Laser Damage Mechanism Research Articles

Femtosecond Laser-Induced Damage Characterization of Multilayer Dielectric Coatings

The laser-induced damage threshold (LIDT) of optical components is one of the major constraints in developing high-power ultrafast laser systems. Multi-layer dielectric (MLD) coatings-based optical components are key parts of high-power laser systems because of their high damage resistance. Therefore, understanding and characterizing the laser-induced damage of MLD coatings are of paramount importance for developing ultrahigh-intensity laser systems. In this article, we overview the possible femtosecond laser damage mechanisms through damage morphologies in various MLD optical coatings tested in our facility. To evaluate the major contributions to the coating failure, different LIDT test methods (R-on-1, ISO S-on-1 and Raster Scan) were carried out for a high reflective hybrid Ta2O5/HfO2/SiO2 MLD mirror coating at a pulse duration of 37 fs. Different LIDT test methods were compared due to the fact that each test method exposes the different underlying damage mechanisms. For instance, the ISO S-on-1 test at a higher number of laser pulses can bring out the fatigue effects, whereas the Raster Scan method can reveal the non-uniform defect clusters in the optical coating. The measured LIDT values on the sample surface for the tested coating in three test methods are 1.1 J/cm2 (R-on-1), 0.9 J/cm2 (100k-on-1) and 0.6 J/cm2 (Raster Scan) at an angle of incidence of 45 deg. The presented results reveal that the performance of the tested sample is limited by coating defects rather than fatigue effects. Hence, the Raster Scan method is found to be most accurate for the tested coating in evaluating the damage threshold for practical applications. Importantly, this study demonstrates that the testing of different LIDT test protocols is necessary in femtosecond regime to assess the key mechanisms to the coating failure.

Preparation of high laser-induced damage threshold sol-gel Nb2O5 films with different additives

In this study, different sol-gel Nb2O5 films were prepared by using NbCl5 as precursor, and acetylacetone (ACAC), diethanolamine (DEA) and citric acid (CA) as additive, respectively. The results showed that the film with ACAC as additive presented the highest absorption and stability, corresponding to the lowest laser-induced damage threshold (LIDT) of 18.9 J/cm2, while that with CA as additive had the lowest absorption, corresponding to the highest LIDT of 24.9 J/cm2. The film prepared with DEA as additive had the smallest surface roughness, and the LIDT was 21.1 J/cm2. Moreover, although the films prepared with three different additives were all subjected to typically defect-induced damages, different types of damage features were observed. Finally, three structural evolution models of the films were proposed, accompanied by establishment of the relationship between the additives and the LIDT, which was of great significance to further understand the laser damage mechanism.

Dynamic behavior modeling of laser-induced damage initiated by surface defects on KDP crystals under nanosecond laser irradiation

The issue of laser-induced damage of transparent dielectric optics has severely limited the development of high-power laser systems. Exploring the transient dynamic behaviors of laser damage on KDP surface by developing multi-physics coupling dynamics model is an important way to reveal the mechanism of nanosecond laser damage. In this work, KDP crystals are taken as an example to explore the mechanism of laser-induced surface damage. Based on the theories of electromagnetic field, heat conduction and fluid dynamics, a multi-physics coupling dynamics model is established for describing the evolution of nanosecond damage processes. The dynamics of laser energy transmission, thermal field distribution and damage morphology during nanosecond laser irradiation are simulated with this model. It is found that the enhancement of light intensity caused by surface defect plays an important role in the initial energy deposition and damage initiation of the laser irradiation area. The evolution of temperature field and crater morphology during subsequent laser irradiation is helpful to understand the laser damage process. The feasibility of this model is verified by the morphology information of typical defect-induced laser damage. This work provides further insights in explaining the laser-induced damage by surface defects on KDP crystals. The model can be also applied to investigate the laser damage mechanisms of other transparent dielectric optics.

