This PDF file contains the front matter associated with SPIE Proceedings Volume 11910, including the Title Page, Copyright information, Table of Contents, and Conference Committee lists.

The ability of short pulse laser ablation in liquids to produce clean colloidal nanoparticles and unusual surface morphology and microstructure has been employed in a range of practical applications. The results of large-scale molecular dynamics simulations help to uncover the key processes that control the structure of laser-modified surfaces and nanoparticle size distributions generated by pulsed laser ablation in liquids. The simulations are performed for one-component metals, bilayer thin films and alloy targets. The predictions of the simulations include the emergence of Rayleigh–Taylor hydrodynamic instability at the interface between ablation plume and liquid environment, as well as the limited elemental mixing in the colloidal nanoparticles generated by laser ablation of bilayer films.

Optical thin films multilayer coating has been driving the advancement of ultrashort laser technology towards ever shorter pulses and broader bandwidth. Deposition and sputtering of dozens of thin film layers with sub-nanometer accuracy. It permits controlling of the spectral phase and amplitude of optical radiation over an optical octave and beyond. We give overview on non-linear effects in multilayer ultrafast coating shortly before damage threshold appear as well as damages. We describe ways how to use or post-pone these effects and damages to higher fluences.

Typical continuous-wave (CW) laser damage, which is based on average power, differs considerably from its ultra-short-pulse counterpart, which is based on maximum field. Despite this fundamental difference, it has recently been found that CW breakdown in the presence of surface particle contamination (i.e. dirt) has some of the dependences of ultra-short pulse breakdown, such as bandgap dependences, albeit based on very different physical mechanisms. Further studies have shown that particles in air are also a danger to surfaces in high average power applications, with laser breakdown potentially occurring at power levels well below that of both clean optics and optics with fixed surface particles.

Low photon energy of mid-infrared radiation supports generation of non-equilibrium free-carrier plasma in transparent optical materials via nonlinear absorption and associated inter-band electron excitation. The collision rate of the plasma may be low so that the free carriers experience from zero to very few collisions per electron per pulse for pulses shorter than 200 fs. Those features of the free-carrier plasma support specific mechanisms of light absorption. We consider solid-state analogs of the Brunel absorption at interfaces, Landau damping, and nonlinear absorption by free-carrier stimulated inverse bremsstrahlung. Parametric scaling of those mechanisms is compared against that of the traditional models.

Using Z-scan and femtosecond 2.35-µm pulses, we study high-order nonlinear effects – multiphoton absorption (MPA) and nonlinear refraction (n2) in several notable mid-IR nonlinear monocrystals. We find that the multiphoton absorption orders N vary from N=4 (GaSe, ZGP) to N=6 (GaP, ZnSe) and correspond to the crystals’ bandgaps.

A simple and cheap technique for post-compression of mid-IR laser pulses relies on nonlinear spectral broadening via SPM in mm-thick optical windows (plates) followed by recompression. Efficient broadening requires peak intensities of ~hundred GW/cm2, which in case of high average power (>10W) drivers puts serious requirements on the used optical materials’ thermal properties, purity, homogeneity, surface quality and LIDT of coatings. Several materials (YAG, fluorides, KBr, Si, ZnSe, ZnS) were tested with 3.2 µm, 13W (130 µJ, 100 kHz), 48 fs pumping. Measured nonlinear absorption, wavefront distortion, spectral broadening and long-term survival as a function of intensity will be shown.

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

Overview of machine learning methods and the practical steps needed for implementation. We will use examples from NIF to demonstrate steps from data preparation to the pros and cons of different machine learning methods.

The development of next-generation laser optics can be guided by studies looking to improve the laser-induced damage threshold of highly-reflective interference coatings. We model intense few-cycle pulses interacting with multilayer HfO2/SiO2 dielectric interference coatings using fully three-dimensional particle-in-cell simulations to which we have added a Keldysh model for photoionization and a dielectric model to include refractive properties of the materials. We explore the reflection, transmission, and absorption of the laser pulses. We use the predicted excited electron density and energy density to estimate damage thresholds for these optics. This research was funded by DOE STTR grant no. DE-SC0019900.

We report on the clear difference in the behavior of laser-matter interaction of tightly focused high-intensity pulses with the bulk of solids when chirping and tuning the wavelength of the laser driver from near-IR to mid-IR. In transparent dielectrics, laser wavelength scaling leads to the change in the heating mechanism which, in turn, leads to a weak dependence of the plasma formation threshold on the pulse duration in mid-IR (4.6 μm) and a significant impact in the case of the near-IR (1.24 μm) excitation. In the case of semiconductors, the ionization mechanism remains within the framework of the multiphoton approximation, but an increase in the photon order for excitation by mid-IR pulses leads to a decrease in delocalization processes and losses in the prefocal region.

