Laser-Induced Damage in Optical Materials 2023

During the more than 60 years from the invention of the laser to demonstration of ignition on NIF, scientists, engineers and technicians at LLNL developed incrementally larger and more energetic/powerful laser systems designed for inertial confinement fusion. Each step forward brought new understanding of issues and new concepts for their solution. A continual effort to solve new and often surprising incidents of optical damage supported this evolution. Insights into problems of scale learned from previous lasers, including Argus (1977), Shiva (1978), Nova (1984) and Beamlet (1995) were combined successfully in NIF to provide this platform for the recent ignition demonstration. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract No. DE-AC52-07NA27344; LLNL-ABS-849703

The first demonstration of fusion in the laboratory required the National Ignition Facility (NIF) to leverage its novel approach to laser-induced damage; managing laser-induced damage rather than avoiding it. Accepting regular damage to the laser’s final optics allows the NIF to deliver about 300% higher energy on its targets. This mode of operation is made viable by a unique optics maintenance strategy and a variety of damage management and mitigation. We will review the prominent damage mitigation and management technologies which are classified in three categories: Damage Initiation Prevention, Damage Management, and Damage Repair. Examples of these technologies are the Fused Silica Debris Shield, AI driven damage detection, and the CO2 mitigation cone, respectively. We introduce a simple empirical model which both quantifies individual and combined impacts of the technologies on NIF’s optics usage. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract No. DE-AC52-07NA27344; LLNL-ABS-849571

On December 5, 2022, Lawrence Livermore National Laboratory’s (LLNL) National Ignition Facility (NIF) made history, demonstrating fusion ignition for the first time in a laboratory setting. A review of the major large optic technologies over the past several decades is presented that have enabled the National Ignition Facility laser to both routinely operate >2MJ and achieve fusion ignition.

An optically addressed light valve is described for high-speed laser beam shaping used in rapid metal additive manufacturing [1]. The resulting Area Printing™ delivers shaped high-power pulses to a metal powder bed that locally sinters and melts to consolidate into a fully dense metal part. This technology and device enable scaling, cheaper additive manufacturing with high spatial resolution and greater efficiency with minimal spatter defects. We address here the unique optoelectronic properties and challenges related to optically addressed photoconducting insulator that control the switching dynamics under high intensity laser irradiation. Further description is presented of the device-level thermomechanical analysis from parasitic absorption of the laser at kW to MW power levels. [1] https://www.seurat.com/area-printing

High-energy laser pulses in the nanosecond regime used to be spectrally broadened to mitigate the stimulated Brillouin scattering known to deteriorate the optical elements. Due to propagating effects, this spectrum broadening lead to FM-to-AM conversion, where the UV laser beam experiences an amplitude modulation at frequencies which are harmonics of the phase modulation frequency. We study the impact of the FM-to-AM conversion on the Brillouin backscattering by applying an amplitude modulation on the UV pump laser beam operating at 351 nm and with a 3 ns pulse duration. Experimental measurements show that adding an amplitude modulation frequency on a phase-modulated laser beam could enhance the stimulated Brillouin scattering and lead to laser damage. Thanks to a theoretical and numerical analysis, we show that this singular behavior originates from a resonance between the frequency of the amplitude modulation and the low orders harmonic frequencies of the phase modulated laser beam.

There are near term plans to increase the energy on NIF (National Ignition Facility) to 2.2 MJ and up to 3 MJ in future years. Managing optics’ damage is one critical aspect of reaching these aggressive goals. Frequency conversion crystals (Third Harmonic Generation) are being examined for readiness to meet the goals for higher energy operations. All aspects of the crystal fabrication process are being studied including growth, processing, laser conditioning, and mitigation to strengthen the crystals against damage. Understanding the difference between bulk and surface damage, is important to limit the number of blocked sites driving the exchange rate.

The achievement of laboratory inertial fusion ignition in Dec 2022 required research- and engineering breakthroughs in the areas of optics, lasers, diagnostics and targets. In this presentation, we will give an introduction of the NIF and the fusion experiments performed at this facility and show the progress of the experiments since NIF started operating in 2010, with a focus on the targets. The cryogenic ignition targets need to fulfill stringent dimensional and thermal requirements at 18K. At the heart of the target is the fuel capsule that holds the deuterium and tritium for the fusion reaction. These capsules must be exceptionally pure, round, uniform, and defect-free. We will cover the specifications, fabrication, and technical challenges in manufacturing these capsules.

High power transmissive optics have become subject to increasing power densities in recent years as laser powers and energies increase. The industry standard material for these applications has been high performing fused silica optics due to their extremely low bulk absorption. However this low bulk absorption comes at the cost of extremely low thermal conductivity. Coherent has developed the capability to produce sapphire aspheres for high contamination and high power density applications. The high thermal conductivity of sapphire, combined with moderately low bulk absorption and high performance IBS coatings allows it to provide comparable performance to fused silica optics when clean, and significantly better performance over time as the surface accumulates contamination.

In many laser applications, there is a higher and higher demand for more efficient coatings with reduced losses, in terms of absorption and scattering as those are contributing factors to diverse laser damage regimes. Ion Beam Sputtering (IBS) is a known technique to provide such high optical quality thin films. Indeed, it allows to achieve high density layers with low absorption and scattering. In this work, various coatings were developed using Bühler IBS technology. Total losses were measured using Cavity Ring Down, absorption using Laser Induced Deflection or Laser thermography, and Total Integrated Scatter using dedicated scatterometers. A correlation between the effect of the chosen deposition method and parameters and the measurement performances were made with the aim of a better understanding of the level and the origin of losses in the coatings. Finally, highly reflecting mirror coatings for 1064 nm wavelength were fabricated with different designs and deposition parameters.

