experimentally, the glasses with different cleaning treatments have been analyzed. The results show that the glass with acid erosion has smaller extinction coefficient, and the cleanliness and the laser damage threshold increase accordingly. Furthermore, we measure the laser-induced damage thresholds of SiO₂ antireflective films coated on several glass substrates above-mentioned. It is indicated that the laser-induced damage threshold under the acid and alkaline cleaning treatment increase by about 80% over the water cleaning treatment.

Waves of change in reflection and conduction (WCRC)was found in 1992. Since that time, this multidimensional wave structure of solitonic types manifests itself in many works of different laboratories. It could be excited by electromagnetic, corpuscular radiation or by mechanical load at some conditions in different condense matter. Between many WRC features one of the most fundamental is 2-times decreasing of the velocity U_i of two following each another pulses, starting from longitudinal sound velocity v_l(km/s), namely, U_iequalsv_l/(2)ⁱ. The last pulse with iequals30 (seen in Cu massive sample) had velocity - micron/s. As a result of nearly constant U_i value and small losses at reflection these pulses exist in the sample for a long time. Here we reviewed hopefully all information about this phenomenon, which could be seen from original records of Boulder Symposiums publications in 1969-1998. This work was sponsored as project 00-02-17249 by RFBR (Russia).

Waves of change of reflection and conduction (WCRC) were suggested at Boulder-1997 as a new phenomenon for precatastrophic indications of laser damage in IR transparent semiconductors. It turned out from results of K. B. Abramova's team (many publications since 1971) that the WCRC-components are exciting in different metals mechanically by flat shock or static load. The velocity measurements were obtained from photomultiplier evolution of visible emission (mechanoluminescence) of the samples. Rapid deformation (approximately 10^-4 s) of Cu-sample by subsonic body excited 5-th WCRC component (U₅ approximately 0.08 km/s). Slow (approximately 400 s) change of the load from 3 to 8 tons (when destruction happened) gave 65% increasing of 20-th WCRC-component (U₂₀ approximately 0.33 cm/s) in the case of Cu-sample. It gives again the possibility to develop the technique for the remote nondestructive precatastrophic indications of damage, now in metals. The work was done with financial support by RFBR, project 00-02-17249-(a), and by KIE.

There are considered manifestations of influence of discreteness of the matter and laser action on laser-induced nonlinear optical and photophysical processes in condensed media. In particular, influence of material discreteness results in small terms appearing in addition to Ewald-Oseen extinction theorem. They depend strongly on type and characteristic size of crystal lattice and on character of electromagnetic interaction between neighboring secondary radiators discretely placed in points of the crystal lattice. Those terms determine near field of material response with fast decay as well as some effects in propagation of super-short laser pulses. It is stated that considered type of discreteness of material response is important for analysis of nonlinear-optical processes in near-field optical systems, low-dimensional quantum structures and for consideration of interaction of super- short laser pulses with materials when optical response is formed by few atomic layers. There are also explained significant differences in results of action of continuos and pulsed-periodic (discrete) scanning laser radiation on surface. They come through general impossibility of asymptotical coming to steady distribution in case of discrete action if laser-induced optical and photophysical processes are coupled through strong feedbacks.

This paper presents both experimental investigations and modeled results on high temperature superconductive exponential microstrip lines. High-Tc ceramic (YBCO) superconducting exponential microstrip lines have been fabricated by sputtering technique, conditions and properties for preparation of exponential microstrip lines are described. The behavior of superconducting microstrip lines approaches that of normal state microstrip line as the strip and ground plane thickness and the substrate thickness become large compared to penetration depth. In order to verify the validity of the implemented macro model for the superconducting exponential microstrip line, a microstrip line structure connected to an inductor having one end grounded with an AC current source used to excite the circuit is simulated. Furthermore we have investigated how a pulse propagates on the superconducting exponential microstrip line and how the circuit parameters of transmission lines affect the propagation characteristic of the line. Simulation results show that the High-Tc exponential transmission lines are more promising for interconnections than the conventional transmission lines by virtue of their lower attenuation and less dispersion, even if dielectric loss of substrate is taken into consideration. This work forms a general basis for the investigation of the exponential microstrip line for use in high speed detection circuit.

