This PDF file contains the front matter associated with SPIE Proceedings Volume 8190, including the Title Page, Copyright information, Table of Contents, International Program Committee listing, Symposium Welcome, Summary of Meeting, and the Summary of the Roundtable Discussion.

By creating nodules from artificial seeds, the damage behaviors of engineered nodules were systematically studied from experimental approaches. The seed diameters, seed absorption and film absorption were varied independently to uncover a single factor's influence on the damage behavior of nodules. First, non-absorbing monodisperse SiO₂ microspheres with five different sizes were used to create engineered nodules in 1.053 μm HfO₂/SiO₂ high reflectors that were prepared by EB process. Laser damage test results showed that the ejection fluences of nodules monotonically decreased with the increase of silica microsphere diameters. And to our surprise, nodules initiating from 0.3 and 0.6 μm silica seeds survived the maximum fluence of 170 J/cm² (10 ns). Film absorption also has big influence on the damage behaviors of nodules. Compared to the nodules in low absorbing reflectors that were prepared by EB process, the nodules in high absorbing reflectors that were prepared by IAD process exhibited a much lower ejection fluences, although the seed diameters for the comprising nodules were same. Moreover, aluminum seeds were also used to create engineered nodules. Laser damage test results showed that the ejection fluences of nodules initiating from aluminum seeds were around 2 J/cm² (10 ns), which is more than an order of magnitude less than the ejection fluences of nodules created from nonabsorbing silica seeds. This result revealed that the seed absorption played a very important role in the laser damage of nodules.

Continued advances in polishing, thin films coating and metrology have facilitated the routine production of optical coatings with losses in the few part per million range. Optical components and coatings with extremely low levels of loss have enabled a number of applications. A brief historical background of ion beam sputtering, substrate requirements, recent results as well as a brief discussion of applications of low loss IBS coated optics are presented.

The femto-second technology gains of increasing importance in industrial applications. In this context, a new generation of compact and low cost laser sources has to be provided on a commercial basis. Typical pulse durations of these sources are specified in the range from a few hundred femtoup to some pico-seconds, and typical wavelengths are centered around 1030-1080nm. As a consequence, also the demands imposed on high power optical components for these laser sources are rapidly increasing, especially in respect to their power handling capability in the ultra-short pulse range. The present contribution is dedicated to some aspects for improving this quality parameter of optical coatings. The study is based on a set of hafnia and silica mixtures with different compositions and optical band gaps. This material combination displays under ultra-short pulse laser irradiation effects, which are typically for thermal processes. For instance, melting had been observed in the morphology of damaged sides. In this context, models for a prediction of the laser damage thresholds and scaling laws are scrutinized, and have been modified calculating the energy of the electron ensemble. Furthermore, a simple first order approach for the calculation of the temperature was included.

HfO₂/SiO₂ multilayers were deposited on single point diamond turned aluminum substrates via modified reactive plasma ion assisted deposition to form a laser durable and environmentally stable dielectric enhanced IR mirror at a wavelength of 1064nm. The effect of the surface quality of the diamond turned aluminum on the optical performance of the dielectric enhanced mirror was assessed. A laser-induced damage threshold up to 11 J/cm² was obtained from the enhanced aluminum mirror tested in pulse mode at 1064nm with a pulse length of 20ns and a repetition rate of 20Hz. Laser damage morphology was revealed by a scanning electron microscopy. The damage mechanism was attributed to nodule defects generated by particle embedded on the aluminum substrate surface.

A new sputter deposition process has been developed based upon remote generation of plasma by a dedicated Plasma Source (PLS). This technique is referred to as high target utilisation sputtering (HiTUS). In contrast to ion beam and magnetron sputtering processes, HiTUS allows fast deposition rates of low stress, high density films from a high percentage (>90%) of the target surface. The process has not previously been applied to thin films for high laser damage threshold applications. The paper will present results of the anti-reflection (AR) coating trials and compare them to two other coating deposition processes - standard e-beam evaporation and hollow cathode ion-assisted e-beam deposition.

Excimer lasers are a critical technology for the $400 billion annual market of manufactured integrated circuits. Other uses of excimer lasers include medical applications such as laser eye surgery and micro-machining industrial applications. Ultraviolet laser mirrors are used for beam steering, therefore high reliability is desired for such commercial industrial applications. A laser damage competition of excimer mirror coatings creates the opportunity to survey private industry, governmental institutions, and university labs allowing a direct laser resistance comparison of samples tested under identical conditions. The major requirement of the submitted coatings was a minimum reflectance of 97% at 193 nm at normal incidence. The choice of coating materials, design, and deposition method were left to the participant. Damage testing was performed with a 193 nm excimer laser at a pulse length of 13 ns. A double blind test assured sample and submitter anonymity so only a summary of the deposition process, coating materials, layer count and spectral results are presented. In summary, a 70× difference was seen in the twelve submitted mirror samples, with the highest laser resistant sample being deposited by resistive heating and composed of three materials (LaF3, AlF3, & MgF2). Laser resistance was strongly affected by substrate cleaning, coating deposition method, and coating material selection whereas layer count had a minimal impact.

