Laser-induced Damage Mechanisms Research Articles

Behaviors of laser-induced damage of dye cell windows at low fluences and their mechanism analysis

In most liquid dye lasers, dye cell windows are the optical components most susceptible to laser damage even at very low fluences. Although various mechanisms and mitigation techniques have been explored, damage still occurs at some point, and some behaviors of the process cannot be effectively explained. In this work, we investigated three special behaviors of dye cell window damage which cannot be explained by ordinary laser-induced damage mechanisms. We proposed that damage to dye cell windows can be caused by a contamination process similar to laser-induced contamination (LIC) on space optics, and used the LIC mechanism to analyze the cause of those damage behaviors. Additional experiments and calculations were conducted, providing more evidence in support of the analysis. In addition, it was found that dye molecules, which are necessary for dye lasers, are the primary contaminants in the LIC process. This explains the inevitable damage to dye cell windows.

Effect of THz-bandwidth incoherent laser radiation on bulk damage in potassium dihydrogen phosphate crystals

The laser-damage performance characteristics of potassium dihydrogen phosphate (KDP) samples under exposure to a distinctive broadband incoherent laser pulse are investigated. A laser system providing such pulses is intended to explore improved energy-coupling efficiency on the target in direct-drive inertial confinement fusion experiments and provides incoherent bandwidths as large as 10 THz in a nanosecond pulse. A consequence of this bandwidth is very rapid fluctuations in intensity capable of reaching maxima much larger than the average intensity within the pulse. A custom damage-test station has been built to perform measurements with broadband incoherent pulses in order to determine what effect these fast and high-intensity oscillations have on laser damage. A set of experiments under different bandwidth and beam configurations shows the effect to be minimal when probing bulk damage in KDP. Modeling indicates this behavior is supported by long electron-relaxation times compared to the source-field fluctuations, following excitation of individual electrons in the conduction band. The results help better understand the laser-induced–damage mechanisms in KDP, and its ability to operate in broadband temporally incoherent high-energy lasers that may be particularly suitable for future laser-fusion energy systems.

Properties of nonlinear optical absorption and refraction of rapidly grown KDP crystals

Potassium dihydrogen phosphate (KDP) single crystals grown under different conditions were obtained by the “point seed” technique. Third-order nonlinear optical properties of type-II samples which were prepared from the pyramidal and prismatic sections of crystal, respectively, were systematically investigated using the Z-scan technique at λ = 532 nm. Some heat-treated samples have been studied simultaneously. The results show that the nonlinear optical absorption and nonlinear optical refraction have been reflected, meanwhile, the nonlinear refractive index (n2) is positive, suggesting the generation of self-focusing effect. Furthermore, the results indicate that the nonlinear absorption and nonlinear refraction were associated with the growth conditions of the crystal. The prismatic sample grown under the relatively low growth temperature range (45∼25 °C) and small supersaturation (4 %) possessed a smaller nonlinear absorption coefficient (β) and n2 (β=4.73 × 10−13 m/W, n2=2.82 × 10−13 esu) than that of the others. In addition, the values of nonlinear absorption and nonlinear refraction of the samples can be further reduced after thermal annealing. These results may provide an assist to understand the mechanism of laser-induced damage of KDP crystals.

Nanosecond laser-induced multi-focusing damage in the bulk of fused silica

A time-resolved imaging system was utilized in this work to study the focusing behavior during laser-induced filamentary damage in the bulk of fused silica. The results show that focusing behavior has a significant dependence on laser energy. With the increased laser energy, single focusing is changed into multi-focusing. The probability of multi-focusing appearances also increases. During this process, the plasma distribution interval increases, which expands the area affected by the damage. The difference in the stress wave velocity indicates that the multi-focusing formation has temporal and spatial order. Finally, the multi-focusing damage’s physical mechanism and formation process are discussed. The results provide a reference for the mechanism of nanosecond laser-induced multi-focusing damage to transparent media.

Laser damage evolution by defects on diamond fly-cutting KDP surfaces

The unavoidable surface defects on the practical processed potassium dihydrogen phosphate (KDP) crystals, especially the most common protuberance defects, which are prone to damage under intense laser irradiation, seriously limit the application of KDP optics in laser fusion facilities. In this work, a defect-induced laser damage explosion model that couples the electromagnetic field and stress field was constructed to reproduce the dynamic damage process and reveal the laser-induced damage mechanisms innovatively. First, the near-field optical modulation to incident lasers was considered to describe the effect of structural parameters of surface defects on the optical transmission performance. A multi-physics field coupled failure model was then established for the first time to explore the laser damage mechanism through the dynamic behaviors of laser damage that reproduced by this model. It was proved that the protuberance defects on KDP crystal surface would lead to severe laser damage with the characteristics of large transverse spalling and longitudinal cracks due to the Rayleigh and stress waves, which well agrees with the results of laser damage experiments. The established damage model is capable of revealing the hazards of protuberance defects, and providing effective guidance for the suppression of surface defects introduced in the optical ultra-precision machining processes.