Layer by layer exposure of subsurface defects and laser-induced damage mechanism of fused silica

Inert ion beams with large incident angle were used for layer by layer etching of chemical-mechanical polished fused silica. The evolution of the impurities, subsurface defects, surface roughness, and surface molecular structure after ion beam etching and their effects on the laser-induced damage threshold (LIDT) were investigated to understand the laser damage mechanism of fused silica. The impurity elements are mainly distributed at a depth of 0–200 nm of the sample surface, which will strongly absorb ultraviolet (UV) laser energy. The result is consistent with the measurement of weak photothermal absorption. After 500 nm removal of the fused silica surface, the deposition layer of polishing powder is removed, and the subsurface defects are exposed. Correspondingly, the amount and size of the defects reach the maximum, and result in the greastest surface roughness and deterioration of surface quality, which enhances the local optical field. As the removal depth increases, the passivation and removal of subsurface defects lead to a decrease in surface roughness and improvement of surface quality. In addition, the density of structural defects and the SiOSi bond angle will decrease after ion beam etching, which yields a densified and strengthened surface of fused silica. The results indicate that metallic impurities are the key factors that limit the improvement of LIDT. Moreover, subsurface defects restrict the further enhancement of LIDT. Therefore, ion beam etching can be developed to remove subsurface damage for improving the laser resistant capacity of fused silica.

In-situ high temperature laser-induced damage of sol-gel Ta2O5 films with different dual additives

Using TaCl5 as the precursor, two types of Ta2O5 sols were synthesized with employment of different dual additives, namely, diethanolamine (DEA) with acetylacetone and DEA with polyethylene glycol (PEG). The prepared sol-gel Ta2O5 films were featured by the low surface roughness, high transmittance, low absorption and high laser-induced damage threshold (LIDT). The highest LIDT of 29.1 J/cm2 was obtained in the film prepared with the DEA and PEG additives. This should be attributed to the chelation effect of DEA and the steric hindrance imposed by PEG, which synergistically resulted in a more regular three-dimensional network structure and consequently lower film internal defects. In the case of temperature increase to 150 °C, the LIDT levels of both the films decreased. The damage morphologies indicated the defect-induced mechanism for the films irradiated either at the room temperature, or at the high temperature of 150 °C. Moreover, an evolution model was proposed to reveal the effect of dual additives on the structure of Ta2O5 films. This study was conducive to a better understanding of the fundamental laser damage mechanism and exploring the potential application of sol-gel films in high temperature environment.

Experimental study of 355 nm laser damage ignited by Fe and Ce impurities on fused silica surface

Laser-induced damage on fused silica surface is often ignited by absorbing impurities introduced by polishing processing. In order to analyze laser damage mechanism induced by Fe and Ce impurity on fused silica surface, this paper presents laser induced damage threshold of three types of fused silica surface dealed with traditional chemo-mechanical or magnetorheological finishing and carries out the surface impurity analysis, photo-thermal absorption analysis and gray haze damage mechanism analysis. The results show the significant different Ce and Fe impurities on the surface of the three kinds of samples. The two impurities both have a serious influence on laser-induced native damage. The analyses of photo-thermal absorption on the surface of optical element show that the haze damage pits morphology is related to the absorptivity of nanoparticles. The native surface damage threshold induced by Fe nanoparticle is lower than by Ce nanoparticle by analyzing atomic force microscopy images and scanning electron microscopy images. The conclusion is different from previous reports which believe Fe element shows a weak relation with laser-induced damage. This paper also analyzes the relationship between gray haze damage and native damage for the first time. The results are helpful to understanding laser induced damage of fused silica ignited by impurities.

17‐2: Invited Paper: Laser Safety Considerations in Laser Beam Scanning Light Field Displays

Laser Beam Scanning (LBS) technology is one of the light field display technologies which are considered cures for the vergence‐accommodation problem in Virtual Reality (VR), Augmented Reality (AR) and Mixed Reality (MR). When Lasers are applied to the human eye, special caution is needed. This paper will discuss the different laser damage mechanisms with the human eye under LBS settings. Traditionally rated Class 1 lasers are not always safe in this case.

Continuous-wave laser damage mechanism of a spectral combining grating.

With increased power of spectral beam combination, surface heat distortion of multilayer dielectric gratings (MDGs) could occur. In this study, the damage morphology of MDGs was initially analyzed under a continuous-wave laser irradiation. Subsequently, the surface distortion and temperature rise of different MDGs were tested experimentally. The experimental results showed that the initial damage of MDGs was caused by the thermal stress. Further, the thermal stress of the multilayer dielectric films on the MDG surface was analyzed theoretically. The calculated results were in good agreement with the experimental results. The conclusions indicated that with the increase of the MDG surface temperature, the stress in the HfO2 layers initially reached the stress damage threshold of the dielectric films and, therefore, the damage occurred.