Isaac Bass, Eyal Feigenbaum, Darin Anderson, Gabe Guss, Wren Carr High energy and power laser systems such as NIF operate near the damage threshold of its optical components. Stable operation requires control of the initiation and growth of the inevitable damage that will occur on its high-value optics due to factors such as beam contrast, non-linear effects, and contamination. This is accomplished through an optics recycling loop.[1] A key element in this loop is the Final Optics Damage Inspection (FODI) [2] system that monitors damage on the final optics preceding the target chamber. When the damage size reaches an upper limit, the optic is removed from the system and transported to the Optics Mitigation Facility (OMF). For fused silica optics, the damage is removed by a machining process using a CO2 laser.[3] This leaves a conical shaped depression in the surface of the optic. The core of the FODI system is an articulated telescopic imager at the center of the target chamber that can interrogate the final optics in any of the 192 beam lines entering the chamber. The detector in the imager is a two-dimensional charge coupled device (CCD) array. Individual optics in any beam line can be separately interrogated using an illuminator that injects light into the optic through one of its edges. This light then scatters or reflects from damage, defects, and cones to the FODI imager. The walls of the cones on the input surface of the optic, because of their orientations, are particularly efficient reflectors/scatterers of light. In many instances, the magnitude is sufficient to highly saturate the CCD pixels. This produces the well know blooming effect in CCDs when the charge exceeds the storage capacity of a pixel and spills into adjacent pixels. This result is a long line of saturated pixels when the charge is read out. The area of this line can be sufficiently large to obscure the growth of damage to sizes exceeding the upper allowable limit for removal of the optic. This can lead to either loss of the optic for recycling, or loss of the optic for any further use. The issue may become exacerbated by the prospect of increased use of even larger input cones to control the growth of exit surface damage by shadowing.[4] We will report on our study of light scattering by input surface cones and techniques to suppress the blooming effect it produces on the FODI imager. It includes high resolution imaging of the scattered light, its dependence on cone size and shape, modeling of the scattering process, and diversion of the scattered light by opposing exit surface cones. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. LLNL-ABS-??????. References: [1] Optics Recycle Loop Strategy for NIF Operations above UV Laser-Induced Damage Threshold, M. L. Spaeth, P. J. Wegner, T. I. Suratwala, M. C. Nostrand, J. D. Bude, A.D. Conder, J. A. Folta, J. E. Heebner, L. M. Kegelmeyer, B. J. MacGowan, D. C. Mason, M. J. Matthews & P. K. Whitman, Fusion Science and Technology Volume 69, 2016 - Issue 1 [2] Kegelmeyer LM, Clark RD, Leach RR, McGuigan DL, Miller Kamm V, Potter D, Salmon JT, Senecal JD, Conder AD, Nostrand M, Whitman PK. Automated optics inspection analysis for NIF. "Fusion Engineering and Design", Volume 87, Issue 12, December 2012, Pages 2120-2124 [3] I. L. Bass, G. M. Guss, M. J. Nostrand, and P. L. Wegner, “An improved method of mitigating laser-induced surface damage growth in fused Silica using a rastered, pulsed CO2 laser,” Proc. SPIE 7842, 7842202010. [4] R. N. Raman, R. Garcha, M. C. Rushford, G. Guss, and C.W. Carr, “A shadowing technique to arrest laser-induced damage growth on exit surface silica,” Proc SPIE 11173, 1117303 (2019)

Laser systems capable of delivering energetic nanosecond pulses with large incoherent bandwidth spanning more than 10 THz are currently under development for the study of laser–plasma instabilities. A consequence of such a spectrum is random intensity fluctuations, which may have a significant effect on laser damage thresholds. A special damage test station has been built for studying the effect this unique radiation has on common laser materials. We will share measurements of damage thresholds in coated optics, KDP, transmission gratings, and bare materials under various conditions aimed at understanding the role temporal fluctuations play in laser damage.

We present the development of large aperture fabricated input surface sites (~1-2 mm size) designed to “shadow” damage sites on the exit surface and arrest their further growth. The inherent expanding wave which leads to exit surface damage, motivates our diversion from a conic shape and development of more intricate geometries. We discuss the design considerations, the fabrication and resulting geometries, the optical modeling of light propagation through these geometries, the imaging of the shadowed region as well as the damage-inducing expanding wave intensification they lead to, and the large beam damage testing and their analysis.

This papers reports on experiments designed and conducted to support the development of the proposed U.S. national laser damage standard. The key aspects of the proposed standard requiring experimental verification are test fluence profile, test scan design and test statistics. The paper will discuss the need and influence of each of these experiments on proposed standard. We will also discuss the experiment's designs and results. This paper will conclude with any revisions to the proposed standard informed by the experimental results.