As large amounts of heat need to be dissipated during laser operation, some diode pumped solid state lasers (DPSSL), especially Yb:YAG laser, operate at cryogenic condition. This work investigated the laser induced damage of coatings (high-reflective and anti-reflective coatings) on Yb:YAG crystals at cryogenic temperature and room temperature. The results show that the damage threshold of coatings at cryogenic temperature is lower than the one at room temperature. Field-emission scanning electron microscopy (FESEM), optical profiler, step profiler and Atomic force microscope (AFM) were used to obtain the damage morphology, size and depth. Taking alteration of physical parameters, microstructure of coatings and the environmental pollution into consideration, we analyzed the key factor of lowering the coating damage threshold at cryogenic conditions. The results are important to understand the mechanisms leading to damage at cryogenic condition.

Optical properties and laser damage characteristics of thin-film aluminized Kapton were investigated. Optical absorption of virgin and irradiated samples was measured from the Kapton side using a Cary 5000 Grating Spectrophotometer and an ABB/Bomem MB157S FTIR Spectrometer with a combined range of 0.2 to 15 μm at both room-temperature and 150°C. Laser-induced damage parameters of penetration time and maximum temperature were measured in a vacuum environment using an IPG Photonics continuous-wave solid-state laser operating at 1.07 μm and an electric-discharge CO₂ laser operating at 10.6 μm. Rather large differences in damage behavior at the two wavelengths were observed due to the variability in starting absorption properties between the NIR and LWIR. A FLIR Systems Quantum Well Infrared Photometer at 8-9.2 μm was used to remotely examine the thin-film temperature evolution based on a known LWIR band of nearly-constant emissivity. A dual-detector FTIR spectrometer was also employed during testing in order to extract spectral emittance information from high-temperature irradiation exposures. Surface emittance was found to change after the material heated past approximately 500°C and during subsequent post-test cooling. This evolving spectral emittance with temperature successfully predicted increases in absorption that led to more rapid penetration times and higher heating rates at increased 1.07-μm laser power. A simplified one-dimensional thermal conduction and radiation model replicated the remotely-sensed temperature as a function of time in tests with constant absorptance and no material breakdown. With the result of evolving emittance data, this model could be modified to capture more realistic heating trends at higher irradiances whereby damage occurs and absorption properties vary spectrally.

Laser-induced damage thresholds for dielectric and metal single-layer coatings at different temperature conditions (123-473 K) were measured by 1064-nm wavelength and 4-ns pulses to elucidate the effects of initial temperature to laser damage mechanisms. SiO₂, MgF₂, gold, silver and copper single-layer coatings were prepared as experimental samples. In the experimental results, temperature dependence of LIDTs for optical substrates and all dielectric single-layer coatings indicated same trend as that for bulk silica glasses, which increased linearly with decreasing the temperature. However, all metallic coatings had the inverse trend of the dependence for dielectric coatings. The effects of initial temperature to laser damage mechanisms were considered with separated processes from the experimental results. In the conclusions, free-electron generation and electron multiple caused difficultly at low temperature and the laser-induced damage thresholds increased. On the other hand, plasma heating caused easily at low temperature and the laser-damage thresholds decreased.

Different dispersive coatings were tested in terms of laser-induced damage threshold by using a Ti:Sapphire laser yielding 1 mJ, 30 fs pulses at 500 Hz repetition rate at 790 nm central wavelength. The beam was focused down to 140 μm. Single layer coatings of Au, Ag, Nb₂O₅, SiO₂, Ta₂O₅ and mixtures of Ta₂O₅ and silica were examined as well as different dispersive coatings. We observed a direct dependence of the damage threshold on the band gap of the materials used to produce the different samples. The damage threshold values for the dispersive coatings employing the same high index material lay within a range of 30% of each other.

Damage tests were carried out to measure the laser resistance of Al₂O₃/TiO₂ and Al₂O₃/HfO₂ antireflection coatings at 1064nm grown by atomic layer deposition (ALD). The S-on-1 and R-on-1 damage results are given. It's interesting to find that ALD coatings damage performance seems closed to those grown by conventional e-beam evaporation process. For Al₂O₃/TiO₂ coatings, the grown temperature will impact the damage resistance of thin films. Crystallization of TiO₂ layer at higher temperature could play an importance role as absorption defects that reduced the LIDT of coatings. In addition, it is found that using inorganic compound instead of organic compound as precursors for ALD process can effective prevent residual carbon in films and will increase the LIDT of coatings.

In nanosecond laser damage investigations, the specific defect density in the optical component or thin film plays the key role in triggering optical breakdown. UV irradiation can induce additional defects in optical materials before the damaging event takes place. This increased defect density can even be the main cause for UV laser damage as shown before in fused silica. Moving on to oxide thin films, this contribution will present studies on SiO₂, Al₂O₃, and HfO₂ ion beam sputtered coatings. Pure material single layers as well as composite material single layers comprised of two oxides have been investigated concerning their tendency to generate additional defects resulting from UV laser irradiation. Within this work, tests at 355 nm and 266 nm have been performed and are compared.