There has been a concern with the typical Laser-Induced Damage Threshold (LIDT) testing due to the small allowable spot size and the small required tested area. With the current tests, it is possible to actually miss defects entirely, resulting in a higher LIDT value. Experiments conducted in this study compare traditional LIDT measurements in the nanosecond regime made with a broad variant in spot sizes. These same spot sizes are also used to run larger area scans to evaluate the damage sites per area. Experimental results will be presented and discussed in the broader scope of developing a reliable LIDT standard.

The development of composite materials opens the way to vary material properties, continuously. The application of mixture materials allows changing the material properties, continuously. The effects of the annealing temperature on the optical and structural properties of (TiO2:Ta2O5) composite coatings were studied. The experimental results show that, the highest LIDT was obtained for 20% TiO2 content at an annealing temperature of 600 ℃. The low content composite coatings annealed at 600 ℃ shows a similar LIDT and a higher refractive index as compared to the pure Ta2O5, which provides significantly support for the preparation of reflective coatings with a higher femtosecond damage threshold.

We propose to survey the state-of-the-art broadband, near-IR multilayer mirrors designed for ultra-short, pulsed laser applications. Specifically, mirrors must meet a minimum reflection of 99.5% at 45-degree incidence angle from 830 nm to 1010 nm at either S or P polarization and GDD < 50 fs^2. The participants select the coating materials, design, and deposition method. The samples will be damage tested at a single testing facility to enable direct comparison among the participants using a 25±5 fs OPCPA laser system operating at 5 Hz. Details of the deposition processes, cleaning method, coating materials, and layer count will be shared.

Atomic layer deposition offers unique advantages compared to more classical PVD coating processes. These advantages include an inherently high coating thickness homogeneity and conformal coating of both micro-optical structures, such as gratings, and large free-form optics, such as aspheric lenses. The talk will introduce the coating process, compare its properties to those of more classical PVD processes and give an overview of recent developments of ALD coatings in optical applications.

The peak-power of petawatt-class lasers is limited by laser-induced damage to final optical components, especially on the multilayer dielectric pulse compression gratings. To improve laser damage resistance of those components, we numerically showed that for a given etching profile, the mirror design can minimize the electric field intensity in the pillars with significant variations. Three designs were manufactured to perform laser damage tests to assess the numerical results. The measured laser damage thresholds corroborate the calculated electric field distribution and confirm the influence of the dielectric stack on the laser resistance of multilayer dielectric gratings.

Spallation effects caused by shock waves in optical components such as those used in the Laser MegaJoule facility during laser operation can lead to material fracture. One solution could be to use a viscoelastic thin film on these fused silica components to reduce the reflection of shock waves from the rear surface, but it must have excellent optical, mechanical, and power-handling properties. Among the viscoelastic materials investigated were Nafion and polydimethylsiloxane-based ormosil, with ormosil synthesized using a sol-gel process. The materials were characterized optically and especially tested for acoustic attenuation. These materials, as thin films deposited on a fused silica substrate, were studied under shock wave propagation using the laser shock technique. Preliminary results showed that these thin films have interesting properties that could help reduce mechanical damage to optical components.

Here we present a side-by-side comparison of LIDT of dispersive mirror at a central wavelength of 1030 nm produced via MF magnetron- and RF magnetron- sputtering deposition methods. We have use different layer materials like Ta2O5, HfO2, SiO2. All dispersive mirror provides -200 fs2 in wavelength range 930-1120 nm. The dispersive mirror with Ta2O5/SiO2 has LIDT about 0.12J/cm2. Alternating of HfO2/SiO2 has allowed us to improve LIDT up to 0.2 J/cm2 at 1030 nm, 190fs, 1kHz, in vacuum.

Corning aluminum process (CAP) led to 10x increase of LIDT at 1064 nm and 20 ns on single point diamond turned aluminum alloy Al-6061. HfO2/SiO2 multilayers were used to further enhance reflectance and laser damage resistance. Broadband spectral reflectance from the visible to the middle wave infrared with an LIDT of 5.0 J/cm2 was realized. Post LID morphology analysis indicated a transition from absorption-driven damage to defect-driven damage with the increase of HfO2/SiO2 multilayer enhancement. The results suggested that a combination of the CAP-modified Al-6061 and low defect deposition process of the dielectric enhanced layers leads to high laser durability.

ASML is the leading provider of lithography equipment in the semiconductor industry. Lithography is one of the cornerstones on which todays semiconductor technology is built. It’s a journey that started with Moore’s law in the 60’s and has continued until today and will continue in the future. In the latest generation of technology a high power 30kW CO2 laser is used to ignite a tin plasma that emits 13.5nm photons to enable the printing of 7nm and below linewidths. This keynote address will give a small ‘sneak peak’ into what technology enables todays EUV chip manufacturing industry. It will also highlight some of the Laser Optics damage challenges we have with the high power CO2 laser ASML uses. The presentation will also show some of the theoretical understandings on laser damage that were built up along the way and illustrate where there are still some challenges ahead.

Recent studies suggest that fatigue effect in dielectric optical coatings is possibly associated with the presence of strong nonlinear absorption, however, up to now there was only indirect evidence for such hypothesis. One of the reasons for that was a technical rigor to characterize nonlinear absorption losses in optical coatings and a lack of pertinent experimental data. Recent advancement of common-path interferometry and LIDT testing allows us to overcome such limitations. In this study we examine nonlinear response and fatigue effect in single- and multilayer dielectric coatings below single shot damage threshold. Although there is no quantitative model that could predict fatigue from absorptance, we found an interesting correlation between nonlinear absorption and fatigue effect under comparable experimental conditions. These results help us to understand the mechanism of fatigue in optical coatings and possibly make more durable femtosecond optics.

