Nanosecond Laser-induced Damage Research Articles

Comparing the laser-induced damage distribution in POFs with raytracing simulations

The understanding of the laser-induced damage behavior in polymer optical materials is of high interest to prevent their damage and to increase the laser damage resistance of optical components. Moreover, compared with optical components made from high-quality glass materials, nanosecond laser-induced damage for wavelengths in the visible and near-infrared (NIR) occurs inside the bulk material of PMMA and not at its surface. This phenomenon complicates the determination of the laser-induced damage threshold (LIDT) in PMMA fibers. Since the bulk material itself determines the LIDT, knowledge of the intensity distribution in the multimode fiber is of utmost importance. Our fibers were irradiated at a wavelength of 532 nm with an ns-pulsed laser system with a 10 Hz repetition rate. To investigate the damage behavior in polymer optical fibers, we applied different imaging and analysis techniques. To our knowledge, those techniques are used here for the first time in order to study damaged polymer materials. With the help of a Nomarski microscope, axial and radial damage distributions within the multimode PMMA fibers were determined and compared with ray-tracing simulations of the intensity distribution within the optical fiber. Moreover, extruded PMMA plates were irradiated with the aim of comparing the damage behavior of materials with different manufacturing. In addition, the planar geometry of the plates allows for a more reliable application of the different measurement methods. Overall, investigations with a thermal imaging camera and EDX analysis indicate that the damage behavior of polymer optical material is thermally driven during the ns-pulsed irradiation. Furthermore, voids are formed during the damaging process within the polymer optical fibers and plates, as indicated by both SEM images and X-ray computed microtomography (µ-CT) investigations. Finally, we investigated the damages in fiber preforms and PMMA plates using two photon-microscopy. By doing this, we detected fluorescence signals from the damaged material, indicating that the damage process leads to a major modification of the polymer.

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.

Investigating the material properties of HfO2/SiO2-based anti-reflection coatings during 1064 nm laser-induced breakdown in air and vacuum conditions

It is important to understand the damage process of optical coatings subjected to nanosecond laser ablation. However, investigating the high power laser interaction with optical coatings in a short time and space resolution is challenging. The dynamic evolution of materials ejection and layer peeling-off processes during damage of HfO2/SiO2-based anti-reflection coatings induced by 1064 nm pulsed laser were systematically examined by employing the time-resolved shadowgraph technique. Therefore, materials response imaging was performed by shadowgraphy to better understand the interaction between coatings and laser beam in air and vacuum conditions. The coatings damage in vacuum was not influenced by the plasma plume, which showed markedly different responses to the laser-coatings interaction. By comparing the form of damage crater and layer peeling-off through SEM analysis and step profiler, it was found that more laser energy dissipated into the target with the decrease in ambient air pressure. For the reverse damage process in air and vacuum, materials ejection by the continuous impact of the laser energy was observed and the forms of materials ejection and damaged craters were the same. Investigating the materials response under different air pressure conditions provided valuable insights into the damage mechanisms during the nanosecond laser-induced damage of optical coatings.

Mechanism in damage variation of nanosecond laser-induced damage of germanium sheets in vacuum

Laser-induced damage of germanium (Ge) sheets plays a key role to estimate the lifetime of optical components in the vacuum and space irradiation environment. However, the dependence between the degrees of laser-induced surface damage and nanosecond pulsed irradiation remains unclear for Ge sheet. Here, we investigate damage morphology and the growth characteristic of Ge sheets under different conditions and measure the laser-induced damage threshold (LIDT) in vacuum. Our results reveal that optical reflectivity of Ge in vacuum chamber falls 92% because of the defective sites on the damaged surface. The area, the depth and the size of the damage crater increase but the hillock height and the surrounding thermal diffuse area decrease along with the increase of vacuum degree. The thermal effect, the failure effect of plasma shock wave and the mechanical stress are analyzed to be the main mechanism in the laser damage. The combined experimental and theoretical work gives an insight into the change of surface damage and material failure behavior, hence broadens the potential applications of Ge sheets in areas of laser material manufacturing and laser processing.