Tantalum pentoxide (Ta₂O₅) is the high index material most commonly used in optical coatings for high average-power lasers since high density sputtered oxide films with absorption losses at mid-infrared wavelengths of less than 1 ppm can be obtained. We have chosen this 'model' oxide to investigate the spontaneous and optically induced absorption at λ = 1064 nm by photothermal common-path interferometric (PCI) technique. This technique is capable of detecting sub-ppm levels of optical absorption and its changes at a given wavelength when a second beam is also incident on the thin film oxide sample. In this work dual beam experiments are implemented to assess changes in the optical absorption at λ = 1064 nm Ta2O5 thin films deposited on fused silica with accompanying electromagnetic radiation with wavelengths ranging from 266 nm to 780 nm. The accompanying radiation was found to have a substantial impact on the optical absorption at 1064 nm. The effect is associated with various electron traps existing in the forbidden gap and depends essentially on the film's preparation conditions. The substantially greater effect observed in the case of 266 nm is suggested to be the result of additional excitations from the valence band involved in the process since the photon energy of 266 nm radiation is higher than the forbidden gap.

Laser-induced damage thresholds of 2.2 picoseconds pulse for HR coatings prepared by Japanese optics makers were measured. The damage thresholds were compared with that of 10 nanoseconds pulse measured at last year. In picoseconds pulse, almost HR coatings had an average threshold of about 5 J/cm². This meant that the damage thresholds of picoseconds pulse were unconnected to a little defects and/or contaminants in the coatings.

In this study, single layer hafnium dioxide thin films were prepared by electron beam deposition (EBD), ion assisted deposition (IAD) with End-Hall and APS ion sources, and ion beam sputtering (IBS). The starting materials for EBD and IAD were hafnium and granulated hafnia, whereas the target for IBS was hafnium. Comprehensive characterization of these films such as structural and optical properties, surface topography and weak absorption have been studied via Xray diffraction (XRD), Lambda 900 spectrophotometer, variable angle spectroscopic ellipsometry (VASE), scanning electron microscopy (SEM), ZYGO interferometer, and Laser Calorimeter. The results show that thin film properties have a close relationship with deposition technologies. The EBD and IBS films are largely amorphous, however, the IAD films with different ion sources are all polycrystalline but with different crystal structures. Comparison with EBD films, the IAD and IBS films, of which the structures are very compact, represent higher refractive index and weak absorption. RMS roughness and total integrated scattering (TIS) of IAD and IBS films were lower than the EBD films. All of these results are useful to investigate the laser-induced damage threshold (LIDT) of hafnium dioxide thin films and hafnia/silica high reflectors for high power laser applications.

Laser bulk damage thresholds were measured for both single-crystal YAG and for diffusion-bonded YAG structures using 600 picosecond pulses at 1064 nm. The tested samples included 3-layer sandwich structures with doped cores of various thicknesses. An undoped-YAG end cap was diffusion-bonded on one end of each of the sandwiches. The 1064 nm laser source was focused to a 13 micron diameter spot at the boundary region between the core and the undoped endcap. Measurements included the evaluation of single- and multiple-pulse damage thresholds at single sites, as well as thresholds for continuous 90%-overlap scans. The single-site thresholds at the diffusion-bonded boundary were close to that of single-crystal YAG. However, the continuous scans revealed isolated microscopic sites where the damage threshold was as much as 4 times lower than that of single-crystal YAG.

One of the efficiency-limiting factors of optical devices used at high intensities is photo damage to the optical materials. In devices that use organic dyes, photo damage causes irreversible damage to the chromophores, deteriorating efficiency, and eventually causes failure. Our present work focuses on monitoring degradation and recovery of anthraquinone dye doped PMMA thin films with a digital imaging apparatus, with the goal of understanding the mechanisms. Our results suggest the possibility of making optical components more resistant to photodamage, and capable of self recovery, removing the necessity to constantly replace components damaged by high intensity light.

A Petawatt facility called PETAL (PETawatt Aquitaine Laser) is under development near the LMJ (Laser MegaJoule) at CEA Cesta, France. PETAL facility uses chirped pulse amplification (CPA) technique. This system needs large pulse compression gratings exhibiting damage threshold of more than 4 J/cm² in normal beam at 1.053μm and for 500fs pulses. In this paper, we present our recent progress and developments of such pulse compression gratings. We have shown in previous works that the enhancement of the near electric field inside the pillars of the grating drives the damage threshold. This was evidenced from a macroscopic point of view by laser damage testing. We herein demonstrate that damage morphology during damage initiation at the scale of the grating groove is also consistent with this electric field dependence. Some recent grating designs will also be detailed.

We report observations that dye-doped PMMA polymer with the organic dye Disperse Orange 11 exhibits self healing after photodegradation by continuous optical pumping whereas in liquid solution, degradation is permanent. This observation illustrates the important role of the polymer matrix in facilitating recovery of the dye molecules. In this work, we report on linear optical absorbance studies that confirm the existence of a quasi-stable state that is not formed in liquid solution. Studies as a function of dye concentration and temperature support our hypothesis of the role of molecular interactions in the decay and healing process that is mediated by the polymer host.

In this contribution we present a technology for deposition of interference coatings for optical components designed to operate as active media in power pulsed lasers. The aim of the technology is to prepare crystals for lasers for the HiPER project (High Power laser Energy Research facility) which should demonstrate the feasibility of laser driven fusion as a future energy source. Diode pumped solid state lasers (DPSSL) are the most likely option for fusion ignition. The choice of the material for the lasers' active medium is critical. Some of the most important properties include the ability to be antireflection coated to reduce the energy losses and increase the overall efficiency. This contribution deals with some of the materials considered to be candidates for slabs serving as the active medium of the DPSSLs. We tested Yb:YAG and Yb:CaF₂ samples. As large amounts of heat need to be dissipated during laser operation, cryogenic cooling is necessary. Appropriate coating materials and techniques need to be chosen. Therefore differences between available coating techniques are investigated in terms of adhesion, enduring of stress from temperature shocks, etc. Coated samples were placed into cryogenic environment in order to simulate conditions similar to those in real life operation. Optical microscopy was used for coating investigation after the conducted experiments.