We experimentally study single-shot laser ablation of GaSb, GaAs, GaP and GaN, for laser pulse durations ranging from 200 fs to 20 ps. These materials have bandgaps ranging from 0.73 eV to 3.44 eV. We find that the larger the bandgap, the stronger the dependence of pulse duration. This is expected, as intrinsic laser absorption is mainly linear when the bandgap is small compared to the photon energy, whereas a larger bandgap requires multi-photon absorption. Thus a larger bandgap leads to a stronger influence of the peak intensity of the pulse and therefore a stronger dependence on the pulse duration, when compared to smaller bandgaps.

Contamination particles and damage on the pillars in multilayer dielectric gratings are believed to be responsible for damage initiation and damage growth in high-power laser systems. In this report we present our modeling result using 3-D FDTD for an array of commonly found grating imperfections including fabrication artifacts, contamination particles, and pillar localized partial or total removal with resolution down to 15 nm or smaller pixels in each direction. From these results, we establish estimates for the ensuing electric-field intensification and projected reduction of the laser-damage thresholds as well as the pattern and the mechanism of damage growth.

Optomechanics and surface residues can be sources of contaminants when directly illuminated or subjected to laser scatter, which can then deposit and grow on optical surfaces leading to transmission loss over time. To extend the useful lifetime of laser systems and laser optic assemblies, it is necessary to determine best practices in material selection and handling of components both in and out of the beam path. We present lessons learned from a series of month-long (~100 trilllion shots) experiments performed at 355nm with laser parameters relevant to material processing and research (20ns pulsed ~1kW/cm^2 average). Laser-induced surface contamination and transmission losses of an assortment of optics in controlled and uncontrolled environments were monitored, and contamination mitigation techniques qualified by transmission measurement and microscopy.

By using a commercial PVD coating system with integrated advanced plasma source, we can manufacture subwavelength structures (SWS) on many different substrate materials. In this process, we use an organic material that exhibits self-structuring properties during plasma etching, and the SWS formed are subsequently overcoated with a dielectric material, e.g. SiO2. To be suited for high power laser applications, the organic material must be removed. We are therefore investigating different methods to be evaluated by reflectance, transmittance and absorption measurements. The best method will be applied to SWS fabricated on fused silica, followed by LIDT measurements in the UV range.

Surpassing the diffraction limit is crucial for achieving super-resolution. We propose the application of chalcogenide thin film saturable absorbers for super-resolution applications such as super-resolved laser inscriptions. Using the Z-Scan technique, the saturable absorption behaviour of thin film has been optimised with respect to the material characteristics (thickness and crystalline structure) and excitation parameters (wavelength and pulse duration). Huge saturable absorption behaviour has been observed (10^-7 m/W). Using the best combinations of laser excitation parameters and material characteristics, a 14% of beam size reduction has been simulated. Finally, a technique to execute super-resolved laser-inscriptions has been proposed.

Advancements in telecommunication and laser technologies have enabled free space communication with geostationary satellites. To ensure efficient and reliable photonic systems, it is crucial to develop optics that can withstand high power levels in the continuous wave regime. This study addresses the issue of absorption of optical coatings at 1570nm through Lock-In Thermography and a finite element model. The measurements provide insight into total absorption and photoinduced effects in a thin film stack subjected to high power laser. The results offer valuable information on optical function shifts resulting from photoinduced effects, essential for designing robust photonic systems.

Our study analyzed the laser-damage threshold of liquid crystal alignment materials, including photoaligned azobenzene, rubbed polyimide, and rubbed nylon. We found that the presence of liquid crystal was necessary to observe variation in damage thresholds among alignment materials. Nylon outperformed photoalignment, which outperformed polyimide. We also investigated the polarization dependence of the damage threshold in ordinary and extraordinary modes at a near-infrared wavelength and found that only the photoalignment material demonstrated polarization sensitivity at our statistical power level. Our results can inform the design of high-power beam-shaping devices for various applications, including fusion, 3-D printing, and defense systems.

As the demands for optical coatings in high-power laser applications increase, the presence of impurities in coatings becomes a critical factor affecting key properties such as transmission, absorption, and laser damage resistance. Ion beam sputtering (IBS) is a popular deposition technology used for producing high-quality optical coatings due to its highly energetic process. Impurities in the coatings mainly arise from grid optics and ion overspray in the vacuum chamber. In this study, we are investigating the impact of various impurities on absorption and laser damage in a systematic manner, to provide guidance for optimizing deposition conditions.

Antireflection coatings were designed for 1030 nm wavelength using SiO2 and HfO2 mixtures and fabricated by ion beam sputtering. Laser damage tests were carried out at pulse durations of 10 ps and 77 fs using different test protocols. The laser damage threshold ranges from 3.3 to 4.3 J/cm2 for 106-on-1 tests using 10 ps pulses, and from 1.22 to 1.45 J/cm2 on single and multi-shot tests carried out using 77 fs pulses.

This study aims to establish a correlation between coating stress and LIDT. For this purpose, a coating is deposited by means of a conventional coating process with different degrees of ion assistance, which leads to varying deformations of the surface. Subsequently, the optics are bent by applying an external force in the tensile and compressive direction and examined for their LIDT. This procedure allows the investigation of the relation between stress and the LIDT for different coating parameters and processes.