Laser-induced damage of nodular defects in dielectric multilayer coatings

Laser-induced damage (LID) of optical coatings has been extensively studied since the invention of the laser. It has been found that the defects, which are unavoidable in real-world optical coatings, are the main reason for triggering laser damage of optical components at low fluences. In particular, embedded nodules in dielectric multilayer coatings are the main limiting defects found in reflective optics operating in nanosecond regimes. During laser irradiation, thermomechanical damage occurs preferentially at nodules because of enhanced energy absorption due to electric-field intensity (EFI) enhancement and the degradation of mechanical stability due to discontinuous boundaries. This report reviews the recent studies of the LID due to nodular defects in dielectric multilayer coatings. First, we present statistical studies on the geometric model and laser damage mechanism of nodules in the real world and introduce the solutions to control the formation of nodules. However, the low density and diverse properties of real nodular defects make the systematic study of LID initiating from localized defects a time-consuming and challenging task. In this regard, experimental and theoretical studies of localized defect-driven LID using artificial defects with properties that can be controlled are highlighted. We also present recent research progress on the damage mechanism of artificial nodules interpreted from aspects of mechanical properties and electric-field enhancement. In addition, approaches for modifying the deposition process or multilayer design are examined to reduce the EFI enhancement in the nodules and to improve the laser damage resistance of near-infrared high-reflective coatings based on the deeper understanding of the underlying physics of the damage process.

Local structural investigation of refractory oxide thin films near laser damage threshold

Study of laser induced damage mechanism and effort to increase the damage threshold of oxide thin films is of immense technological importance because as improved laser-damage resistant optical materials and better fabrication technologies are developed, laser designers can increase the system operating energies and powers to the limits of these new materials. Although, many efforts have been made to investigate how stoichiometry or long range order structure plays a role in describing the laser damage process, to the best of our knowledge, efforts to understand the local structural changes in the materials due to laser irradiation around the damage threshold has not been made so far. In this communication, a set of RF sputter deposited dielectric refractory oxide thin film samples have been subjected to laser irradiation across their damage threshold. In case of TiO2, Ta2O5, and HfO2 thin films, the samples were irradiated with optimized laser fluence and different locations on the samples were irradiated with varying number of pulse shots, while in case of Gd2O3 films, different locations of the samples were irradiated with fixed number of pulse shots and with varying laser fluence. The irradiated samples have subsequently been subjected to X-ray absorption spectroscopy (XAS) studies to find out change in the local structure of the samples under laser irradiation. Apart from XAS measurements, various other characterization techniques such as optical microscopic imaging, grazing incidence X-ray reflectivity and grazing incidence X-ray diffraction have also been employed to study the laser irradiated zones. The results indicate interesting local structural evolution in oxide thin films with the onset of laser induced damage which gives novel insight about the laser damage mechanism.

Discussing defects related to nanosecond fatigue laser damage: a short review

Laser damage measurements with multiple pulses at constant fluence (S-on-1 measurements) are of high practical importance for design and validation of high-power photonic instruments. Using nanosecond lasers, it was recognized long ago that single-pulse laser damage is linked to fabrication-related defects. Models describing the laser damage probability as the probability of encounter between the high fluence region of the laser beam and the fabrication-related defects are thus widely used. Nanosecond S-on-1 tests often reveal the “fatigue effect,” i.e., a decrease of the laser damage threshold with increasing pulse number. Most authors attribute this effect to cumulative material modifications operated by the incubation pulses. We discuss the different situations that are observed upon nanosecond S-on-1 measurements that are reported in literature and speak in particular about the defects involved in the laser damage mechanism. These defects may be fabrication related or laser induced, stable or evolutive, cumulative or of short lifetime.

Laser-induced modifications of HfO2 coatings using picosecond pulses at 1053 nm: Using polarization to isolate surface defects

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 recently detailed the role of defects in the ps laser-modifications of silica coatings. We now discuss the similar role of defects in HfO2 1/2-wave coatings and also include the environmental effects on the damage process. By switching between S and P polarizations, we distinguish the effects of defects at the surface from those throughout the material. We find that defects very near the surface are dependent on the environment, leading to worse damage in vacuum than in air. Air suppresses or lessens the effects of these defects, suggesting a photo-chemical component in the mechanism of laser damage in HfO2 coatings.