We have investigated femtosecond laser induced microstructures, gratings, and craters in four different polymers: poly methyl methacrylate (PMMA), poly dimethyl siloxane (PDMS), polystyrene (PS) and poly vinyl alcohol (PVA) using Ti:sapphire laser delivering 800 nm, 100 femtosecond (fs) pulses at 1 kHz repetition rate with a maximum pulse energy of 1 mJ. Local chemical modifications leading to the formation of optical centers and peroxide radicals which were studied using UV-Visible absorption and emission, confocal micro-Raman and Electron Spin Resonance (ESR) spectroscopic techniques.

The stimulated Raman scattering gain coefficient in KDP/DKDP crystals for any orientation with respect to the crystal axis, propagation and polarization of the pump beam can be estimated using a) the spontaneous Raman scattering cross section of the material, b) the spectral profile of the Raman line and, c) the Raman scattering tensor. Of particular interest in ICF class laser systems are the parasitic Transverse Stimulated Raman Scattering effects. In this work we provide experimental results that help advance our ability to estimate these effects at operational conditions for second, third and fourth harmonic generation.

Third harmonic (TH) imaging is inherently suited for optical material characterization. Under linearly polarized illumination the total TH signal is dominated by the signal resulting from material interfaces. For symmetry reasons, circularly polarized illumination of a medium with isotropic or cubic symmetry yields zero TH, and prevailing signals originate from localized anisotropic sample sites. Such anisotropy may result from laser induced stress, crystallinity, or birefringence. Pairing THG with complementary imaging techniques proves to be a useful diagnostic for investigating additional material characteristics. We report TH imaging of 10 nm colloidal gold nanoparticles, 100 mN nanoindentations, nascent film anisotropy, and laser induced material modification of HfO₂ films both pre- and post-laser damage.

Local temporal shot-to-shot variation of a high-energy laser system is measured in order to model the maximum fluence that any location on the optic will be exposed to after N shots (Max-of-N). We constructed a model to derive an equivalent-Max-of-N fluence distribution from a series of shots of differing energy and contrast in order to calculate damage initiation and optics lifetime. This model allows prediction for Max-of-N effects when direct measurements of the fluence distribution are not available. Comparison to different laser systems will be presented in order to gain insight as to the physical origins of the Max-of-N effect.

We measured the single-shot and multiple-shot optical breakdown thresholds leading to optical damage of borosilicate glass, specifically BK7 glass, at 1.064 μm. We used 8-ns, single-longitudinal-mode, TEM00 laser pulses tightly focused inside a BK7 glass window. The radius of the focal spot was measured using surface third harmonic generation; it is equal to 7.5 μm. With this tight focus, the laser power at the breakdown threshold of BK7 glass is below the SBS threshold, and the effect of self focusing is small. We found the single-shot and multiple-shots optical breakdown thresholds to be deterministic. At the single-shot damage threshold, the optical breakdown in BK7 glass occurs on the trailing edge of the laser pulse, in contrast to fused silica in which the breakdown always occurs at the peak of the laser pulse. However, the multiple-shot damage threshold of BK7 glass occurs at the peak of the last laser pulse. Our single shot damage threshold for BK7 glass is 4125 J/cm², and our multiple shot damage threshold ranges from 3974 J/cm² for 2-shot damage to 3289 J/cm² for 31-shot damage. We also compare damage morphologies of BK7 glass with those of fused silica.

Surface thermal lensing (STL) technology was developed into an effective apparatus for the measurement and analysis of weak-absorption optical coatings. Previous work focused on the measurement of the whole sample and was based on the assumption that the film is very thin. In this paper, by changing the modulated frequency of the pump laser from 1 Hz to 101 kHz, we accurately controlled the thermal diffusion length and got both the amplitude and phase of the photothermal signal of different samples. Based on the comparison of these signals, some analysis of 1) the thermal conductivity measurement and 2) the depth localization of strongly absorbing layer was performed.

The influence of laser beam size on laser-induced damage performance, especially damage probability and laser-induced damage threshold (LIDT) is investigated. It is found that damage probability is beam size dependent when various damage precursors with different potential behaviors are involved. This causes damage probability and LIDT are different between tested under large-aperture beam and under small-aperture beam. Moreover, fluence fluctuations of large-aperture laser beam bring about hot spots moving randomly across the beam from shot to shot. Thus it leads to the most probable maximum fluence after many shots at any location across components is several times the average beam fluence. These two effects result in difference of damage performance of components in large-aperture lasers and in small-aperture lasers.

We observed multiple filamentation of a Terawatt fs-laser beam (λ = 800 nm, E = 170 mJ/pulse) after 1 km horizontal propagation in the atmosphere. The interaction of these filaments with the non-transparent targets was studied. The filaments were strong enough to damage the surface of optical windows like Ge and ZnSe even at long distances and under turbulent conditions. The damage effects were analysed by studying the modulation transfer function (MTF), the spectral transmission loss, the ablation depth and the damage threshold. LIBS was applied to estimate the plasma temperature during the interaction process.