The evolution of nano-precursors at film-substrate interface under nanosecond laser irradiation

Functionally graded layers (FGLs) with quasi-continuous interfaces are deposited by ion-beam sputtering (IBS). The laser-induced damage threshold (LIDT) of FGLs is more than twice that of film-substrate interface. Basic time-resolved microscope system is designed to capture the ultrafast dynamics process of nanosecond laser-induced damage in FGLs and film-substrate interface, respectively. Materials ejections and damages craters at the film-substrate interface are observed by focused ion beam (FIB) microscope. The primary sources leading to low LIDTs are the nano-precursors distributed among the subsurface. The nano-precursors formed by the metastable chemical structures (FeOx and CeSixOy) are agglomeratedand grown during migration process because of sputtered atomic collision motion. High normalized electric field intensity enhancement in the subsurface region (about 275 nm) is considered to be an important factor that promotes absorption of the nano-precursors. The experiment first provides a sufficient experimental observation of the laser-induced damage process inside films which deepens people’s understanding of nano-precusors and lays a solid basis for solving the damage of film-substrate interface.

Material ejection and layer peeling-off in HfO2/SiO2 thin-film beam splitters induced by 1ω and 3ω lasers

Nanosecond pulse laser-induced damage is associated with nodular defects and nano-precursors in hafnium oxide (HfO2)/silica (SiO2) multilayer films. In this study, the dynamic evolution of material ejection and layer peeling-off in HfO2/SiO2 thin-film beam splitters used for third harmonic separation were examined by time-resolved shadowgraph technique under the fundamental frequency of 1064 nm (1ω) and third harmonics 355 nm (3ω) laser beam, respectively. The initial material ejection and subsequent peeling-off of layers were studied to clarify most thin films damage events caused by 1ω and 3ω lasers. The peeling-off of films under 1ω laser was mainly caused by thermomechanical effect. In contrast, the peeling-off of films under 3ω laser was mainly affected by laser-induced mechanical damage. The different transmission forms of layers peeled off from the films revealed the influence of mechanical strength and thermal strength on damage progression. The Taylor–Sedov theoretical model was used to analyze the propagation characteristic shock waves after nanosecond laser irradiation of films. In summary, identification of propagation forms and speed of peeling-off layers provides helpful insights into the damage mechanisms during nanosecond laser-induced damage of optical thin films.

Time-resolved imaging of CaF2 poly-crystal response following 355 nm nanosecond laser irradiation

The evolution of 355 nm nanosecond laser-induced damage to C a F 2 poly-crystals is investigated by using the time-resolved pump-probe shadowgraph technique. The damage morphologies of the front surface, rear surface, and interior of the C a F 2 crystal are imaged by optical microscopy. When the rear surface is focused by one laser pulse throughout the front surface, three shock waves (SWs) and one SW are observed in the air beside the front and rear surfaces, respectively. When the laser energies are 40 mJ and 60 mJ, at delay time of 1000 ns the radii of SW fronts beside the front surface are 2569.8 µm and 2831.7 µm, while those beside the rear surface are 1012.9 µm and 1078.1 µm, respectively. The filamentary channels inside the C a F 2 crystal are established before the end of a laser pulse at energies of 25, 40, or 60 mJ. The average propagation velocities of SWs along the filamentary channel are approximately 8.2 µm/ns. The maximum diameters of channels can reach approximately 53 µm and 128 µm for 25 mJ energy and 40 mJ energy, respectively. The experimental results help to explore the mechanism of laser-induced C a F 2 poly-crystal damage in nanosecond regime.

Ultraviolet nanosecond laser-induced damage on potassium dihydrogen phosphate crystal surface originated from different defects

Nanosecond UV laser-induced surface damage of potassium dihydrogen phosphate samples was investigated and discussed by means of defect characterization, in situ damage test, as well as pump–probe shadowgraph imaging. Two distinctive types of surface damage induced by different defects have been demonstrated. Surface damage occurring at relative lower fluence (typically below 5 J / cm2 in our experiment) is highly correlated with fluorescent surface defects, which are considered as fractural structures introduced by surface cutting. The other type of surface damage that always occurs at higher fluence (above 8 J / cm2) is confirmed to originate from the bulk damage precursor located near the crystal surface.