Investigations on variation of defects in fused silica with different annealing atmospheres using positron annihilation spectroscopy

The laser damage resistance properties of the fused silica can be influenced by the microstructure variation of the atom-size intrinsic defects and voids in bulk silica. Two positron annihilation spectroscopy techniques have been used to investigate the microstructure variation of the vacancy clusters and the structure voids in the polishing redeposition layer and the defect layer of fused silica after annealing in different atmospheres. The fused silica samples were isothermally annealed at 1000 K for 3 h in a furnace under an air atmosphere, a vacuum atmosphere and a hydrogen atmosphere, respectively. The positron annihilation results show that ambient oxygen atmosphere only affects the surface of the fused silica (about 300 nm depth) due to the large volume and low diffusion coefficient of the oxygen atom. However, hydrogen atoms can penetrate into the defect layer inside the fused silica and then have an influence on vacancy defects and vacancy clusters, while having no effect on the large voids. Besides, research results indicate that an annealing process can reduce the size and concentration of vacancy clusters. The obtained data can provide important information for understanding the laser damage mechanism and improving laser damage resistance properties of the fused silica optics.

Special Section Guest Editorial:Laser Damage III

Laser damage of optical materials, first reported in 1964, continues to limit the output energy and power of pulsed and continuous-wave laser systems. In spite of some 48 years of research in this area, interest from the international laser community to laser damage issues remains at a very high level and does not show any sign of decreasing. Moreover, it grows with the development of novel laser systems, for example, ultrafast and short-wavelength lasers that involve new damage effects and specific mechanisms not studied before. This interest is evident from the high level of attendance and presentations at the annual SPIE Laser Damage Symposium (aka, Boulder Damage Symposium) that has been held in Boulder, Colorado, since 1969. This special section of Optical Engineering is the first one devoted to the entire field of laser damage rather than to a specific part. It is prepared in response to growing interest from the international laser-damage community. Some papers in this special section were presented at the Laser Damage Symposium; others were submitted in response to the general call for papers for this special section. The 18 papers compiled into this special section represent many sides of the broad field of laser-damage research.more » They consider theoretical studies of the fundamental mechanisms of laser damage including laser-driven electron dynamics in solids (O. Brenk and B. Rethfeld; A. Nikiforov, A. Epifanov, and S. Garnov; T. Apostolova et al.), modeling of propagation effects for ultrashort high-intensity laser pulses (J. Gulley), an overview of mechanisms of inclusion-induced damage (M. Koldunov and A. Manenkov), the formation of specific periodic ripples on a metal surface by femtosecond laser pulses (M. Ahsan and M. Lee), and the laser-plasma effects on damage in glass (Y. Li et al). Material characterization is represented by the papers devoted to accurate and reliable measurements of absorption with special emphasis on thin films (C. Muhlig and S. Bublitz; B. Cho, E. Danielewicz, and J. Rudisill; W. Palm et al; and J. Lu et al.). Statistical treatment of measurements of the laser-damage threshold (J. Arenberg) and the relationship to damage mechanisms (F. Wagner et al.) represent the large subfield of laser-damage measurements. Various aspects of multilayer coating and thin-film characterization are considered in papers by B. Cho, J. Rudisill, and E. Danielewicz (spectral shift in multilayer mirrors) and R. Weber et al. (novel approach to damage studies based on third-harmonic generation microscopy). Of special interest for readers is the paper by C. Stolz that summarizes the results of four “thin-film damage competitions” organized as a part of the Laser Damage Symposium. Another paper is devoted to thermal annealing of damage precursors (N. Shen et al.). Finally, the influence of nano-size contamination on initiation of laser damage by ultrashort pulses is considered in paper of V. Komolov et al.« less

激光对鳞片石墨改性酚醛树脂涂层的损伤机理

激光对鳞片石墨改性酚醛树脂涂层的损伤机理

Study on melting and thermal-stress damage thresholds of silicon induced by long pulsed laser at 0.532, 1.064 and 10.6 μm

In this paper, we present the melting and thermal-stress damage thresholds of silicon induced by long pulsed laser at 0.532, 1.064 and 10.6μm. Physical model of the problem is established based on the classical heat transfer and thermal-elasticity theory, and analytical solutions of temperature, thermal-stress and damage threshold are obtained by using integral transform method. Melting and thermal-stress damage thresholds of silicon were modeled and mechanism of laser damage to silicon was analyzed. Results show that, the damage threshold of silicon due to 0.532μm laser irradiating is lower than that of 1.064 and 10.6μm laser irradiating, and the damage mechanism of silicon material is mainly the melting damage.

Laser damage mechanisms in conductive widegap semiconductor films

Laser damage mechanisms of two conductive wide-bandgap semiconductor films - indium tin oxide (ITO) and silicon doped GaN (Si:GaN) were studied via microscopy, spectroscopy, photoluminescence (PL), and elemental analysis. Nanosecond laser pulse exposures with a laser photon energy (1.03 eV, 1064 nm) smaller than the conductive films bandgaps were applied and radically different film damage morphologies were produced. The laser damaged ITO film exhibited deterministic features of thermal degradation. In contrast, laser damage in the Si:GaN film resulted in highly localized eruptions originating at interfaces. For ITO, thermally driven damage was related to free carrier absorption and, for GaN, carbon complexes were proposed as potential damage precursors or markers.