Light Field Intensification in Optical Films Induced by Intercoupling of Defects and Organic Contamination.

Based on the finite-difference time-domain method, light field intensification in optical films due to the intercoupling of defects and organic contamination was analyzed. The results show that the intercoupling between the defect and the organic contamination droplet leads to an increase in the local electric field and the coupling effect enhances with the decreasing distance between the defect and the organic contamination droplet and the increasing diameter of the organic contamination droplet. The coupling effect of the defect and the organic contamination layer depends on not only the thickness of the organic contamination layer but also the refractive index of the organic contamination layer. With the thickness and the refractive index of the organic contamination layer increasing, the peak value of the electric field decreases. This work deepens the physical understanding of the degradation mechanism of laser-induced damage in optical thin films used in vacuum.

Complicated morphologies and instructive mechanism of laser-induced bulk damage in transparent materials

Laser-induced bulk impurity damage has always been a difficult issue in optical systems, affecting the signal-to-noise ratio, beam quality, and other vital laser parameters. In this study, the generation mechanism of bulk damage morphology is demonstrated through a combination of experiment and theory. The growth mechanism is then represented through parameter control. Finally, a general conclusion of bulk damage generation is obtained alongside a reasonable control range of dangerous parameters. The damage area is described comprehensively, and the guiding conclusions of dangerous parameters are given, providing theoretical support for precision optics processing, high-power laser design and other projects.

The laser-induced damage mechanism of amorphous and crystalline Sc2O3 films

This paper investigated the laser-induced damage mechanism of amorphous and crystalline scandium oxide (Sc2O3) films prepared by ion beam sputtering (IBS) and annealed at different temperatures. A significant increased laser-induced damage threshold (LIDT) at 355 nm wavelength was obtained by amorphous film annealed slightly below the crystallization temperature, which was above 300% higher than those of crystalline films. Meanwhile, the damage morphologies of amorphous and crystalline films were totally different. Measurement of the absorption of films showed that higher absorption led to a large distinction between the LIDTs of films. Energy dispersive spectroscopy (EDS) patterns revealed very uniform distributions of elements without obvious aggregation phenomenon. Positron annihilation spectroscopy (PAS) indicated that vacancy defect increased the absorption and heavily reduced the resistance of films for 355 nm wavelength laser. The investigation in this study would promote the understanding of the laser-induced damage mechanism of amorphous and crystalline films and provide an effective method to increase LIDTs at 355 nm wavelength.

Study on optical performance and 532 nm laser damage of rapidly grown KDP crystals

Type II samples were obtained from the adjacent positions of pyramidal sector and prismatic sector in the same potassium dihydrogen phosphate (KDP) single crystal grown by the “point seed” technique under different temperature range and supersaturation. Some samples were annealed at 160 °C for 24 h. The UV–Vis spectra, especially in the ultraviolet region, show the optimal transmitting ability for the KDP crystal which was grown under relative low temperature range. The structural analysis performed by means of Fourier transformed infrared spectroscopy (FTIR) suggests that the crystal lattice of the KDP grown under relative small supersaturation become compact at the molecular level. The results of photoluminescence (PL) emission spectra show that the PL intensity increases with the rise of supersaturation and growth temperature range, which indicates that the concentration of all the detected micro defects in the KDP is abundant. The damage threshold conducted using 532 nm ns laser pulses shows the same trend with the results of PL spectra. The laser-induced damage threshold (LIDT) has been improved with the decrease of growth temperature range and supersaturation, and its variation has the highest correlation with the concentration of hydrogen vacancy defect. In addition, thermal annealing can effectively reduce the concentration of all the detected micro defects in the sample, and LIDT of the samples can be enhanced further after laser sub-threshold annealing. • KDP single crystals were obtained using the “point seed” rapid growth technique. • The relationship between laser-induced damage threshold and the concentration of micro defects is established. • The synergistic effect of thermal annealing and laser sub-threshold annealing is confirmed. • Enhanced laser-induced damage competence and laser-induced damage mechanism of the samples has been discussed.

Filamentary damage of fused silica irradiated by a 532 nm nanosecond laser

In this study, we use a time-resolved pump-probe shadowgraph technique to investigate the evolution characteristics of filamentary damage in bulk fused silica induced by a nanosecond pulse at 532 nm. The pump laser focuses on the front surface of sample and filamentary damage appears independently in the middle of sample. The whole damage process can be divided into single filament (SF), double filaments (DFs), and long filament (LF) successively. At the same time, the improved moving focus model is proposed by taking into account the temporal shape of the laser pulse and the laser is blocked and reflected by plasma at the critical density. It is in good agreement with the experimental result of filamentary damage process and helps to explore the mechanism of laser-induced filamentary damage in nanosecond regime.