Model Development for Nanosecond Laser-Induced Damage Caused by Manufacturing-Induced Defects on Potassium Dihydrogen Phosphate Crystals.

Nanosecond laser-induced damage on (potassium dihydrogen phosphate) KDP crystals is a complex process, which involves coupled actions of multi-physics fields. However, the mechanisms governing the laser damage behaviors have not been fully understood and there have been no available models to accurately describe this complex process. In this work, based on the theories of electromagnetic, thermodynamic, and hydrodynamic fields, a coupled multi-physics model is developed to describe the transient behavior of laser-supported energy deposition and diffusion accompanied by the surface defect (e.g., surface cracks)-initiated laser damage process. It is found that the light intensification caused by the defects near the crystal surface plays a significant role in triggering the laser-induced damage, and a large amount of energy is quickly deposited via the light intensity-activated nonlinear excitation. Using the developed model, the maximum temperature of the crystal material irradiated by a 3 ns pulse laser is calculated, which agrees well with previously reported experimental results. Furthermore, the modeling results suggest that physical processes such as material melting, boiling, and flowing have effects on the evolution of the laser damage process. In addition, the experimentally measured morphology of laser damage sites exhibits damage features of boiling cores, molten regions, and fracture zones, which are direct evidence of bowl-shaped high-temperature expansion predicted by the model. These results well validate that the proposed coupled multi-physics model is competent to describe the dynamic behaviors of laser damage, which can serve as a powerful tool to understand the general mechanisms of laser interactions with KDP optical crystals in the presence of different defects.

Comparison of 355-nm nanosecond and 1064-nm picosecond laser-induced damage in high-reflective coatings

The damage properties of multilayer coatings tested with 1064-nm 30-ps pulses are similar to those tested with 355 nm, nanosecond pulses. A kind of HfO2 / SiO2 high-reflective (HR) coating is prepared by electron beam evaporation. Laser-induced damage of HfO2 / SiO2 HR coatings is tested by 355-nm 7-ns pulses and 1064-nm 30-ps pulses, respectively. Damage morphologies and cross-sectional profiles are characterized using a scanning electron microscope and focused ion beam, respectively. The laser-induced damage thresholds and morphologies in the two tests are compared. The developing processes and damage mechanisms are discussed. Many similarities are found in the two tests: the typical damage morphologies in both tests appeared as micrometer-sized pits when irradiated by low-fluence pulses, while it turned out to be layer delamination when irradiated by high-fluence pulses. Damage onset is nearby the peak of the E-field in the two tests. Damage pits in both tests may be related to thermal stress caused by nanometer-sized isolated absorbers. There are also some differences in the damage properties between two tests: damage pits in 1064-nm 30-ps tests have a much higher density than that in 355-nm 7-ns tests. The detail features and the developing processes of the pits are different.

Correlation between defect absorption and nano-second laser-induced damage of HfO2/SiO2 based high reflective coatings at 1064 nm

The distribution of defect weak absorption and nanosecond pulse laser induced damage characteristics of HfO2/SiO2 multilayers were investigated. A combination of in-situ measurements of defect weak absorption distribution and laser induced damage threshold (LIDT) map was proposed, whereby the evolution of defect weak absorption and the laser induced damage morphology under laser irradiation was studied. During the laser irradiation, some nodular and scalded defects can be removed or passivated by the low flux laser, and the absorption of these defect types was reduced accordingly. Besides, catastrophic destruction defects with enhanced absorption after laser irradiation would develop into catastrophic damage. Further, two-dimensional scanning of the sample surface was used to study the relationship between defects weak absorption and the LIDT. The results indicated that the correlation of defects weak absorption intensity distribution and LIDT map depended on defect evaluation properties during the laser irradiation.