Determination of ultra-short laser induced damage threshold of KH2PO4 crystal: Numerical calculation and experimental verification

Rapid growth and ultra-precision machining of large-size KDP (KH2PO4) crystals with high laser damage resistance are tough challenges in the development of large laser systems. It is of high interest and practical significance to have theoretical models for scientists and manufacturers to determine the laser-induced damage threshold (LIDT) of actually prepared KDP optics. Here, we numerically and experimentally investigate the laser-induced damage on KDP crystals in ultra-short pulse laser regime. On basis of the rate equation for free electron generation, a model dedicated to predicting the LIDT is developed by considering the synergistic effect of photoionization, impact ionization and decay of electrons. Laser damage tests are performed to measure the single-pulse LIDT with several testing protocols. The testing results combined with previously reported experimental data agree well with those calculated by the model. By taking the light intensification into consideration, the model is successfully applied to quantitatively evaluate the effect of surface flaws inevitably introduced in the preparation processes on the laser damage resistance of KDP crystals. This work can not only contribute to further understanding of the laser damage mechanisms of optical materials, but also provide available models for evaluating the laser damage resistance of exquisitely prepared optical components used in high power laser systems.

Assessment of mono-shot measurement as a fast and accurate determination of the laser-induced damage threshold in the sub-picosecond regime.

Standard test protocols need several laser shots to assess the laser-induced damage threshold of optics and, consequently, large areas are necessary. Taking into account the dominating intrinsic mechanisms of laser damage in the sub-picosecond regime, a simple, fast, and accurate method, based on correlating the fluence distribution with the damage morphology after only one shot in optics is therein presented. Several materials and components have been tested using this method and compared to the results obtained with the classical 1/1 method. Both lead to the same threshold value with an accuracy in the same order of magnitude. Therefore, this mono-shot testing could be a straightforward protocol to evaluate damage threshold in short pulse regime.

Experimental and computational study of damage pocess induced by 1064 nm nanosecond laser pulse on the exit surface of fused silica

Material response and the launch of laser plasma during the 1064 nm nanosecond laser pulse induced damage to the exit surface of fused silica are investigated. Employing a polarization-based two-frame shadowgraphy setup with ～ 60 fs probing resolution, the transient material responses from the rising part of nanosecond pumping pulse to several hundred nanosecond timescale are captured. Using a shearing interferometry setup, the evolution of transient phase shift of laser plasma in the expansion process to the ambient air is also investigated. Inhomogeneous distribution of phase shift caused by the electrons and neutrals in the plasma is quantitatively resolved by employing the fast Fourier transform based filtering algorism. To demonstrate the evolutions of important plasma parameters such as pressure, temperature and density, a continuum hydrodynamic model is numerically solved. The initial pressure of plasma is estimated according to the point-explosion model, and the initial plasma temperature is achieved by calculating the difference between simulating shockwave front radius and experimental value at the same delay. The optimal temperature is chosen when the radius difference is minimal. Main conclusions are as follows. 1) Abundant suprathermal electrons are excited in the early energy deposition process. Part of these electrons contribute to the thermal transport process and produce the laser supported solid-state absorption front (LSSAF) which propagates into the bulk silica. Other electrons escape to the air side and contribute to the formation of air plasma through the impact ionization process. Plasma expansion speed is about 20 km/s during this phase. 2) When the pump pulse is terminated, the LSSAF and air plasma lose their energy supplied and experience a rapid decline of the temperature and expansion velocity. As a result, the final damage crater depth exhibits seldomly no increase compared with the transient crater depth during this phase. Hot bulk plasma formed in this phase becomes the damage precursor and induces the ejection of abundant neutrals probably due to the phase explosion mechanism. Inhomogeneous distribution of stress is formed by Rayleigh-Taylor instability at the interface between hot bulk plasma and surrounding bulk material during the expansion of LSSAF. Radial and circumferential cracks are formed due to the release of stress. 3) Evolution of air plasma follows the conventional evolution process of laser-induced plasma, i. e. , internal pressure, temperature and density decrease quickly with time delay. The simulated transient highest pressure is about 600 MPa. Simulation also predicts the formation of the internal shockwave. Our work will be helpful in understanding the laser damage mechanism of the fused silica optical window.