Full optical loss characterization for an ultra-high reflecting mirror with R > 99.995% around 633nm and AOI = 0° is presented to proof the energy balance 1 - R = A + S + T. Total loss/reflectivity is measured using cavity ring-down (CRD) technique at 639 nm and AOI = 0° resulting in about 20 ppm total mirror loss. Absolut absorption losses have been measured using laser induced deflection (LID) technique at 638 nm. A scatterometer MLS10 - developed at Fraunhofer IOF – has been applied in full spherical scan mode at the wavelength 632.8 nm to measure both, the total scattering and transmission of the ultra-high reflecting mirror.

We explore the laser-damage behavior of gallium alloy-based liquid metal mirrors for their potential to provide higher-damage-threshold performance. One of the key advantages of using liquid metal mirrors is the self-healing potential following perturbations arising from exposure to high-power laser pulses. In this work, key performance metrics, such as reflectivity and the laser-damage initiation mechanism and initiation threshold, were investigated using fused-silica cells filled with three different Ga liquid metal alloys. The results suggest that irreversible modification (damage) under 355-nm, 6-ns pulses are associated with the formation of gallium oxide, taking place at a fluence significantly higher than that for damage initiation in conventional metal mirrors. This exploratory work is the first of its kind and highlights the strong performance of gallium alloy metal mirrors.

This work uses analysis of the laser induced damage threshold (LIDT) and laser damage volume to determine the effects of magnetic field-assisted finishing (MAF) with different tool types (polishing pad and iron-particle tool) on surface contamination. Little differences in the LIDT and volume of the damaged area were observed regardless of the polishing tool types. However, contamination on the surface polished with the iron-particle tool may be redeposition of chips on the surface. By eliminating redeposited contamination, multi-stage polishing with an iron-particle tool resulted in an LIDT improvement of 63% over the commercially polished surface.

Yttrium aluminum garnet (YAG) is an important gain medium material for solid state lasers. Contamination of YAG crystal surfaces after classical polishing process often becomes a limitation in terms of laser damage resistance characterized by laser-induced damage threshold (LIDT) due to residual extrinsic defects. Therefore, we investigate the possibility to remove those defects by surface plasma etching. LIDT is measured as a function of etching depth by using repetitive laser pulses of picosecond duration. We demonstrate that using optimized etching conditions significant enhancement of LIDT is achievable by reaching intrinsic LIDT of the bulk also on the crystal surface.[1] 1. Belosludtsev, Alexandr, Andrius Melninkaitis, and Giedrius Abromavičius. "Significant enhancement in laser damage resistance of YAG crystal surface by plasma etching." Optics Letters 48.9 (2023): 2226-2228.

During laser irradiation, several changes in optical coatings can be induced. Catastrophic damage can easily be detected in situ and after the test. While the discolorations and laser induced contamination are difficult to observe during the measurements by only analyzing reflected/scattered light intensity. Different high reflectivity mirrors designs, deposited by either sputtering or evaporation technologies, have been tested for optical resistivity and laser induced contamination using ultrafast pulses. Imaging ellipsometry measurements of irradiated sites reveal changes in reflected polarized light. Quantitative comparison of different mirror designs has been performed in order to determine how electric field distribution and material affect the laser induced contamination phenomena.

The Fused Silica Debris Shield (FSDS) is a disposable optic introduced to reduce the damage initiation rate on the NIF’s Grating Debris shield (GDS), enabling higher power and energy to be delivered to NIF’s targets. We will define the FSDS exchange criteria, describe the diagnostic tools used to measure the FSDS exchange parameters, and evaluate the damage initiation mechanisms of the FSDS. The surface damage morphologies will provide insight into the effects of the various mechanisms on the FSDS lifetime. These results will help in our analysis to optimize the FSDS exchange criteria to further extend GDS lifetime and help NIF operate more efficiently.

PETAL (PETawatt Aquitaine Laser) is an ultra-high power laser delivering sub-picosecond pulses. A way to improve the PETAL operation is to increase the Laser-Induced Damage Threshold (LIDT) of transport mirrors. Three approaches are being considered to improve the LIDT of these components : changing the design, the materials and the deposition processes. Monolayers of SiO2, HfO2, Sc2O3 and mixed materials of HfO2/SiO2 and Sc2O3/SiO2 were elaborated by magnetron sputtering. The LIDT of the materials was determined using 1 on 1 procedure tests. These tests exhibit that the Sc2O3/SiO2 mixture is the best material in terms of laser damage resistance.

Nonlinear index of refraction is of great importance in evaluation of laser-induced damage threshold, as it relates to parameters defining self-focusing critical power and avalanche breakdown. An accurate method to measure the nonlinear refraction can help restrict the damage caused by these mechanisms. Beam-deflection (BD) method is a useful tool to calculate nonlinear refractive index. The critical parameters in the calculations are probe-to-excitation spot size ratio, r, and the relative displacement of the two overlapping beams. Similar to the Z-Scan method, an empirical function, solely dependent on r, is obtained which provides a relation between the BD signal and the phase shift.

High diffraction-efficiency nonpolarizing gratings with CW damage thresholds in the range of 100 kW/cm2 and higher are necessary for scaling spectral-beam-combined (SBC) high-energy laser (HEL) systems to MW powers and above. We present the results of a campaign to improve capabilities of multilayer dielectric gratings for these applications. Current capabilities are demonstrated by showing the peak temperature, damage threshold, defect density, and diffraction efficiency (where applicable) of 51 MLD gratings and 17 coatings when illuminated at 1070 nm at intensities up to 3 MW/cm2.