A model SiO₂ thin film system with nanoscale absorbing defects (gold nanoparticles) is employed with the goal of unraveling the connection between the pulsed-laser-energy absorption process inside a single nanoscale defect and the resulting film damage morphology. For this purpose, gold nanoparticles are lodged at a well-defined depth inside a SiO₂ monolayer film. Particle sites, as well as damage craters generated at these locations after 351-nm pulsed- laser irradiation, are mapped by means of atomic force microscopy. The results of this mapping confirm mechanism of damage that involves initiation in the nanoscale defect followed by absorption spreading out to the surrounding matrix. At low laser fluences (below optically detected damage onset), the probability of damage crater formation and the amount of the material vaporized is, to within +/- 25% of the average value, almost independent of the particle size. Inhomogeneities in the particle environment are held responsible for variances in the laser-energy absorption process and, consequently, for the observed particle/damage crater correlation behavior. The nanoscale damage threshold is introduced as a laser fluence causing localized melting without significant vaporization.

A technique for inhibiting the growth of laser-induced surface damage on fused silica, initiated and propagated at the 351-nm laser wavelength, has been investigated. The technique exposes the damage sites to single pulses of a CO₂ laser operating at the 10.6 micrometers wavelength at or near beam focus. This method results in a very localized treatment of the laser damage site and modifies the site such that laser damage does not propagate further. A laser damage site initiated with a single pulse of 355-nm laser light at approximately 45 J cm^-2 and 7.5-ns pulse duration grows rapidly upon further illumination at 8 J cm^-2 with 100% probability. Treatment of these sites with single pulses of 10.6 micrometers laser light for one second at a power level of between 17 and 37 Watts with a beam diameter of 5 mm alters the damage site such that it does not grow with subsequent 351-nm laser illumination at 8 J cm^-2 10-ns pulse duration for > 1000 shots. The technique has been found to be 100% effective at stopping the growth of the laser damage.

A program to identify and eliminate the causes of UV laser- induced damage and growth in fused silica and DKDP has developed methods to extend optics lifetimes for large- aperture, high-peak-power, UV lasers such as the National Ignition Facility (NIF). Issues included polish-related surface damage initiation and growth on fused silica and DKDP, bulk inclusions in fused silica, pinpoint bulk damage in DKDP, and UV-induced surface degradation in fused silica and DKDP in a vacuum. Approaches included an understanding of the mechanism of the damage, incremental improvements to existing fabrication technology, and feasibility studies of non-traditional fabrication technologies. Status and success of these various approaches are reviewed. Improvements were made in reducing surface damage initiation and eliminating growth for fused silica by improved polishing and post- processing steps, and improved analytical techniques are providing insights into mechanisms of DKDP damage. The NIF final optics hardware has been designed to enable easy retrieval, surface-damage mitigation, and recycling of optics.

In this paper we present the results of damage tests performed at 1064 and 355-nm at 8-10 ns on conventional and rapid growth DKDP tripler crystals. The crystals were laser conditioned prior to damage testing by raster scanning using either Nd:YAG (1064 and 355 nm, 8-10 ns) or excimer lasers at 248, 308 or 351 nm with pulse durations of approximately 30-47 ns. The results show that it is possible to attain increases in 355-nm damage probability fluences of 2X for excimer conditioning at 248 and 308 nm. However, these wavelengths can induce absorption sufficient to induce bulk fracture by thermal shock when impurities such as arsenic, rubidium and sulfur are present in the crystals in sufficient quantity. Tests to evaluate the efficiency of 351-nm conditioning (XeF excimer) show improvements of 2X and that thermal fracture by induced absorption is not a problem. We also discuss our recent discovery that low fluence raster scanning at UV wavelengths leads to 1064-nm damage thresholds of over 100 J/cm² (10-ns pulses).

We have investigated the flash of light that accompanies laser damage using time-resolved spectroscopy. Damage events were initiated in the bulk of both fused silica and DKDP crystals with 355-nm 3-ns pulsed radiation. Spectra from the accompanying flash were recorded in the 200-500 nm wavelength range with 5-ns temporal resolution. Ten ns following damage initiation, the spectra were found to be roughly blackbody with temperatures on the order of 5000 K to 7000 K, depending on the material studied and excitation energy used. The observed temperatures and cooling rates can be related to the size and electron density of the plasma fireball that initiates the damage event.

This work is an experimental investigation to evaluate the potential of fluorescence microscopy as a tool to detect surface contamination as well as reveal surface damage precursors on DKDP and SiO₂ optics. To achieve these technical objectives, microscopic imaging systems were built that also incorporate in-situ damage testing capabilities. Fluorescence imaging experiments were performed using 351-nm laser excitation while damage testing was performed at relatively high laser fluences. The experimental results demonstrated the potential of this technique to address the aforementioned technical issues.