Cr³⁺ co-doped Nd:YAG is very useful for the flashlamp or solar pumped laser through Cr³⁺ ion has broad absorption bands in the visible region and an efficient energy transfer caused from Cr³⁺ to Nd³⁺. But the process of the energy transfer is not understood well. It is also a problem that the absorption at near infrared is appeared with changes of Cr³⁺ to Cr⁴⁺. We investigated these problems by measuring the temperature dependence of fluorescence characteristics (lifetime, excitation fluorescence and absorption) and the change after UV irradiation. The excitation fluorescence spectra at 450 K increased because the Cr³⁺ absorption depended on temperature. The fluorescence lifetime excited at the Cr³⁺ absorption bands shortened with increase temperature, that is, the energy transfer time will be short. Solarization was appeared in Nd/Cr:YAG ceramics irradiated by KrF excimer laser pulses at 248 nm.

Magnesium fluoride (MgF₂) single crystal is expected as the alternative of Quartz for polarizing materials in high power lithography system. MgF₂ is anisotropic crystal and its physical properties are different along each crystal axes. Therefore it is difficult to make large diameter single crystal by using Bridgman method which is mainly used for growth of fluoride crystals. We have been studying on making large diameter and high quality single crystal by using Czochralski (CZ) method [1,2]. Previously we reported the stable growth of it with 150mm diameter. This time we succeeded to grow the crystal with over 200mm diameter. Additionally, by improving the purification process and growth process, we succeeded to reduce 75 percent of the amount of color center induced by irradiation of ArF laser.

We report on the characterization of AR coatings on fused silica as well as AR coated LBO crystals used in high power NIR/VIS laser applications, mainly by means of LID (laser induced deflection) absorption measurements. The comparison of different LBO crystals shows that there are significant differences in both, the AR coating and the LBO bulk absorption. These differences are much larger at 515 nm than at 1030 nm. Results from first absorption spectroscopy measurements combining LID technique with a high power OPO laser system indicate that the coating process affects the LBO bulk absorption properties. Additionally, an emphasis is placed on the importance of the independent calibration procedure. Here, the electrical calibration is compared to two other approaches that use either doped samples or highly absorptive reference samples in combination with numerical simulations. As example, LBO crystals and fused silica are taken to show the complexity and the existing diversity of the material's photo-thermal response and its influence on choosing the appropriate measurement concept. Finally, a new concept is introduced to significantly increase the LID sensitivity for optical materials featuring a low photo-thermal response. In the case of CaF2, a sensitivity enhancement of larger than factor 6 is obtained.

We have laser conditioned a couple of KDP-SHG and DKDP-THG samples thanks to a facility which delivers 6 ns fundamental (1,053 nm, noted 1ω) pulses, and the harmonics generated by the crystals themselves. The conditioning ramp has been established according to a model coupling statistics and heat transfer, in order to minimize the generation of bulk laser damage during the process. Then the efficiency of this procedure has been evaluated for both samples using two laser damage testing setups, and compared to the best monochromatic conditioning process known to date. For the KDP-SHG, it appears that this procedure is less efficient than the monochromatic conditioning. But it raises the resistance to laser damage of the SHG to a level compatible with the use on megajoule-class high power lasers. For the DKDP-THG, the efficiency of both procedures is quite similar. And even if the conditioned THG still exhibits laser damage within the range of high power laser working fluences at 351 nm, the density is only a few per mm3.

In this paper, we report on the second installment of our ongoing investigation into the nature of the laser survivability curve (LSC). The LSC has been traditionally viewed as a curve in the plane defined by fluence,φ , and the number of shots, N, which defines the frontier of assured survival. In this year's report we expand the concept of the survivability curve to a surface of survival, the laser survival surface (LSS), which is in turn used to develop a conditional probability estimate for survival. This conditional probability viewpoint is discussed as a possible basis for a cost-efficient life time test. The LSS is developed for test results at 1064 nm wavelength taken at atmospheric pressure and at vacuum.

Cavity ring-down (CRD) techniques based on measuring the rate of decay of light intensity inside the optical cavity, are widely used for trace gas analysis and high reflectivity measurement. In this presentation a filtered optical feedback CRD (FOF-CRD) technique employing a multi-longitudinal-mode continuous-wave diode laser is investigated for measuring high reflectivity of high reflective mirrors. The original spectrum of the diode laser without the effect of FOF has two longitudinal modes covering tens of the free spectral ranges (FSR) of the ring down cavity (RDC). Due to the relatively broadband spectrum, the theoretical efficiency of coupling the laser power into the RDC is less than 0.05%. In the FOF-CRD scheme, on the other hand, the FOF induced overall spectrum broadening is experimentally observed, with the diode laser running with several longitudinal modes. However the bandwidth of each longitudinal mode is significantly reduced. The coupling efficiency of the laser power into the RDC is higher than 20% in FOF-CRD technique. The enhancement of the coupling efficiency induced by the FOF effect is nearly three orders of magnitude. High accuracy measurements of high reflectivity are achieved with this simple FOF-CRD scheme.