The Matter in Extreme Conditions Upgrade (MEC-U) project is a major upgrade to the MEC instrument on the Linac Coherent Light Source (LCLS) X-ray free electron laser (XFEL) user facility at SLAC National Accelerator Laboratory. The envisioned MEC upgrade will significantly enhance the capabilities of the pump laser sources, increasing the intensity and repetition rate of the short pulse laser to Petawatt (PW) at 10 Hz rate, and increasing the energy of the shock driver to the kJ level. Building such high power/energy pump laser systems presents many challenges to prevent and minimize damage to the laser optics. In this talk, I will discuss the various optical damage threats encountered in the laser design and present approaches to mitigate these risks.

The long-term performance of high-power laser systems is adversely affected by particle contaminants that are introduced into the system during the manufacturing of optical components and the handling during installation and operation of the laser system. Such particles can absorb or focus laser energy, reducing the laser-induced–damage threshold (LIDT) values. We developed ultrathin coatings that can decrease the overall load of contamination and aid with the removal of the already-accumulated particles using simple gas-flow cleaning. These coatings do not alter the intrinsic LIDT values, and they remain stable over time and during the system operation.

In this study, both established excision techniques and the relatively novel shading methods of damage mitigation are evaluated for repair of isolated and clustered filamentary damage on a NIF Wedge Focus Lens which varies in thickness from ~10 mm to 50mm. The repair success criteria (preventing of damage growth during a series of high fluence and intensity 351-nm laser shots) is evaluated by comparison of microscopy from before and after laser exposure. The challenges and success rate of the methods are compared for various filamentary damage morphology.

This work explores reactive ion etching parameters in order to identify and optimize key characteristics in gratings that govern their overall performance, including minimization of sidewall and trench structural defects and modification of fused silica via intrinsic molecular-level defects. This study is performed using grating-like samples with 5-µm-wide lines and trenches generated in fused silica by the photolithographic process and inductively coupled plasma-reactive ion etching. The analysis compiles metrology, simulation, and damage-testing results to obtain a better understanding of how to modify the fabrication process of gratings toward achieving better laser-induced–damage performance.

For most of the laser applications is optics equipped with antireflective coatings must. Therefore, laser damage resistance and stability at high energies of used components is a key performance limiting factor at the large portion of the user cases. In UV region, issue of laser damage is particularly enhanced as many optical materials tends to degrade at longer exposure and any contamination may accelerate that. In the following paper will be disseminated laser damage performance of selected commercially available optical windows equipped with AR coatings, designed for high-power lasers in UV region. Damage threshold measured with mm-size laser beam will be compared and influence of the long exposure to ultrashort pulses will be considered.

For pulse lengths between 1 and 60 ps, laser-induced modifications of optical materials undergo a transition from mechanisms intrinsic to the materials to defect-dominated mechanisms. Elucidating the location, size, and identity of these defects will greatly help efforts to reduce, mitigate, or eliminate these defects. We discuss our work that detailed the role of defects in the ps laser-modifications of SiO2 and HfO2 1/2-wave coatings. For HfO2 coatings, we included a study of environmental effects on the damage process. We found that the response of defects very near the surface are dependent on the environment, leading to worse damage in vacuum than in air. One or more constituents of air, most likely oxygen and/or water, suppress or lessen the effects of these defects during laser exposure. We discuss the implications of these findings for defect-driven laser-induced damage for ps to ns laser pulses and for mechanisms for laser-induced damage initiation.

Understanding the physical mechanism behind the laser-induced damage of multilayer dielectric interference coatings is essential for developing ultra-high intensity laser systems. The previous work reported high damage thresholds of MLD mirrors and blister formation near the threshold. Here, we present the cross-sectional study of the blisters using transmission electron microscopy and focused ion-beam processing. The measurement shows evidence of void formation and phase transformation under the surface, interdiffusion, and intermixing at the interfaces. These findings provide valuable insights into the mechanisms behind laser-induced damage, facilitating the development of more robust and reliable optics for high-power laser applications.

Cones machined into the surface of the final fused silica optics on the NIF have been used to remove laser induced damage from exposure to high fluence 351 nm laser light. When applied to the input surface of an optic, a shadow is created on the exit surface due to the divergence of the laser light by the cone walls. In recent years input surface cones have been utilized to shadow exit surface damage and thus arrest its continued growth. The expanding waves from the cone walls interfere with the incident beam to create a high fluence intensification at the exit surface. This intensification has the characteristic periodic spatial variation on a scale of the order of the 351 nm wavelength. The question arises as to how the damage density probability, (), is affected by this variation as compared to a uniform fluence. Does it follow the local periodic variation, or is it averaged over that variation. We consider both cases, how it can be predicted by direct measurement of the intensification as opposed to costly damage tests, and how we might measure the effect directly.

Stacks of amorphous oxide mixtures and SiO2 conform the interference coatings of current gravitational wave detectors (GWD). This talk will discuss a comprehensive study of amorphous oxide mixtures and nanolaminates that has led to the identification of a new alloy, TiO2 doped GeO2 which is a promising material to engineer next generation coatings for gravitational wave detectors with lower coating thermal noise.

In general, there are just a few dielectric materials available which are suitable to prodouce laser optics. Such binary dielectric films are limited by fixed optical properties e.g. optical bandgap or intrinsic LIDT. The current paper gives an overview of recent achievements to manipulate the optical properties of dielectric layers precisely. One approach is given by the deposition of ternary composites. The other approach is well known for crystalline materials and takes benefit of quantized effects. In recent years it could be shown that such effects can be utilized in amorphous dielectric layers as well, known as quantized nanolaminates.