Embedded gold and mechanical deformation in silica were used to investigate initiation of laser-induced damage at 355 nm (7.6 ns). The nanoparticle-covered surfaces were coated with between 0 and 500 nm of SiO₂ by e-beam deposition. The threshold for observable damage and initiation site morphology for these engineered surfaces was determined. The gold nanoparticle coated surfaces with 500 nm SiO₂ coating exhibited pinpoint damage threshold of <0.7 J/cm² determined by light scattering and Nomarski microscopy. The gold nanoparticle coated surfaces with the 100 nm SiO₂ coatings exhibited what nominally appeared to be film exfoliation damage threshold of 19 J/cm² via light scattering and Nomarski microscopy. With atomic force microscopy pinholes could be detected at fluences greater than 7 J/cm² and blisters at fluences greater than 3 J/cm² on the 100-nm-coated surfaces. A series of mechanical indents and scratches were made in the fused silica substrates using a non-indentor. Plastic deformation without cracking led to damage thresholds of approximately 25 J/cm², whereas indents and scratches with cracking led to damage thresholds of only approximately 5 J/cm². Particularly illuminating was the deterministic damage of scratches at the deepest end of the scratch, as if the scratch acted as a waveguide.

High power laser pulses can produce damage in high quality fused silica optics that can lead to its eventual obscuration and failure. Current models suggest the initiation of a plasma detonation due to absorbing initiators and defects, leading to the formation of shock waves. Recent experiments have found a densified layer at the bottom of damage sites, as evidence of the laser-damage model. We have studied the propagation of shock waves through fused silica using molecular dynamics. These simulations show drastic modifications in the structure and topology of the network, in agreement with experimental observations.

This paper describes the effect of 355-nm laser conditioning on the concentration of UV-laser-induced surface damage sites on large-aperture fused silica optics. We will show the effect of various 355-nm laser conditioning methodologies on the reduction of surface-damage initiation in fused silica samples that have varying qualities of polishing. With the best, generally available fused silica optic, we have demonstrated that 355-nm laser conditioning can achieve up to 10x reduction in surface damage initiation concentration in the fluence range of 10-14 J/cm² (355- nm at 3 ns).

We have applied a carbon dioxide (CO₂) raster scanning laser polishing technique on two types of fused silica flat optics to determine the efficacy of CO₂-laser polishing as a method to increase the 351-nm laser damage resistance of optic surfaces. R-on-1 damage test results show that the fluence for any given 355-nm damage probability is 10-15 J/cm² higher (at 3 ns pulse length, scaled) for the CO₂-laser polished samples. Poor quality and good quality surfaces respond to the treatment such that their surface damage resistance is brought to approximately the same level. Surface stress and the resultant effect on wavefront quality remain key technology issues that would need to be addressed for a robust deployment.

We have investigated the surface degradation of bare and sol-gel coated deuterated potassium dihydrogen phosphate (DKDP) crystals when exposed to 351-nm laser pulses in atmospheric air and nitrogen and at pressures ranging from atmospheric down to 10^-5 Torr vacuum. Optical microscopy, surface topography, surface chemical analyses, 351-nm pumped photoemission maps, and photometry results have been used to characterize these samples. We report the occurrence of two potentially linked surface degradation phenomena: the development of increased photoemission and the development of unacceptable surface roughening in the region exposed to the beam in vacuum. We note no degradation for surfaces exposed in air or nitrogen at pressures exceeding 1 Torr. Diamond-turned DKDP surfaces show a ubiquitous, low-intensity photoemission signature before exposure to any laser fluence. The observed reduction of this emission signal as a function of operating pressure and accumulated laser energy when crystals are exposed to 351-nm laser pulses in air can be correlated with the removal of surface carbon.

Commercial thin borosilicate glass sheets have been evaluated for use as a single-shot optic debris shield to separate the radiation and contamination produced by the inertial confinement fusion (ICF) experiment from the expensive precision laser optics which focus and shape the 351-nm laser beam which irradiates the target. The goal of this work is identification of low cost materials that can deliver acceptable beam energy and focal spots to the target. The two parameters that dominate the transmitted beam quality are the transmitted wave front error and bulk absorption. This paper focuses on the latter. To date, the materials with the lowest linear 351-nm absorption have also generally demonstrated the lowest nonlinear absorption. Commercial materials have been identified which approach the beam energy and focus requirements for many ICF missions.