Operation of high fluence pulsed laser systems in space imposes various risks to optical components involved. Volatile organic components are omnipresent in vacuum vessels housing space-borne laser systems and can be the source for selective contamination of optics. Laser systems may respond very sensitively to absorption increases of their multiple optical surfaces leading to inacceptable transmission losses and system degradation. In the recent past, thorough and long term laser tests, performed at the optics qualification laboratories at DLR and at ESTEC using space relevant and model substances, have revealed the onset, the built-up, and the later stages of the deposition process. It was found that these deposits tend to accumulate preferably on the laser footprint area of the optic. Observed thicknesses are on the order of several tens of nanometers, which can be sufficient to induce noticeable absorption. Sensitive techniques for insitu and ex-situ monitoring of these molecular contaminative effects under vacuum conditions were developed and are applied successfully. They are summarized in this paper, along with the phenomena, which are significant for the appearance of deposits. In addition, adverse conditions, which are favorable for provoking deposits, are communicated. Finally, mitigative and preventive methods are discussed.

Plumes were generated by ablation of graphite using a 248 nm excimer laser in the presence of low-pressure argon at 50-1000 mTorr. The pulsed laser deposition of energy on carbon/graphite targets at fluences of 1-5 J/cm² in low pressure argon backgrounds yields emissive plumes with large kinetic energies (estimated between 10-200 eV ), driving the formation of a shock front with large Mach numbers (M). The plumes were investigated using element specific imaging (filtered and gated ICCD camera), time-of-flight experiments, and UV - VIS - IR spectroscopy. We expect to see contributions from atomic carbon as well as the C2 diatomic. Studies showed the importance of plume/substrate interaction in causing secondary excitation/interaction phenomena. The propagation of the shock front is independent of ionization species and adequately characterized by the Sedov-Taylor shock model during the early life-time of the plume if the dimensionality is allowed to deviate from ideal spherical expansion. The ideal efficiency of energy conversion from laser pulse to shock expansion is investigated. The low background pressures between 50 and 1000 mTorr are sufficient for the generation of a strong shock front with significant thickness, but may be too low to develop three-dimensional flow. It can be shown that shock strength is proportional to the Mach number.

In this paper, we demonstrate the formation of self-organized periodic nanogratings on the titanium surface under the irradiation of a single-beam femtosecond laser. We vary various laser parameters such as the laser fluence and the number of laser pulses in each spot to fabricate self-organized nanogratings on the titanium surface. We investigate that the direction of the nanogratings is perpendicular to the direction of laser polarization. We also report on the dependence of the nanogratings' period, produced on titanium surface, on the laser fluence and the number of irradiated laser pulses in each spot. Nanogratings' period show increasing trend with the increase of the laser fluence, whereas show decreasing trend with the increase of the number of applied laser pulses. Furthermore, we briefly explain the formation mechanism of the self-organized periodic nanogratings, produced on the titanium surface. The reasons behind the dependence of nanogratings' period on the laser fluence and the number of laser pulses are explained as well. The self-organized nanogratings are mainly produced due to the interaction of the high-intensity incident laser beam and the laser induced plasma waves. Above certain threshold energy, phase explosion takes place, which in turn causes the formation of selforganized nanogratings on the titanium surface.

All dielectric HR mirror coatings consisting of AlF₃/LaF₃/Oxide layers were deposited on DUV grade fused silica and CaF₂. A novel technique was employed to measure the absorption of these mirrors during irradiation by a 193nm ArF excimer laser source. The method involves the application of a photothermal measurement technique. The setup uses a Shack-Hartmann wavefront sensor to measure wavefront deformation caused by the heating of the coating by the ArF beam. Laser calorimetric measurements of absorption were used to calibrate the wavefront sensor. Gage R&R (Repeatability & Reproducibility) measurements were done to show that this is a practical test technique for use in production. The new test setup was used to investigate HR mirror coatings both before and after exposure to high average power ArF laser beams. HR mirror samples were irradiated by a 193 nm kilohertz laser source for either 500 million or 18.6 billion pulses. The spatial resolution is sufficient to make wavefront distortion measurements both inside and outside of the laser beam footprint. The differences between wavefront distortion measured inside the beam footprint compared to measured outside the beam footprint can be explained by compaction of the coating in the area heated by the ArF laser. Interesting wavefront distortion results from testing mirrors with either fused silica or CaF₂ substrates can be explained by considering the figure of merit of these materials for excimer laser mirror substrates.

We report on a comparative study of a variety of fused silica materials for ArF laser applications, which differ regarding their OH content. Laser induced deflection (LID) technique is applied to measure directly and absolutely the absorption coefficient in fused silica materials at 193 nm as a function of the incident laser fluence in the range 1...3 mJ/cm² before and after applying 20 million shots at a fluence of 5 mJ/cm². In addition the laser induced refractive index change is detected by interferometer measurements after the prolonged irradiation for all samples. Prior to the long term irradiation, low OH containing fused silica ([OH] < 80 wt-ppm) exhibits both, the lowest absorption coefficient and the lowest absorption increase with fluence (dk/dH) in the range 1...3 mJ/cm². During 20 million laser pulses at 5 mJ/cm², however, the absorption and the dk/dH values show a strong increase for the low OH containing fused silica. In contrast, the absorption of the medium OH containing samples ([OH] = 200...650 wt-ppm) is highest prior to the long term irradiation but is remarkably lowered throughout the 20 million laser pulses. High OH containing fused silica ([OH] > 900 wt-ppm) shows an intermediate absorption level, which only slightly increases or decreases during the irradiation with 20 million laser pulses. The ArF irradiation induced refractive index change is positive (= compaction) for all samples at the fluence 5 mJ/cm². For analysis, a particular material classification is taking into account. For low and medium OH containing samples, referred to as compaction-dominated, the compaction factor increases with the OH content. For high OH containing samples, referred to as rarefaction-dominated, the resulting compaction factor decreases with increasing H₂ content.