Optical coatings are an essential part of laser components and the requirements placed on them cover a wide range of physical properties. With the limited choice of available coating materials the tunability of the physical properties, among them for instance the refractive index, is limited. However by mixing the materials, referred to « mixtures » coatings, a broad range of possibilities is available. Since many years, a variety of applications have been established in practice for mixtures. An example of this is the application to minimize the loss in high end coatings for highest finesse cavities : mixtures of titanium oxide and tantalum oxide are mainly used as high refractive material. Another important application is the suppression of crystallization during coating. Such phenomena can be observed, for example, in hafnium oxide and titanium oxide, and can be suppressed by using small amounts of aluminum oxide or silicon oxide. The most important area of application, however, is the laser damage resistance of the coatings. The basic idea here is to stabilize the sensitive layers in which high field strengths occur by using mixtures. A large number of studies have been carried out in this area in recent years. Some important developments have been carried out on the material deposition, characterization and design strategies. This work forms the main part of the overview that will be given in the presentation.

Quantization effects in nanolaminate structures of oxide materials were proposed and experimentally demonstrated only recently. In this paper we will investigate the material combinations of Ta2O5-SiO2 and amorphous silicon-SiO2 deposited by magnetron sputtering and show that the quantization effect is observed in both materials. We will describe the deposition process and demonstrate the tunability of the refractive index and the bandgap energy. Quantized nanolaminates (QNL) composed of Ta2O5-SiO2 in combination with SiO2 were used as high and low refractive materials in optical interference coatings forming an antireflection and a mirror coating, whereas QNL with aSi-SiO2 as the high index material were used in a log pass filter with edge at 720nm. All designs could be deposited successfully with close match to the design. The aSi-SiO2 based filter showed a blocking range throughout the visible spectrum whereas a comparable filter based on SiO2-TiO2 only blocked 500-700nm.

Quantizing nanolaminates are an interesting alternative to classical coating materials with greater independence of refractive index and the optical bandgap energy. This leads to more flexibility and considerable potential to increase the laser-induced damage threshold in the ultra-short pulse regime. The following study presents and compares the design choices, characterization, and LIDT testing of different quantizing nanolaminates for the ultraviolet spectral range to classical coating materials.

In our recent experiment, it is discovered that the interaction of resonant metasurfaces and mid-infrared femtosecond laser pulses can realize damage patterns that overcome this limitation, and the geometry of the trench can be controlled by the pulse intensity, polarization, and the total pulse number. In this study, we report the particle-in-cell (PIC) simulation of the interaction of the M-shape metasurface and 200-fs mid-IR laser pulses with high intensity. The simulation results suggest that localized lattice heating and explosions occur at a scale and in the relationship with the polarization consistent with the experimental data.

Laser-induced damage is a statistical process, so it is critical to understand the errors of the testing method. In this study a Gaussian damage precursor fluence distribution is created from randomly generated numbers and then evaluated using the ISO and raster scan damage test protocols. Building on previous work, the impact on beam shape, pointing, and fluence variation will be investigated as well as doubling the number of ISO test sites. Finally, the impact of non-growing damage sites at low fluence, previously defined as a functional laser-induced damage threshold, will be evaluated for both testing protocols.

Ultrafast lasers are very useful for surface engineering of semiconductors. Here we used a Scanning Tunneling Microscope (STM) to map in situ topography and spectra of hydrofluoric acid-etched silicon (100) damaged by an ultrafast pulsed Yb:KGW laser at 1030nm with 70fs duration in high vacuum. We observed absence and presence of laser induced periodic surface structures with single and multiple shot irradiation, respectively. Surface morphology were captured with atomic resolution, which can help understand the subtle changes to surface ultrafast lasers can cause near the laser induced damage threshold fluence. The results demonstrate the potential of STM for in-situ studies of laser damage on clean surfaces in ultra-high vacuum.

We investigate the unique surface fractures of CaF2 found after single shot laser irradiation using 77 femtosecond, 1030 nm laser pulses. Optical microscopy and atomic force microscopy revealed elevated rectangular structures across laser-ablated craters. The underlying mechanism for this unique morphology may be related to anisotropic thermal conductivity and laser-induced defects. Our findings provide insights into the fundamental mechanisms of laser-induced damage in CaF2 and have implications for the design and optimization of high-power laser systems.

The Laser Interferometer Space Antenna (LISA) will be the first space based gravitational wave observatory. LISA consists of 3 spacecrafts, which use time-delay interferometry to measure distance variations with picometer accuracy. Thus, even a nanometer-sized deposit on an optical element could affect the gravitational wave detection performance. To mitigate risks due to laser-induced contamination (LIC), we have carried out an extensive LIC test campaign, including a series of short duration tests with different test parameters, as well as a long-duration test. A remaining concern is whether LIC could occur in the presence of metallic particles on optical surfaces. Our ongoing research thus aims at testing for a possible deposit formation in a combined LIC and metallic particulate contamination test.