When discussing laser-induced damage phenomena, the main parameter of interest is the laser-induced damage threshold (LIDT). Since LIDT is a function of irradiation time (or number of pulses), its characterization is of critical importance when designing reliable industrial or medical laser systems or even planning long-term space missions involving high power lasers. Within laser-induced damage community, decrease of LIDT with increase in irradiation time (the so-called fatigue effect) is often estimated by using the S-on-1 test procedure described in the ISO 21254-2 standard. However, due to measurement limitations, S-on-1 tests are usually carried out for relatively small numbers of pulses, therefore additional extrapolation methods must be used in order to predict lifetime of optical components. The simple extrapolation procedure provided by the ISO 21254-2 standard no longer meets the demands of the community as it ignores existence of multiple failure modes and data censoring resulting from different damage detection techniques, therefore a new approach for LIDT extrapolation is much needed. In our previous work [1], we explored application of accelerated lifetime testing (ALT) techniques on S-on-1 test data. ALT approach treats S-on-1 experiments as sets of lifetime (or time to failure) distributions at different fluence levels instead of damage probability curves at different numbers of pulses. We have shown that this approach, combined with Bayesian inference and Markov chain Monte Carlo (MCMC) sampling, is well suited for extrapolation and uncertainty evaluation of S-on-1 experiments. However, proper application of these methods require empirical knowledge about lifetime distributions for different material types and irradiation parameters. Therefore, in this work an attempt is made to experimentally characterize lifetime distributions of highly reflective dielectric and metallic laser optics at both nanosecond and femtosecond pulse durations. UV and IR wavelengths as well as influence of irradiation fluence and polarization state are also considered. The results of this study provided insights into lifetime distributions of laser-induced damage and helped to shape guidelines for extrapolating S-on-1 experiments to longer irradiations. [1] L. Smalakys, A. Melninkaitis, Predicting lifetime of optical components with Bayesian inference, Opt. Express 29, 903-915 (2021).

The recent development utilizing automatic microscopy to identify damage sites on an optic for damage mitigation at the National Ignition Facility (NIF) has resulted in a large set of damage dataset for growth analysis. In this work we will examine how the pre and post installation damage sizes are used to analyze the cumulative probability of growth for damage sites that have been exposed to multiple laser shots. The analysis can form the basis to derive single-shot probability growth behavior of fused silica damage site as a function of size.

The design principles of a 20-fs, near-infrared laser-induced–damage testing system are presented. In addition to typical damage-initiation threshold tests such as S-on-1 and raster scan, this system is capable of studying other performance-relevant metrics, such as damage growth and functional thresholds. Experimental results for a hybrid mirror sample are presented as a demonstration.

Temporal and spectral Laser-induced UV-loss studies in DUV-fibers with undoped synthetic silica core, mainly low-OH silica and modifications, were carried out with an automated set-up using pulsed 213 nm, 266 nm and 355 nm Nd-YAG Lasers. A multiple Gaussian band approximation was used to separate the individual but superimposed optically active UV defects leading to new temporal results of the individual UV defects. The curve shape of spectral losses was found to be different and strongly influenced by the Laser wavelengths, which are within absorption bands in low-OH silica, reducing the correlated defect at the beginning of irradiation significantly. In addition to solarization caused by 266 nm Laser damaging, annealing of some defects was observed. Depending on the fiber type, including hydrogen-treated types, different time constants were determined. Obvious transformations were observed during damaging with Lasers and/or broadband D₂-lamp. Finally, an improved fitting of the UV attenuation or UV-induced losses in UV-fibers around 250 nm (ODC-II region) was possible by introducing two bands with slightly different peak wavelengths and temporal behaviors.

Laser users desire optics that not only have a high laser induced damage threshold (LIDT), but also perform consistently over time. In fact, some aerospace and military applications can require long lifetime optics for remote environments where replacement is untenable. UV laser optics are susceptible to slow performance degradation with exposure to sub- Laser Induced Damage Threshold UV laser light due to the process of UV fatigue. Although UV fatigue of optics is well documented, the mechanisms are not well understood. Design and construction of a UV lifetime testbed to investigate the mechanisms of UV fatigue, preliminary results, and future directions are presented.

Chemical vapor deposited (CVD) Zinc Selenide (ZnSe) window are often used in the high power laser system because of their extremely low (near-/far-)infrared absorption, and large transmission range (0.5um-22um). A good CVD process enables us to achieve the extreme purity needed for high-powered optics: 99.999% purity. Zinc vapor and H2Se gas react to form sheets of ZnSe on graphite susceptors. Different grain-sized polycrystalline ZnSe plates are precisely prepared and tooled to form optical windows or optical gain media. In recent years, in the area of few-cycle laser interaction with optic components, many new phenomena were found and investigated. Electron dynamic calculations were clearer and richer in this time domain[1-2]. Due to the potential applications in NIR few-cycle laser or attosecond laser generation, the single shot and multi-shot laser-induced damage threshold are tested in a multiplate supercontinuum-based few-cycle laser platform (12fs 1030nm centered) in CREOL according to ISO 21254. The damage threshold of different pulse numbers was carefully extracted. Laser-induced damage evolution along with the pulse number and grain boundary density are carefully characterized by SEM, AFM, and confocal Raman spectroscopy. Our experimental results will show the impact of grain boundary density on the laser-induced damage threshold of polycrystalline materials, which is of great value for future ultrashort laser pulse applications.