Wavelength dependence of the mechanisms governing the formation of nanosecond laser-induced damage in fused silica.

The influence of the wavelength on the morphology of nanosecond laser-induced damage on the exit surface of fused silica is investigated. A combination between the typical features of damage sites initiated at 1064 nm and 355 nm is observed at 532 nm, including ring patterns sporadically exhibited, in good agreement with calculations of the development of an electron avalanche at this wavelength. The associated ring appearance speed scales as the cube root of the laser intensity, and is ~10.5 km/s while it is ~20 km/s when initiated by infrared pulses. The whole set of results sheds light on the different wavelength-dependent mechanisms governing damage formation.

Multi-wavelength growth of nanosecond laser-induced surface damage on fused silica gratings

The nanosecond laser-induced damage growth phenomenon on the exit surface of fused silica grating is investigated at 1064 nm and 355 nm separately and also simultaneously. Experiments are first carried out on damage sites on a plane fused silica sample showing two different morphologies, and a damage type is selected for ensuring the repeatability of the subsequent tests. Comparing the mono-wavelength growth results on a grating and a plane fused silica sample, the periodic surface structure is found to be an aggravating factor for damage growth. This is highly supported by calculations of the enhancement of the optical electric field intensity thanks to Finite-Difference Time-Domain simulations. Finally, the mono-wavelength results enable us to quantify a coupling occurring in the multi-wavelength configuration, which could originate from the heating of the plasma (more likely produced in the ultraviolet) preferentially by the infrared pulse. This study provides interesting results about the involvement of the surface topography in damage growth, and paves the way towards the comprehension of this phenomenon at high-energy nanosecond laser facilities where fused silica gratings are simultaneously irradiated at several wavelengths.

Increase in nanosecond laser-induced damage threshold of sapphire windows by means of direct dielectric barrier discharge plasma treatment.

In this Letter we present a novel approach for increasing the nanosecond laser-induced damage threshold (LIDT) of sapphire windows. It is shown that after direct dielectric barrier discharge plasma treatment at atmospheric pressure for 90 s the LIDT is increased by a factor of 1.5 with respect to untreated samples. Several possible underlying mechanisms are introduced. For instance, organic contaminants and residues from polishing agents were removed by the plasma as ascertained by XPS measurements.

Comparison of defects responsible for nanosecond laser-induced damage and ablation in common high index optical coatings

Spatiotemporally resolved optical laser-induced damage is an experimental technique used to study nanosecond laser damage and initiation of ablation in dielectric metal-oxide films used for optical coatings. It measures the fluence (intensity) at the initiation of damage during a single laser pulse. The technique was applied to coatings of HfO2, Sc2O3, and Ta2O5, which were prepared by ion-beam sputtering, and HfO2 which was prepared by electron-beam evaporation. Using the data obtained, we were able to retrieve the defect density distributions of these films without a priori assumptions about their functional form.

Influence of vacuum on nanosecond laser-induced surface damage morphology in fused silica at 1064 nm

The influence of vacuum on nanosecond laser-induced damage at the exit surface of fused silica components is investigated at 1064nm. In the present study, as previously observed in air, ring patterns surrounding laser-induced damage sites are systematically observed on a plane surface when initiated by multiple longitudinal modes laser pulses. Compared to air, the printed pattern is clearly more concentrated. The obtained correlation between the damage morphology and the temporal structure of the pulses suggests a laser-driven ablation mechanism resulting in a thorough imprint of energy deposit. The ablation process is assumed to be subsequent to an activation of the surface by hot electrons related to the diffusive expansion of a plasma formed from silica. This interpretation is strongly reinforced with additional experiments performed on an optical grating in vacuum on which damage sites do not show any ring pattern. Qualitatively, in vacuum, the intensity-dependent ring appearance speed V∝I1/2 is shown to be different than in air where V∝I1/3. This demonstrates that the mechanisms of formation of ring patterns are different in vacuum than in air. Moreover, the mechanism responsible of the propagation of the activation front in vacuum is shown to be outdone when experiments are performed in air.