We will present a temporally and spatially resolved photoluminescence (PL) measurement technique developed to rapidly characterize fused silica damage sites and determine their propensity to grow under subsequent laser irradiation. A diffusional model will be used to describe the observed PL dynamics and correlation to the local damage morphologies. We believe that our measurement and analysis approach can allow rapid identification of growth-prone damage sites, providing a pathway to fast, non-destructive predictions of laser-induced damage growth and enable selective damage site mitigation. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

Large-aperture potassium dihydrogen phosphate (KDP) crystals possessing freeform surfaces provide a pathway for polarization smoothing of high-peak-power laser beams used for inertial confinement fusion. These optics improve beam uniformity by producing spatially-variable polarization states induced by the surface thickness variations. However, the figured surface also affects the far-field spot shape which is not desirable in all cases. To avoid spot shape distortion, the freeform surface can be planarized with an index-matching material that possesses a sufficient laser-induced damage threshold in the ultraviolet regime. Methods and materials for this planarization process will be presented.

Our custom-built STM system, in conjunction with a femtosecond laser, enables the study of ultrafast laser dynamics with materials at the atomic level. Our system operates in ultrahigh vacuum (UHV), providing freedom from contamination. In our initial work, we studied in situ laser damage of clean silicon surfaces. In-situ damage was done using pulse duration of 77fs laser focused by our objective inside the chamber onto the silicon surface with spot size less than 20um. To study the mechanisms for laser damage, we varied the pulses from 1 to 10 shots at fluence between 0.78J/cm2 and 1.38J/cm2 on surface. To study the morphology of laser modification on the silicon surface, we performed STM scanning on in-situ laser damage areas and compared them to damage-free silicon. We have developed a fiducial system that allows for the registration of the laser-irradiated areas with the STM tip, and we have successfully registered in-situ laser damage.

In this investigation we tested different coating materials on fused silica substrates with a diameter of 25 mm regarding the high-power continuous wave laser damage threshold. Overall, three coating batches each consisting of a different high refractive multilayer stack material were manufactured using an ion assisted deposition process. Furthermore, we performed measurements regarding the beam diameter dependence on hybrid mirrors varying the beam diameter between 90 and 420 µm. The damage and temperature behavior of the hybrid mirrors showed a mostly defect driven damage mechanism which we compared with numerical heat transfer simulations and a suitable defect driven damage approach.

In the present study, we investigated the damage behavior of PMMA optical fibers. We used a 10 Hz nanosecond pulsed laser system emitting at 532 nm. Our studies show that the damage distribution in the optical fibers is in good accordance with raytracing simulations regarding the intensity distribution inside the fiber. We further investigated the laser induced damages using various analysis techniques among them the most informative ones being EDX, SEM, X-Ray microtomography and the multiphoton microscopy.

Large fusion scale laser facilities aim at delivering megajoules laser energy in the UV spectrum and nanosecond regime. Due to the extreme laser energies, the laser damage of final optics of such beamlines is an important limitation. Once a damage site initiates, it grows at each laser shot, which decreases the quality of the optical component and spoil laser performances. Operation at full energy and power of such laser facilities requires a perfect control of damage kinetics and laser parameters. In order to prepare for the Laser MegaJoule upgrade in energy, experiments are carried out with specific optics.

Glass drilling and cutting is crucial for optics, consumer electronics, and Micro-Electro-Mechanical System (MEMS) devices. Speed and reproducibility are issues common to traditional glass cutting methods. We use a femtosecond laser to efficiently and accurately cut interior shapes in glass. Unlike a traditional Gaussian beam, which has a shallow focal range and cannot penetrate deep into materials, Bessel beams have a much longer focal range, up to millimeters. With a Bessel beam, we can cut straight through without the need for mechanical cleaving or moving the sample through the focus, improving reproducibility and speed. The cut surfaces are analyzed with optical microscopy, atomic force microscopy, and scanning electron microscopy to observe any structural/morphological changes to the materials near the laser affected regions. Our 260fs laser operates at 10kHz, with 1030nm central wavelength, depositing 1.4W on target. An axicon generates the Bessel beam with a FWHM central spot size of 6±1µm and a fluence of up to 41Jcm-2. Our study has the potential to open new technological pathways for integrated electronic and photonic platforms.

Laser damage of optical components is a critical issue for high power laser facilities. One of the key ingredients of the laser damage is the photo-ionization mechanism occurring over the first instants of the laser-optical components interaction. To model the photo-ionization of solids, we use to consider the Keldysh theory [1] developed for a monochromatic laser beam characterized with a linear polarization. We will present our theoretical development for the ionization of crystal by a circularly polarized laser beam, and particularly, the photo-ionization cross sections that we have evaluated in the low intensity regime. We will compare theoretical expressions modeling the ionization of solids irradiated by linear or circular polarization in the low intensity regime. A particular attention will be devoted to the influence of the laser polarization and the laser wavelength on the photo-ionization of solids in the context of laser damage.

An alternative method to identify the onset of the damage on optical components in vacuum was proposed based on the Langmuir probe measurement of the plasma emitted by the sample. The vacuum measurements based on the Target Current (TC) and Langmuir Probe (LP) current measurements are compared with the spatial modulations of the beam-pulse profile that are used to qualify the LIDT in air. Several materials in thin film form were irradiated by a fs laser beam on a wide range of fluences. Our proposed approaches for the LIDT monitoring system provide measurable signals for fluences below the LIDT measured in air, with a clear signature when damage is induced on the irradiated materials. The in situ Langmuir probe approach for monitoring the degradation of the optical components placed in the vacuum can make a big difference in high-power laser facilities, as it can accelerate the transition towards industrial-grade lasers through a safer operation.

The poster will present the MELBA setup located at CEA CESTA (France). The MELBA laser delivers a nanosecond UV centimeter-sized laser beam dedicated to the study of laser-induced damage and damage growth in the context of the Laser MégaJoule. Laser pulses are spatially, temporally and spectrally both shaped and characterized. A dedicated imaging system can measure the non-linear propagation in samples and its consequence on laser-induced damage and filamentation formation. Recently, it was made possible to adjust the beam polarization from linear to circular.