Fused silica an optical material of choice for high energy laser applications offers a wide range of advantages in the UV and NIR wavelength range. Increasing laser performance (especially with critical fluences) require detailed information on laser induced damage threshold of the optical material itself, and not just the coating. Therefore, S-on-1 measurements according to ISO 21254-2 were commissioned from two different, experienced, and well-respected laboratories. The resulting LIDT (laser induced damage threshold) values are discussed in the context of current publications and differences are worked out. The measurements focus on the ns-pulse regime at 1064 and 355nm. The results differ up to a factor of 10 between different laboratories. In comparison to the literature study, we noticed a spread of measured values around a factor of 20. Non-binding recommendations of the standard as well as the optical setup have significant influence on the obtained LIDT values. However, depending on the optical design of the application, one or the other optical setup with the corresponding LIDT value can be relevant. In this respect there is no such thing as a single, generally applicable value. Nevertheless, a standard should enable material comparisons under sufficiently standardized conditions.

Interactions of liquid crystals (LC’s) with polarized light have been studied widely and have spawned numerous device applications across wide regions of the electromagnetic spectrum. To date, little is known about the effect of incident light polarization state on laser-induced damage thresholds (LIDT) for LC’s exposed to high-peak-power, nanosecond-pulsed lasers. This work describes the LIDT dependence on incident polarization for light encountering a LC mesophase in a high degree of molecular orientation. In one example chiral nematic LC, the 1053 nm, 1.4 ns LIDT ranged from 17.7-30.5 J/cm2 in the same device, depending on the input polarization handedness and ellipticity.

Laser-induced damage experiments on HfO2 and Nb2O5 were performed with 500 fs pulse duration at 1030 nm wavelength. Threshold fluences in dependence of beam size have been determined within the range of effective beam diameters from 40 µm up to 220 µm. The results suggest no dependence of beam size effect on effective beam diameters at least between 80 µm to 160 µm. The LIDT results indicate importance of appropriate focusing conditions and beam measurement to qualify the optics for use in lasers with large beam sizes.

Dielectric cavity mirrors for storage-ring free-electron lasers (FELs) in the VUV range have to overcome several challenges including resistance to synchrotron radiation and long-term stability in a contaminated vacuum. While oxide materials are not suitable below 190 nm, pure fluoride materials are not stable in this challenging environment. It has not been possible until now to generate lasing below 176 nm in a storage ring FEL. Duke University/TUNL and Laser Zentrum Hannover have successfully developed new strategies and produced mirrors with a high reflectivity of 95 %, which enabled FEL lasing between 168,6 and 176,7 nm.

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

The effect of oxygen partial pressure on the properties of Al₂O₃ films deposited by electron beam evaporation has been investigated through a combination of spectrophotometric and interferometric characterization techniques. As oxygen partial pressure increases, a decrease in the refractive index is observed, as well as a shift towards less tensile films once they are exposed to ambient conditions. This decrease in tensile stress was observed to be correlated with water content in the films. Increasing oxygen partial pressure during deposition improved film stoichiometry, absorption, and laser induced damage threshold (LIDT) at 351 nm.

Following customer demand, silicon oxynitride optical coatings were produced by ion beam sputtering (IBS). The multilayer sequence of the samples was designed such that the low refractive index was provided by SiO2, whereas for different designs a different composition of silicon oxynitride (n≈1.5–1.9) was used as high refractive material. This was realized by varying the ratio of the reactive gas components N2 and O2. LIDT measurements were carried out using a pulsed laser (9ns FWHM) @1064nm. Additionally, comparative measurements were carried out with frequency doubled pulses at 532nm. The performances of the designs were compared with their oxide-based QWOT-equivalents.

One third of the exchanged final turning mirrors on the National Ignition Facility have been laser damaged by 3ω (351nm) target backscattered light. The mirrors are spectrally complex hafnia and silica multilayer dichroic coatings with high 1ω reflection for the forward propagating beam and low 3ω reflection to suppress propagation of target backscatter. Alumina, a wider bandgap high index material than hafnia, was selected to improve the 3ω laser damage resistance despite a lower refractive index. A 2-4x improvement in 3ω LIDT was demonstrated with adequate 1ω laser damage resistance. Nodule ejection was the dominate laser damage morphology. An 18% decrease in the refractive index between alumina and hafnia results in coating designs that are 3x thicker. Similarly, a decrease in refractive index also decreases the angular bandwidth resulting in increasing the number of mirror types from two to six over an angular range of 16 degrees. The significant improvement in 3ω LIDT has justified beginning a scale up demonstration to full aperture (0.3m²).

Multilayer dielectric gratings（MLDGs）have been widely used in chirped pulse amplification due to their high laser induced damage thresholds（LIDTs）. The quest for MLDG LIDT improvement is endless. For MLDGs applied in picosecond(ps) lasers, damage shows the characteristics of both thermal effect and nonlinear effect. The thermal damage of multilayer dielectric films (MLDFs) and MLDGs were investigated using a 1064 nm laser with a duration of 8 ns in our study. Differently from previous 1-on-1 studies, Raster Scan method is adopted to investigate the effect of low-density defects on the laser damage resistance of MLDFs with different top layers and MLDGs. The results show that the LIDTs of MLDGs are half of those MLDFs. For MLDFs with the top layer of HfO2, the damage behaves the ablation of the top layer material due to the surface strong electrical field. For MLDFs with top layer Ta2O5 and SiO2, the typical morphologies are nodule ejections. The initial damage of MLDGs fabricated by etching these three kinds of grating films are similar, and all behave nodule ejections. This indicates that reducing nodule defects can help the MLDGs LIDT improvement in ps pules. These results provide guidance for process optimizations of MLDG fabrication.