Mechanisms of picosecond laser-induced damage from interaction with model contamination particles on a high reflector

The interactions of microparticles of different materials located on the surface of a multilayer dielectric mirror with intense 1053-nm laser pulses of varying fluence and duration (10 and 0.6 ps) are investigated. The particles caused localized intensification of the electric field, which becomes the dominant mechanism for the onset of damage and secondary contamination of the mirror at fluences far below the pristine (without particles) laser-induced-damage threshold of the mirror. Several interaction mechanisms leading to material modification and damage are identified, including the localized field intensification by multibeam interference and particle-induced microlensing, plasma-induced scalding, and secondary contamination via nanoparticle generation and particle melting. The resulting morphologies were observed to be vulnerable to damage growth and additional damage initiation when irradiated by subsequent pulses.

Recent Advances in Laser-Induced Surface Damage of KH2PO4 Crystal

As a hard and brittle material, KDP crystal is easily damaged by the irradiation of laser in a laser-driven inertial confinement fusion device due to various factors, which will also affect the quality of subsequent incident laser. Thus, the mechanism of laser-induced damage is essentially helpful for increasing the laser-induced damage threshold and the value of optical crystal elements. The intrinsic damage mechanism of crystal materials under laser irradiation of different pulse duration is reviewed in detail. The process from the initiation to finalization of laser-induced damage has been divided into three stages (i.e., energy deposition, damage initiation, and damage forming) to ensure the understanding of laser-induced damage mechanism. It is clear that defects have a great impact on damage under short-pulse laser irradiation. The burst damage accounts for the majority of whole damage morphology, while the melting pit are more likely to appear under high-fluence laser. The three stages of damage are complementary and the multi-physics coupling technology needs to be fully applied to ensure the intuitive prediction of damage thresholds for various initial forms of KDP crystals. The improved laser-induced damage threshold prediction can provide support for improving the resistance of materials to various types of laser-induced damage.

Mechanisms of picosecond laser-induced damage in common multilayer dielectric gratings.

The modifications of multilayer dielectric (MLD) gratings arising from laser-induced damage using 0.6-ps and 10-ps laser pulses at 1053 nm are investigated to better understand the damage-initiation mechanisms. Upon damage initiation, sections of the affected grating pillars are removed, thereby erasing the signature of the underlying mechanisms of laser damage. To address this issue, we performed paired studies using macroscopic grating-like features that are 5 mm in width to reveal the laser-damage morphology of the different grating sections: pillar side wall, sole, and pillar top. The results suggest that, similarly to MLD coatings, there are two damage-initiation mechanisms corresponding to the different pulse durations.

Laser-induced damage thresholds and mechanism of silica glass induced by ultra-short soft x-ray laser pulse irradiation.

Laser-induced damage thresholds (LIDTs) of silica glasses obtained by the femtosecond soft x-ray free-electron laser (SXFEL, 13.5 nm, 70 fs) and the picosecond soft x-ray laser (SXRL, 13.9 nm, 7 ps) are evaluated. The volume of the hydroxyl group in the silica glasses influenced its LIDTs. The LIDTs obtained in this research by the femtosecond SXFEL and the picosecond SXRL were nearly identical, but were different from that by the nanosecond soft x-ray pulse. The photoionization processes of silica glass in context of the laser-induced damage mechanism (LIDM) are also discussed. In the ultra-short soft x-ray pulse irradiation regime, the LIDM can be speculated to include the spallation process with a scission of bondings.

The characterization of laser-induced thermal damage mechanism of mid-infrared optical coatings with surface contaminants

Understanding the laser-induced thermal damage mechanism is important to the development of high power continuous wave (CW) laser. In this paper, we monitor the evolution of damage via a self-build optical element testing platform and build a correspond theoretical model based on the temperature field and heat conduction theory. The waveband of optical coatings (ZnSe and YbF3) under test is dedicated for the mid-infrared. Using a 10 kW level mid-infrared CW laser, the thermal stress damage process of the optical coatings caused by surface contaminants is recorded. The Finite Element Method was employed to calculate the thermal damage mechanism of mid-infrared optical coatings. The calculated thermal damage mechanism agrees very well with our experiment. To the best of our knowledge, this is the first comprehensive study on thermal damage mechanism of mid-infrared coatings with surface contaminants induced by CW laser.

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.