We report a summary of the surface damage, growth mitigation effort at 3(omega) for fused silica optics at LLNL. The objective was to experimentally validate selected methods that could be applied to pre-initiated or retrieved-from- service optics, to stop further damage growth. A specific goal was to obtain sufficient data and information of successful methods for fused silica optics to select a single approach for processing NIF optics. This paper includes the test results and the evaluation thereof, for several mitigation methods for fused silica. The mitigation methods tested in this study are wet chemical etching, cold plasma etching, CO₂ laser processing, and micro-flame torch processing. We found that CO₂ laser processing produces the most significant and consistent results to halt laser-induced surface damage growth on fused silica. We recorded successful mitigation of the growth of laser-induced surface damage sites as large as 0.5-mm diameter, for 1000 shots at fluences in the range of 8 to 13 J/cm². We obtained sufficient data for elimination of damage growth using CO₂ laser processing on sub-aperture representative optics, to proceed with application to full- scale NIF optics.

Thermo-mechanical surface damage initiation and growth in fused-silica 3(omega) (355 nm) optics are important performance and cost issues for high-power lasers (fluences of 4-14 J/cm²) in the few ns pulse length regime. We are working to characterize and identify the extrinsic origins of damage initiation: impurities, particulates, and manufacturing defects. We have performed a materials characterization survey approach using transmission electron microscopy to identify the chemistry and morphology of particles, and structural defects. TEM offers high chemical or elemental specificity and small analytical spot size yielding complementary materials characterization data and powerful clues to manufacturing improvements. We will report on our characterization of the near surface of one commercially manufactured fused silica optic, where the results indicate both the efficacy and potential value of this approach.

Laser-induced damage initiation on fused silica optics can limit the lifetime of the components when used in high power UV laser environments. For example in inertial confinement fusion research applications, the optics can be exposed to temporal laser pulses of about 3 nsec with average fluences of 8 J/cm² and peak fluences between 12 and 15 J/cm². During the past year, we have focused on optimizing the damage performance at a wavelength of 355-nm (3(omega) ), 3-nsec pulse length, for optics in this category by examining a variety of finishing technologies with a challenge to improve the laser damage initiation density by at least two orders of magnitude. In this paper, we describe recent advances in improving the 3(omega) damage initiation performance of laboratory-scale zirconium oxide and cerium oxide conventionally finished fused silica optics via application of processes incorporating magnetorheological finishing (MRF), wet chemical etching, and UV laser conditioning. Details of the advanced finishing procedures are described and comparisons are made between the procedures based upon large area 3(omega) damage performance, polishing layer contamination, and optical subsurface damage.

We determined the damage thresholds and lifetimes of several materials using 157- and 193-nm excimer lasers and a beam profile technique similar to that described in ISO 11254-2. We made these measurements to select an appropriate absorbing material for use in our primary standard laser calorimeter for 157-nm excimer laser energy measurements. The materials we tested were nickel-plated sapphire, chemically-vapor-deposited silicon carbide (CVD SiC), nickel-plated copper, and polished copper. Applied pulse energy densities (or dose) ranged from 80 to 840 mJ/cm². We determined the applied dose from a series of laser beam profile measurements. Silicon carbide had the highest damage threshold: 730 mJ/cm² per pulse. For this reason, and because of its high thermal and electrical conductivities, we have chosen silicon carbide as the absorber material for the 157-nm calorimeter. We also conducted long-term exposure studies in cooperation with MIT Lincoln Laboratory at a pulse energy density of 5 mJ/cm² to simulate typical calorimeter operating conditions. No aging effects or other surface changes were observed at these dose levels after 500 million pulses, corresponding to a projected calorimeter lifetime of 50 years.

This paper is a survey of experimental results in laser- induced damage observed mainly at State Optical Institute (St. Petersburg, Russia) and at School of Optics/CREOL (Orlando, FL) which expounds conditions of observation of an intrinsic breakdown of high-purity silicate glasses and proposes the general idea of its mechanism. It is shown that the surface laser-induced breakdown of dielectrics is resulted from photo- and thermo-ionization of surface defects but not from interaction of laser radiation with dielectric material itself. Conditions of thermal ionization of the volume of dielectric materials are determined in dependence on features of absorption of material and temporal features of laser radiation. Statistical properties of laser-induced breakdown of high-purity glasses are caused by statistical properties of laser radiation while the breakdown itself is a deterministic process. Elimination of impact of self-focusing on the results of the breakdown threshold measurements is observed if the spot size of laser radiation in focal plane is less than the wavelength. No photoionization of glass matrix is detected before laser- induced breakdown, and there is no effect of photoionization of impurities and defects on intrinsic breakdown. A mechanism of intrinsic laser-induced breakdown is proposed which is a spasmodic transformation of the electronic level structure in a wide-bandgap dielectric caused by the electric field of laser radiation. This is a collective process converting a transparent material to the opaque state but not an individual process of any type of ionization.