When designing femtosecond laser mirrors, one tries to minimize the peak intensity within the high-index layers to increase the laser-induced damage threshold. Typically, the optimization procedure utilizes the electric field distribution in the layer stack generated by monochromatic irradiation. This approach is sufficient for certain structures like high-reflectors based on quarter-wave films. More complex structures require to take into account the exact evolution of the pulse parameters such as peak intensity and duration within the multi-layer system. We exemplify this by discussing a merit function that can be included in femtosecond optics design.

The temperature distribution of a slightly absorbing optical thin film atop a glass substrate subjected to a laser pulse train of pulse duration 1ns is derived from the classical heat equation. The critical fluence defined by the point at which thermal damage occurs is derived and the dependence of laser induced damage threshold (LIDT) on repetition rate, pulse duration, wavelength, thermal properties, beam and optic dimensions is discussed. A comparison is made between the theoretical LIDT and well known experimentally observed scaling laws for both Gaussian and ”Top-Hat” pulse profiles. It is found that the ratio of beam to optic diameter is an important parameter in LIDT determination. Larger substrates are found to have a lower LIDT and it is suggested that LIDT follows an inverse scaling rule with respect to repetition rate.

We investigate contamination induced in grating-like structures during the etching process as a possible precursor to laser-induced damage. Our experimental model utilizes 5-mm line structures fabricated in E-beam–deposited coatings of silica using reactive ion etching (RIE) and reactive ion beam etching (RIBE). This makes it possible to compare the behavior in the pillars and trench regions. The results suggest that surface contaminants are primarily fluorinated polymers, while embedded contaminants consist primarily of carbon with very low detection of fluorine. Samples fabricated by the RIBE method exhibit significantly reduced roughness in the trenches, yet still present similar embedded contamination.

In Laser Powder Bed Fusion (LPBF), there are high demands on the cleanliness of the protective window (PW). During processing, however, process-by-products may accumulate on the PW surface and thereby change the process boundary conditions. In this work, a methodology to assess the condition of PWs in a LPBF machine using a line scan camera is presented. Image processing is used to detect the degree of contamination on the PW surface by using a thresholding algorithm. The line scan camera results are further validated by ex-situ microscopic images of the contaminated surface of the PW. The higher resolution of these images enables the quantification of particle count and size distribution. Moreover, the influence of the contamination degree of the PWs on the beam quality is investigated. A laser focus shift of up to 22.6 mm against the direction of beam propagation within a caustic measurement as well as a minor decrease in relative density of 0.08 pp in 316L samples were detected due to the contamination. Based on this work, methods for monitoring the health condition of PWs in LPBF can be derived.

Optical components are the main parts in laser systems, which limits the total generated output power due to laser-induced damage. At nanosecond laser pulses materials experiences thermal expansion phenomena, therefore optical coatings gain stress leading to breakdown. Moreover, the main resistance to laser radiation is limited by material itself (band gap). Glancing angle deposition method is presented to produce porous nanostructured coatings, which are characterized by low inner stress. Optical resistivity dependance on porosity of several materials such as aluminium, niobium and silicium oxides singlelayers was evaluated. Furthermore, all-silica Bragg mirror is formed and optical properties investigated in different environments to achieve stable and superior optical resistance.

Organic contaminants on optical components can degrade optical properties, thus limiting the energy enhancement of highpeak- power laser systems. It is still challenging to remove organic contaminants on the SiO2 sol-gel antireflection film and avoid damage. In this work, a low-pressure air plasma in-situ cleaning technique and a chemical reaction model of plasma cleaning were proposed to study the removal of organic contaminants. The optical properties of sol-gel AR films suffered from organic contaminants were evaluated by transmittance and laser-induced damage threshold, which recovered completely after 5 minutes of plasma cleaning without damage. Meanwhile, the hydrophilicity of the surface and the surface free energy were significantly increased after plasma treating. Surface composition analysis indicated that the CH and C-C bonds were reduced considerably, while abundant C=C and C=O bonds were produced after plasma cleaning. Optical emission spectrum analyzed the reactive species and its concentration in the air plasma as a reference for simulation. The chemical interaction process of oxygen radicals with organic contaminants was simulated by reactive molecular dynamics. The results can provide a guide for optical components in-situ cleaning.

We present a method for producing spatially invariant glass engraved meta-surfaces, which is scalable, has high mechanical stability, and has high laser damage durability. The process is based on dewetting a thin metal film on a glass, followed by dry etching and metal mask removal. We will present masking technology that enables deeper etching while maintaining sub-wavelength feature sizes, performance of the optimized metasurfaces as antireflective layers, mechanical stability and laser durability of the fabricated surfaces, and discuss ongoing work.