The detailed evolution of transparent material modifications and damage morphologies due to Ultrafast lasers is complex and highly material-dependent. Mitigation of unwanted damage or enhancement of desirable modifications is often guided by ex post facto observation long after the laser material interactions have occurred. We present time-resolved imaging of shockwave and crack evolution in brittle transparent materials due to absorption of single, or multiple, ultrafast laser pulses. We achieve temporal resolution down to a few hundred femtoseconds and spatial resolution to a few microns. These images allow for observation during material modifications and offer new insight into material behavior.

Laser-induced UV HR mirror degradation has become a problem for laser system integrators. The purpose of the presented study was to compare commercial-grade standard UV mirrors operating in the fs-pulse regime with coatings made of HfO2 & SiO2 mixture and to identify the key features that may accelerate component ageing. As a result, after completion of the experimental work, we demonstrated how color-change damage behaves across the coating's surface and achieved high LIDT coatings with catastrophic damage at 0.719 J/cm2 (10 000 000-on-1) that showed minimal fatigue.

The performance of thin-film interference filters is determined by the refractive index of the materials used. The greater the refractive index contrast between the optical materials the better the performance of the filters. In this study, we show how material refractive indices of larger than 3 for the NIR spectral range can be achieved by using quantized nanolaminates. The theoretical basics as well as the coating technique based on IBS and the measurement results are presented. Experimentally, a 100nm gap blue shift was proven. In addition, HR-mirrors and AR coatings were produced with this concept.

The continuous wave laser-induced damage threshold of germanium is measured for a 5s exposure of 1.07 µm light focused to a spot size with 1/e2 diameter of 1044 µm, following the International Organization for Standardization standards.

A unique dual-beam laser-damage test station has been constructed for testing materials with broadband incoherent radiation. Utilizing a novel optical parametric amplifier-based light source capable of delivering an incoherent bandwidth of up to 10 THz, bulk damage in KDP has been characterized for a number of different bandwidth conditions to help understand the role that the resulting intensity fluctuations play. The results largely suggest that damage mechanisms in KDP do not rely heavily on nonlinear processes and are supported by established models of plasma generation and growth.

Laser-induced damage test was conducted on super polished excimer ArF-laser-grade CaF2 window. Surface defect-driven damage was revealed by scanning electron microscope. Further laser-induced damage threshold (LIDT) test was performed on SSD-free laser-durable-grade (LDG) CaF2 with and without Gen 3 protective coating for CaF2 (PCCF3) using ISO S-on-1 method. The LIDT was determined by the least fluence failure method. Relationship between the LIDT and the pulse count was established. Effect of plasma densification of the PCCF3 on the LIDT was revealed. An accelerated lifetime damage test (ALDT) methodology was discussed to further enhance lifespan predication of the demanding laser optics.

With numerous manufacturers providing different Laser-Induced Damage Threshold (LIDT) values in the nanosecond regime, a simple ranking based on numbers alone may not provide a clear picture of the best choice. Variations in testing procedures, albeit following the ISO 21254 standard, further complicate the selection process. By employing a comprehensive 1-on-1 test procedure, it becomes possible to observe various parameters that influence LIDT values. When sharing test results within the community, adhering to good practices and meticulous attention to the error budget and its contributors are crucial. Above all, laser optics users must comprehend the intricacies of laser-induced damage testing and seek detailed information instead of relying solely on numerical comparisons. This study explores the challenges and considerations in selecting and testing laser optics, emphasizing the importance of a comprehensive approach.

Surface damage of silica optics routinely limits the operation of high energy laser systems.NIF, which exposes ~100 m2 of fused silica optics surface to high energy nanosecond laser light on every full system shot, provides an ideal platform to study the growth behavior of laser-induced damage. By measuring damage sites when an optic is removed for recycling, microscopy allows the study of thousands of sites before and after exposure to various shot sequences on the NIF laser. A multi-shot model was fit to predict the likelihood of exit surface damage site growth under exposure from 3ω nanosecond regime pulses.

Nonlinear absorption processes play a significant role in the ultra-short pulse behavior of thin film dielectric optics. The primary mechanisms are typically multiphoton absorption and photon tunneling, resulting in avalanche ionization and damage to the optic. Nonlinear absorption measurements provide an alternative to obtain an LIDT estimate without using destructive methods. By using an LCA-measurement to track the increased absorption of an optic with respect to the irradiation intensity, it may be possible to obtain an estimate of the LIDT. This absorption can be characterized by several theoretical models. Additionally, the effect of imbedded particles in the film are discussed.

Possible linear-to-circular polarization conversion had been studied for the Laser MégaJoule. We measured the consequences of such polarization conversion on laser-induced damage using the MELBA testbed. The MELBA laser is located in CEA CESTA (France) and delivers a nanosecond UV centimeter-sized laser beam. Experimental comparison of polarizations states showed a significant decrease of damage densities in circular polarization. Thanks to the particular imaging setup, we were able to explain this by both a reduction of the Kerr effect (supported by theory) and a reduction of the intrinsic absorption of silica optics defects.

In this work we present a novel approach to filling damage sites close to the surface. We explore laser parameters and thermal distribution for material reflow.

We present a scalable method for producing all-glass meta-surfaces with high mechanical stability and robust resistance to laser-induced damage. The process is based on dewetting a thin metal film on a glass substrate, followed by dry etching and metal mask removal. We will present advances of the method which enable formation of lenses, antireflection surfaces, and birefringence elements.