Photoluminescence spectrum study of defects of potassium dihydrogen phosphate crystals irradiated by different laser fluences

The laser-induced damage to potassium dihydrogen phosphate (KDP) crystal restricts the development of high power laser systems and attract the attention of researchers. The defects are essential for the understanding of the laser-induced damage to KDP crystals. The defects in KDP crystals are commonly related to <inline-formula><tex-math id="Z-20200908091635-1">\begin{document}$ \rm H_2PO_4^{-} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20200482_Z-20200908091635-1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20200482_Z-20200908091635-1.png"/></alternatives></inline-formula> groups. The defects of KDP crystal have been studied extensively, however the changes of defects of KDP crystal with low fluence and high fluence have not been investigated sufficiently. The synchrotron radiation technology is a sensitive method of detecting the defects. The vacuum ultraviolet photoluminescence (PL) emission spectra can provide microscopic structural changes in KDP crystals. In this work, we investigate the defects of KDP crystals irradiated with different fluences by vacuum ultraviolet PL emission spectra. The vacuum ultraviolet spectra are obtained at the 4B8 beam line in Beijing synchrotron radiation facilities. Each KDP crystal spectrum is measured from 200 to 400 nm and 400 to 800 nm. The emission spectra of KDP crystal irradiated with different fluences are fitted for illustration. Each Gaussian curve represents a kind of defect. Comparing the retired components with KDP crystal irradiated by 11.5 J/cm<sup>2</sup>, the new band at 231.55 nm emerges in the spectra of KDP crystal irradiated by 9.0 J/cm<sup>2</sup>. The intrinsic luminescence band is assigned to the radiative annihilation of self-trapped excitons. According to our previous work, the short chain structures mainly exist in the crystal irradiated by 9.0 J/cm<sup>2</sup>, and the long chain structure is mainly in the crystal irradiated by 11.5 J/cm<sup>2</sup>. The retired components have the short, medium and long chain. The length of P—O bond in the short chain is shorter than that in the long chain structure. The overlap between phosphorus 3<i>s</i> orbitals and oxygen 2<i>p</i> increases, and the radiative annihilation of STEs becomes stronger. So the band at 231.55 nm emerges in the spectrum of KDP crystal irradiated by 9.0 J/cm<sup>2</sup>. It suggests that the structure of the retired component and the structure of KDP crystal irradiated by 9.0 J/cm<sup>2</sup> are different. The results provide an insight into the defects in KDP crystals. It is meaningful to study the mechanism of laser-induced damage to KDP crystal.

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.

Laser-induced Damage to Polysilicon Microbridge Component

Based on the typical pixel structure and parameters of a polysilicon uncooled bolometer, the absorption rate of a polysilicon microbridge infrared detector for 10.6 µm laser energy was calculated through the optical admittance method, and the thermal coupling model of a polysilicon microbridge component irradiated by far infrared laser was established based on theoretical formulas. Then a numerical simulation study was carried out by means of finite element analysis for the actual working environment. It was found that the maximum temperature and maximum stress of the microbridge component are approximately exponentially changing with the laser power of the irradiation respectively and that they increase monotonically. The highest temperature zone of the model is gradually spread by the two corners of the bridge surface that are not connected to the bridge legs, and the maximum stress acts on both sides of the junction of the microbridge legs and the substrate. The mechanism of laser-induced hard damage to polysilicon detectors is the melting damage caused by high temperature. This paper lays the foundation for the subsequent study of the interference mechanism of the laser on working state polysilicon detectors.

The impact of nano-bubbles on the laser performance of hafnia films deposited by oxygen assisted ion beam sputtering method

Hafnia is a high refractive index material used in the manufacturing of dielectric coatings for next generation lasers. The formation of defects during deposition is the major barrier to realizing high laser-damage resistant coatings for future high energy density laser applications. Understanding the precursors responsible for laser-induced damage in hafnia is therefore critical. In this work, we investigate the mechanism of laser-induced damage in 90-nm thick hafnia films produced by an oxygen assisted dual ion beam sputtering (IBS) process. Under pulsed, nanosecond ultraviolet laser exposure (355 nm, 8 ns), the laser-induced damage onset is found to be strongly dependent on the amount of argon and excessive oxygen entrapped in the nanobubbles within the hafnia films. The presence of nanobubbles is revealed and confirmed by small angle X-ray scattering and scanning/transmission electron microscopy coupled with high-angle annular dark-field. The damage onset is stable initially but decreases as the energy of oxygen goes beyond 100 eV. The damage initiation is ascribed to a laser-induced plasma generation within the nanobubbles through multiphoton ionization. The results reveal that nanobubbles formed in the IBS produced coatings are a potent precursor. Although nanobubbles are commonly present in IBS films, their negative impact on laser damage resistance of hafnia films has not been previously recognized. Our findings provide a fundamental basis for the development of potential mitigation strategies required for the realization of laser damage resistant hafnia films.