The laser-induced damage threshold (LIDT) of nematic liquid crystals is investigated in the femtosecond regime at 1030 nm. The thickness and breakdown of freely-suspended thin films (approximately 100 nm) of different mixtures (MLC2073, MLC2132 and E7) is monitored in real time by spectral-domain interferometry. The duration of laser pulses was varied from 180 fs to 1.8 ps for repetition rates ranging from single-shot to 1 MHz. The dependence of the LIDT with pulse duration suggests a damage mechanism dominated by ionization mechanisms at low repetition rate and by linear absorption at high repetition rate. In the single-shot regime, LIDTs exceeding 1 J/cm² are found for the three investigated mixtures.

For the semiconductor and liquid crystal display manufacturing process, resist removal by using laser irradiation has been investigated instead of conventional processes such as oxygen plasma and chemical method. An advanced laser resist stripping method for the positive-tone diazonaphthoquinone (DNQ) / novolak resist was successfully developed without causing the laser damage to the Si wafer. The pulsed laser irradiation in water can improve the resist stripping effect when compared with that of conventional atmosphere irradiation, however, the mechanism has yet to be clarified. In this study, we investigated the analysis of resist stripping phenomenon by using a high-speed laser imaging system. A pulsed laser at 640 nm (pulse duration: 40 ns) was used as an illumination laser and a CCD camera detected the reflectance image on the sample. Time resolution of this system depended on the pulse duration of illumination laser. Time-resolved images were acquired based on the “1-on-1” method. Time-resolved images were acquired from 40 ns to 10 us after the laser irradiation. At the laser irradiated spot, changes of the resist were observed after 40 ns from the laser irradiation. The resist was completely stripped from the Si wafer surface after 10 us. The duration of resist removal phenomenon in the water condition was longer than that in the normal atmosphere condition. A resist stripping mechanism could be elucidated by combining experimental high-speed laser imaging and a finite element (FE) analysis. The mechanism of the resist stripping in the water condition will be presented.

Degradation and resulting photo luminescence (PL) are sometimes induced by irradiation with UV lasers. The mechanisms of the degradation are dependent on the grades of silica glass and their production conditions. In this study, relationship between degradation phenomena and photo luminescence during the laser irradiation of pulsed 266 nm was evaluated with several glades of silica glass. Synthetic fused silica containing ca. 1000 ppm of OH (ES), silica glass produced from synthetic silica powder melted with oxy-hydrogen flame (S), and plasma torch (SD) were used　for this measurement. Pulsed 266 nm laser with 80 kHz was irradiated at the inside of silica glass by using lens with a focal length of 50 mm. Degradation at the focused area was observed from the transmitted laser power. PL from the focused area was also detected with a spectrometer. SD needs nine times longer time for degradation than ES. PL around 640 nm related to the NBOHC (SiO) was gradually increased with degradation change, and rapidly increased when the laser damage occurred at the focused area. The mechanism of the degradation during the laser irradiation of pulsed 266 nm will be presented.

The paper aims to compare the results obtained with the same laser source with a large beam and with small beams. These latest were shaped from phase objects implemented to obtain several small beams from a single larger beam. The consistency of the results from both sets of measurements is shown. It validates the assumptions made and the specific mathematical treatments implemented to establish the link between the two approaches. It also validates and strengthens the approach developed from the rasterscan procedure used to measure damage densities from the scanning with beams of small dimensions. This shows that small beam tests are reasonably representative of tests carried out with large beams.

The development of high power lasers implies specific needs of optics with very low absorption to avoid detrimental thermal effects up to damage of optical components, and associated metrology of these optics. In this work, we discuss on our recent developments of a thermographic-based technique to measure absorption: specifically, the Lock-In Thermography (LIT) technique applied in a multipass configuration to increase the effective power on the tested samples. We will present the system developed at the Institut Fresnel that is based on 1.5 kW Ytterbium fiber, that allows to submit samples to 5 kW effective power and measure absorption levels down to the ppm on optical coatings.

The pulse compression grating (PCG) is one of the most critical components of a high power chirped pulse amplification laser system to achieve the shortest pulse duration. Compared to metal gratings, a multi-layer dielectric (MLD) grating is a possible solution to improve the laser induced damage threshold (LIDT) of PCGs. Our previous work reported simulations of electron excitation dynamics in the interaction of MLD mirrors and femtosecond pulses (<100 fs). Here we present the study of the interaction of a MLD grating and a 50-fs pulse using a 2D dynamic simulation modeling both the E-field enhancement and transient material responses.

Antireflection coatings, containing alternating layers of hafnia (HfO₂) and silica (SiO₂), were deposited using electron beam (e-beam) evaporation for use in laser operations at 532 nm and 1064 nm in the nanosecond regime. The e-beam evaporation process produces coatings that are porous and therefore absorb water from the ambient environment. Consequently, humidity may affect the spectral performance of the coatings, and the laser damage resistance of the coatings may be affected as well. The purpose of this study was to compare the laser-induced damage thresholds of the antireflection coatings measured in the ambient environment at 0% and 40.5% relative humidity. At 1064 nm, the laserinduced damage thresholds at 0% and 40.5% relative humidity were almost the same. However, at 532 nm, the laserinduced damage thresholds at 40.5% relative humidity were nearly twice as high as those measured at 0% relative humidity. This indicates that humidity can inhibit lower-fluence precursors that would lead to laser damage at 532 nm in the nanosecond regime, thereby improving the durability of the coatings in a humid environment.