A measurement system for quantitative determination of both surface and bulk contributions to the photo-thermal absorption in DUV optics was developed. It is based upon a Hartmann- Shack wavefront sensor with a sensitivity of ~λ/10000 rms, accomplishing precise on-line monitoring of wavefront deformations of a collimated test laser beam transmitted perpendicular through the excimer laser-irradiated site of a cuboid sample. Caused by the temperature dependence of the refractive index as well as thermal expansion, the initially plane wavefront of the test laser is distorted into a cylindrical shape, with bending ends towards the surface. Sign and magnitude depend on index change and expansion. By comparison with thermal theory, this transient wavefront distortion yields a quantitative absolute measure of bulk and surface absorption losses in the sample. First results for fused silica are presented.

Significant effort is pursued for the development and optimization of lithography grade materials aiming for ultra-low optical losses. Nowadays, very sophisticated crystal growing techniques are available. The surface finish of these DUV substrates has to be considered in an analogous manner, as the performance of thin film optical coatings may directly be influenced by the surface composition. Using laser calorimetry according to ISO 11551, the treatment-dependent surface contribution to the overall absorption of lithography grade substrate materials is deduced. The sensitivity enhanced test setup allows for a detailed study at ultra-low fluences - typical for current deep ultraviolet lithography applications. The results on absorption measurements are supported by an innovative surface qualification method, deriving both, characteristics on roughness and near surface stoichiometry, which are the footprints of applied polishing methods and, further, handling conditions on the one hand side, and a consequence of cleaning procedures and dose dependent exposure to DUV-radiation on the other side.

The energy of a laser beam, irradiating a surface, is primarily absorbed by electrons within the solid. In actual transparent materials, absorption is low. High-intensity lasers may, however, be absorbed by initially bounded electrons through nonlinear processes. The increase of free electron density leads eventually to dielectric breakdown and the material becomes highly absorbing. We present theoretical studies on the dynamics of electrons in dielectrics under irradiation with a visible high-intensity laser pulse. We consider microscopic processes determining absorption, redistribution of the energy among electrons and transfer of energy to the crystal lattice. We review different aspects of electronic excitation, studied with time-resolved models as the Boltzmann kinetic approach and the time and spatial resolved multiple rate equation. Further we investigate criteria for damage thresholds. Two concepts compare, namely a critical free electron density and the melting threshold of the lattice. We show that in dielectrics both criteria are fulfilled simultaneously. Optical parameters depend on the density of free electrons in the conduction band of the solid, so the free electron density directly leads to an increased energy absorption causing material modification. We present results on the spatial dependence of dielectric breakdown.

Transparent dielectric layers on semiconductors are used as anti-reflection coatings both for photovoltaic applications and for mid-infrared optical elements. We have shown recently that selective ablation of such layers is possible using ultrashort laser pulses at wavelengths being absorbed by the semiconductor. To get a deeper understanding of the ablation mechanism, we have done ablation experiments for different transparent materials, in particular SiO₂ and SixNy on silicon, using a broad range of wavelengths ranging from UV to IR, and pulse durations between 50 and 2000 fs. The characterization of the ablated regions was done by light microscopy and atomic force microscopy (AFM). Utilizing laser wavelengths above the silicon band gap, selective ablation of the dielectric layer without noticeable damage of the opened silicon surface is possible. In contrast, ultrashort pulses (1-2 ps) at mid-infrared wavelengths already cause damage in the silicon at lower intensities than in the dielectric layer, even when a vibrational resonance (e.g. at λ = 9.26 μm for SiO₂) is addressed. The physical processes behind this, on the first glance counterintuitive, observation will be discussed.

Growth of laser induced damage on the surface of fused silica plays a major role in determining the optics lifetime in high power laser system. Previous studies proved that the size of the crater increased under successive laser shots, but that of the gray haze and CO2 laser mitigation spot remained constant. In this study, Scanning electron microscopy (SEM), focus ion beam (FIB) and profiler were applied to observe their vertical and horizontal cross sections. Energy dispersive spectrometers (EDS) micro-analysis technique and fluorescent microscopy were used to detect the differences of chemical composition and molecular structure among the three. Results showed that the absorbing defect and crack was found in the crater, which did not exist in the gray haze and mitigation spot. Finite difference time domain (FDTD) was applied to calculate the light intensity distribution. It's found that the peak light intensity around the crater was much higher. Based on the above analysis, a growth mechanism of laser induced damage in fused silica was proposed.