Nanosecond multi-pulse laser-induced damage mechanisms in pure and mixed oxide thin films

We report on nanosecond laser-induced damage of pure and mixed oxide thin films deposited by ion beam sputtering. Silica, hafnia and alumina as well as their binary mixtures have been tested in S-on-1 mode at 355nm and 266nm using a multiscale approach. The results were analyzed qualitatively to discuss the different fatigue behaviors observed. The absence of a multi-photon absorption step in the 1-on-1 damage thresholds as a function of the band gap indicates defect-mediated damage mechanisms. During the multi-pulse experiments we observed laser-induced defects that cause fatigue effects and preexisting low-density defects, which are insensitive to multiple pulse irradiation. Depending on material and beam size both types of defects (preexisting and light-induced) may contribute equally to the observed damage probability. Comparing the fatigue behavior of the mixtures to their constituting pure oxides, we found that, in general, the fatigue behavior of binary mixtures cannot be interpolated from the behaviors of the pure oxides.

Multi-wavelength study of nanosecond laser-induced bulk damage morphology in KDP crystals

This study reports on experimental results on the morphology of laser-induced damage in the bulk of potassium dihydrogen phosphate (or KDP) crystals by nanosecond pulses. The resulting morphology of bulk laser damage is structured in four different zones whose characteristics depend on laser parameters such as wavelength, laser polarization, and fluence. To have a better understanding of bulk formation and structure of KDP laser damage, accessing the mechanical properties of KDP is a promising way which departs from a thermal approach which is mainly used in various models well documented in the literature. In that way, we have revisited various mechanical models of fracturation frequently used to evaluate the toughness of optical materials. Our experiments are compared to these models where the toughness is the criterion, which has been chosen to validate the modeling part of this study. We conclude that experiments are in agreement with the common values obtained by indentations tests proposed in the literature, allowing us further developments with one of these models to describe the damage formation in KDP crystals. Also, it appears that laser damage events split into two separated groups depending on the wavelength, with on one side 351-nm damage sites and on the other side 1064- and 532-nm damage sites. That suggests the existence of different physical mechanisms responsible for bulk damage initiation in KDP crystals with respect to the wavelength.

Effects of different post-treatment methods on optical properties, absorption and nanosecond laser-induced damage threshold of Ta2O5 films

Ta2O5 films deposited by ion beam sputtering were post-treated with different methods. The optical properties, microstructures, surface morphologies, absorption and laser-induced damage threshold (LIDT) at 1064nm and 12ns of the films were comparatively studied. Annealing conduced to larger changes of optical transmittance and surface morphologies whereas it led to smaller changes of absorption and LIDT than laser conditioning. As it was shown, all the three post-treatment methods in this study were beneficial to the laser damage resistance of the films, yet the details of the defect elimination caused by these methods were different. A defect evolution model was proposed, which revealed the different effects of post-treatment methods on the defect repair of the films. The highest LIDT achieved by the combination of annealing and laser conditioning was due to their complementary effect which resulted in the lowest defect density and the smallest defect size of the film.

Nanosecond laser-induced damage at different initial temperatures of Ta2O5 films prepared by dual ion beam sputtering

Ta2O5 films were deposited by dual ion beam sputtering method. The nanosecond laser-induced damage threshold (LIDT) at different initial temperatures and time of the films was investigated by an in situ high temperature laser-induced damage testing platform. It was shown that, when the initial temperature increased from 298 K to 383 K, the LIDT at 1064 nm and 12 ns significantly decreased by nearly 14%. Then the LIDT at 1064 nm and 12 ns decreased slower with the same temperature increment. Different damage morphologies were found at different initial temperatures. At low initial temperatures, it was the defects-isolated damage while at high initial temperatures it was the defects-combined damage. The theoretical calculations based on the defect-induced damage model revealed that both the significant increase of the highest temperature and the duration contributed to the different damage morphologies. With the initial temperature being increased, the thermal-stress coupling damage mechanism transformed gradually to the thermal dominant damage mechanism.