We are presenting our observations regarding laser induced degradation of coated optics exposed to the burst-mode pulsetrain of the optical pump-probe laser systems (PP-laser) at the European X-ray Free Electron Laser (EuXFEL). Degradation happens on HR and AR coatings from various suppliers during the course of prolonged exposure to the 515 nm, 800 fs burst-mode radiation. Since the lasers typically operate 24/7, coatings degrade within a few weeks to several months, depending on the requested burst lengths and number of pulses and hence integrated burst energies. Reduced performance of pointing- and pulse energy-stability and a drop of average power of the non-collinear optical parametric amplifiers (NOPAs) result. As user facility, the EuXFEL strives to maximize up-time and to reliably provide high quality x-ray and optical laser beam lines for user experiments, which is compromised to some extent by the observed coating degradation. In this report we show our investigations into the effect so far. In collaboration with the coating manufacturer Layertec GmbH, an improved mirror coating was found, which prolongs optics life time by nearly a factor of 3.

The laser-induced damage threshold (LIDT) of anti-reflective (AR) coatings is a limitation when it comes to the maximum laser power. Furthermore, LIDT data for AR coatings on nonlinear crystals are rather scarce in scientific literature, mainly available for ns pulse durations. Lately, however, ps and fs pulse durations are also becoming important. Thus, in this work, four LBO crystals were IBS coated on both sides with two distinct coatings, namely, AR@1064 nm and AR@355 nm. These coatings were then LIDT tested for both UV and IR wavelengths with ns, ps, and fs pulses.

Silica substrates coated with organic thin films were exposed to stainless steel and silica micro-particles to determine the effectiveness in preventing particle contamination and cleaning efficiency by air flows. Three specially designed monolayers coatings were developed and tested. Laser induced damage tests were conducted to confirm that the coatings do not affect the LIDT values. The results suggest that although the accumulation of particles is not significantly affected, the coated substrates exhibit significantly improved cleaning efficiency with air flow. A size distribution analysis was conducted to study the adsorption and cleaning efficiency of particles of different sizes.

The developments of ultra-high purity material which resist the damage by DUV laser are strongly required. According to the general mechanism of laser-induced damage, some kinds of defects and contaminations on the optical material are very important factors for DUV laser-induced damage. The borate crystal SrB4O7 (SBO) was reported to be a nonlinear optical material with a wide transparency down to 130 nm. In this study, we grew a high-quality SBO single crystal and measured the surface DUV laser-induced damage threshold (LIDT) in several polished conditions. The SBO crystals grown over 13 days was 60 x 6 x 30 mm3 (a x b x c) without cracks or other defects. Two (020) plates were cut from the SBO crystal, and the both faces of the plates were optically polished. After that, we introduced catalyst-referred etching (CARE) to the one plate in order to atomically produce flat and damage-free SBO surfaces. As a result of the CARE treatment at a removal rate of 364 nm/h, the surface condition changed drastically, and a linear step-and-terrace structure was grew with a step height of 0.2 nm. The surface LIDT in several polished conditions were measured with a 1-on-1 method at 266 nm (5 ns pulse width). The polarization direction was parallel to the c-axis of the (020) sample. Synthetic silica was also evaluated for comparison. The surface LIDT (17.3 J/cm2) of SBO after optical polishing is 4.3 times that of synthetic silica (4.0 J/cm2). In addition, the surface LIDT (24.1 J/cm2) of CARE-treated SBO is 6.0 times that of synthetic silica. This suggests that CARE-treated SBO crystals are a promising material for optical components in high-power DUV laser systems.

Effect of polishing contamination and surface defects for the LIDT has already been extensively studied in fused silica based transparent optics. The presence of contamination and damage layers on the surface of polished fused silica contributes to a large reduction in LIDT at ultraviolet wavelengths. The magnetic field-assisted finishing (MAF) technique has been shown to be successful in the fine finishing of optical components such as transparent Nd:YAG ceramics. Magnetic field-assisted finishing (MAF) with several tools has previously been applied to fused silica and was shown to improved surface LIDT at 266 nm. In this paper, the damaged surface of fused silica with enhanced damage resistance after MAF was analyzed to classify the MAF processed condition. Irradiated energy density and damaged volume calculated from depth geometry were measured with a white color interference microscope (Zygo: Zegage). Fused silica substrates polished with CeO2 compounds were prepared as workpieces, and the surface roughness was about 0.3 nm Sq after optical polishing. Material removal over 100 nm occurred with the MAF process, however the final surface roughness did not change. The LIDT at a laser wavelength of 266 nm of processed surfaces was about 1.4 times higher than for the as-polished surface. The damaged volume of as-polished surface was linearly increased as increase in the irradiated energy density. In contrast, MAF processed surface showed little change for the damaged volume. The damage morphology will be also discussed.