Photoluminescence excited by 325 nm laser light is used to investigate defect populations existing in different surface flaws in high purity fused silica and to achieve a better understanding of laser damage mechanisms. Luminescence bands peaking at 1.9, 2.1, 2.3, 2.7 and 3.1 eV have been detected in the spectral area ranging from 1.6 up to 3.6 eV. According to the literature, the 2.3 eV band would be due to STE's (Self Trapped Excitons) relaxation. In order to study this hypothesis, temperature dependent experiments have been driven in the 90 K-300 K range. For indentations as well as laser damage, we show the evolution of luminescence spectra with temperature. Contrarily to the well known behavior of STE's, which shows a change of several orders of magnitude for luminescence intensity, the 2.3 eV band is weakly influenced by temperature decrease, from the ambient down to 90 K. The Gaussian decomposition of spectra allows dividing the five luminescence bands in two categories. The first one corresponds to bands showing a significant intensity enhancement with temperature decrease, and the second one to bands remaining insensitive to the fall in temperature. That classification may provide helps in order to establish links between luminescence bands and defects.

In the past decade it was demonstrated experimentally that negatively-chirped laser pulses can lower the surface LIDT for wide band-gap materials by decreasing the number of photons required for photoionization on the leading edge of the pulse. Similarly, simulations have shown that positively-chirped pulses resulting from selffocusing and self-phase modulation in bulk dielectrics can alter the onset of laser-induced material modifications by increasing the number of photons required for photoionization on the leading edge of the pulse. However, the role of multi-chromatic effects in free-carrier absorption and avalanching has yet to be addressed. In this work a frequency-selective model of free-carrier dynamics is presented, based on a recently extended multi-rate equation for the distribution of electrons in the conduction band. In this model free-carriers gain energy from the field by single-photon absorption at the instantaneous frequency, which varies as a function of space and time. For cases of super-continuum generation it is shown that a Drude-type absorption can vary from 50% to over 200% the absorption rate as evaluated at the central pulse frequency only. Simulations applying this model to ultrafast laser-plasma interactions in fused silica explore how pulse chirps may be used as a distinguishing parameter for LID resulting from otherwise identical pulses.

Due to its high nonlinear coefficients, KTP (KTiOPO₄) is one of the most important nonlinear optical materials for frequency doubling of Nd:YAG lasers. Former studies suggest a certain cooperativeness of the laser induced damage mechanism between the 1064 nm and the 532 nm wavelengths present during second harmonic generation. We report on experiments that allow confirming and quantifying the cooperativeness of the laser damage mechanism in this material and compare it to known data from KDP. A damage scenario based on the formation of color centers, which are also responsible for the gray tracking effect, will be presented.

The dynamics of electrons and holes in potassium dihydrogen phosphate ( KH₂PO₄ or KDP) crystals and its deuterated analog (KH₂PO₄ or DKDP) induced by femtosecond laser pulses is investigated at λ = 800nm. To do so, experiments based on a femtosecond time-resolved interferometry technique have been carried out. It is shown that two relaxation dynamics exist in KDP and DKDP crystals. In particular, it appears that one dynamics is associated with the migration of proton/deuteron in the crystalline lattice. Both of the dynamics correspond to physical mechanisms for which the multiphoton order required to promote valence electrons to the conduction band is lower than the one of a defect-free crystal. These results suggest the presence of states located in the band gap that may be due to the presence of defects existing before any laser illumination or created in the course of interaction. In order to interpret the experiments, a model based on a system of rate equations has been developed. Modeling results are in good agreement with the experimental data, and allow one to obtain fundamental physical parameters governing the laser-matter interaction as multiphoton absorption cross sections, capture cross sections, recombination times, and so forth. Finally, it will be shown how these results can be used to the understanding of laser-induced damage by nanosecond pulses in inertial confinement fusion class laser aperture.

This paper reports a study on correlation between stress field generated by extensive crystalline defects such as dislocation or growth boundaries and laser damage. It is found that stress fields decrease laser damage resistance. This result is compatible with the hypothesis that laser damage precursors consist of clusters of punctual defects. Indeed, such defects are affected by stress fields as their concentration varies in order to minimize the free energy of the crystal. Chemical analysis carried out on one of the crystal tend to show that the punctual defects involved are intrinsic rather than extrinsic.

Avalanche ionization plays a crucial role in the photoionization of dielectric materials and as such is important for optical damage. Although it has been investigated closely during the last years, there is little experimental evidence of how the impact ionization parameter changes with electron density and/or incident pulse intensity. One reason is that in most dielectric materials there are several competing ionization and relaxation processes. Here we present an UV-pump IR-probe experiment that allowed us to isolate the avalanche ionization from other major ionization processes, especially multiphoton ionization, electron tunneling, and relaxation into traps and their re-excitation. We have measured the intensity dependence of a transmitted IR pulse, propagating through a thin sample of UV-grade sapphire (α-Al₂O₃), after seeding electrons in the conduction band with a UV pulse. We show that the assumption of an intensity independent impact ionization factor α cannot explain the results. Application of a simple avalanche ionization model within the flux-doubling approximation requires an intensity dependent coefficient a(I) to explain the data. We also determined the two photon absorption coefficient of sapphire at 266 nm (β⁽²⁾ = (2.7 ± 0.1) • 10^-11cm/W) as well as the "free" electron absorption cross section for 800 nm of conduction band electrons in sapphire (δ₀ = (12.5 ± 0.2) •10^-18 cm²).