Laser-induced contamination (LIC) can lead to optical losses or laser-induced damage on optical components and limits the reliable operation of high repetition rate industrial lasers. In our work, we used MHz sub-ps laser source at 515 nm to test dielectric oxide materials in air environment in terms of LIC formation. We found significant difference in the LIC deposit rate in dependence on coating material and deposition technique. The results could be used for new designs of optical components that will be more resistant to LIC formation.

Molecular contamination due to outgassing of organic materials in vacuum can be a source of significant degradation for space equipment, especially for sensitive optical instruments. Thus, on-ground contamination measurements in vacuum are essential to ensure the flight hardware cleanliness and for making in-flight performance predictions. A suitable detection technique shall work in-situ and in real-time, i.e. it shall be possible to measure the real-time build-up of contamination layers of a few nanometers and less on surfaces in vacuum in the presence of an outgassing source. In this paper we address a new experimental setup for controlled contamination and high-sensitivity spectroscopic analysis of the contaminants on representative material for typical space optics. The central method of choice for the detection of organic contaminants incorporated in this setup is laser-induced fluorescence (LIF), which provides the demanded distinguishability of the applied contamination species. Different excitation wavelengths in the UV spectral range are used. LIF measurements are accompanied by in operando verification of amount and species of the contaminants by a thermoelectric quartz crystal microbalance (TQCM) and a mass spectrometer (MS). This parameter study aims for exploring the laser parameter dependent detection limits and the temperature-dependent condensation process with respect to various substrate-contaminant combinations, in order to reveal strategies to prevent contamination for space applications.

The potassium dihydrogen phosphate (KDP) crystals suffer from nanosecond pulse laser irradiation and are susceptible to damage during the operation of ICF system. In particular, the microcracks on the surface of KDP crystals caused by the single-point diamond fly-cutting (SPDF) process are more likely to cause serious damage under the subsequent laser irradiation. However, the mechanism of laser damage is still unclear. A model that can well represents the laser damage response is very important to reveal the mechanism of laser-induced damage. In this work, the electromagnetic field, stress field and temperature field are coupled, the mechanical characteristics of KDP material are considered, and the reasonable strength equation is applied to model the laser damage response of KDP crystal. Then, the conical crack is taken as an example to explore the laser damage response process of KDP crystal caused by surface defects under laser irradiation. It is found that the surface conical cracks have a great influence on the response process and the morphological characteristics of the laser damage. The existence of surface conical crack defects would lead to the extension of the longitudinal cracks beneath the damage crater, which has great disadvantages for the repairing of the laser damage sites. This work is of great guidance for avoiding the defects-induced damage and improving the service life of the crystal applied in ICF systems.

During the grinding and polishing processes of hard-brittle fused silica optics, the defects would be inevitably formed on the finished surface. Fused silica has a high absorption coefficient for far-infrared lasers, which makes the CO₂ laser processing to be the potential repairing technology for machining-induced defects on fused silica surfaces. In this work, using a low-power CO₂ laser, a new repairing method to heal the machining-induced micro-defects on the surface of fused silica is proposed. Then, based on the nonlinear thermodynamic parameters of fused silica material, a thermal transfer model under CO₂ laser irradiation and a dynamic defect healing model were established. On basis of that, the influence of CO₂ laser parameters on the maximum surface temperature and the temperature distribution inside the silica material was investigated. It is found that, under the low-power and near-continuous CO₂ laser irradiation, the maximum melting depth can be obtained under the non-evaporative condition. The defect healing process under various laser powers was explored as well. It is found that the defects would be more difficult to be healed under a laser with higher-power, smaller beam size or shorter pulse width. This work can provide theoretical guidance for the determination of the optimal parameters in the laser healing process and the optical manufacturing strategies of fused silica optics.

Potassium dihydrogen phosphate (KDP) crystal has been regarded as the solely irreplaceable component in laser-driven inertial confinement fusion (ICF) facilities. Nevertheless, the laser-induced damage on KDP crystal surfaces under highenergy laser irradiation considerably restricts the output power of ICF facilities. The laser damage event on KDP surface is an extremely complex process, among which the non-heat initial energy deposition is regarded as the major absorbed energy source, determining the subsequent thermal damage process and final damage morphology. The initial energy deposition process is a non-heat stage, where the plasmas are generated from ionization processes under intense laser irradiation. However, there is still no available model that can well reproduce the dynamic interaction behaviors between the high-energy laser and plasmas in the initial energy deposition process, resulting in the laser-induced damage mechanisms on KDP crystal surface still not fully revealed. In this work, a Particle-In-Cell (PIC) model is established to investigate the initial dynamic damage behaviors of KDP crystals under intense laser irradiation. On basis of this model, the crater formation process and the particle ejection dynamics involved in the laser damage event are reproduced. The reproduced characteristic parameters of laser damage craters on KDP input and output surfaces, and the obtained particle ejection angles are consistent with the previously reported laser damage morphology, which verifies the effectiveness of the established PIC model. This work could provide theoretical means for investigating the initial energy deposition process and also offer further insights in understanding the laser-induced damage mechanisms of KDP crystal components.