Laser-induced Damage Morphology Research Articles

Laser Induced Damage Study of Dye Doped Polystyrene Films deposited in Glass Substrate by Dip Coating Technique

In the present work, the Laser-induced Damage Morphology of Dye-doped Polystyrene films is investigated. The focus is on the Front and Rear side Damage Morphology at two different Pulse Repetition Frequencies, namely 1 pps and 20 pps. For this experiment, films were deposited on both sides of Borosilicate Glass. It was found that at 1 pps, the front side film is damaged without affecting the glass, while the modified beam propagates through the glass to damage the rear side film. Dendrimer-like microstructures are found on both the front and rear sides of the damaged spots, along with pits and craters. At 20 pps frequency, the front side film is damaged, including damage to the glass, resulting in a large crater on the front surface. The morphological features were studied using Fresnel equations. Polymeric films were fabricated using the Dip Coating Technique, and Film thickness was evaluated using the Prism Coupling technique.

Laser-induced damage of black glass before and after surface treatment by containing impurities-SiO2 film during ultra clean manufacturing

Ultra-clean manufacturing is an indispensable key technology for inertial confinement fusion (ICF) to generate clean and sustainable energy, but the pollutants from laser-induced damage are a threat to ultra-clean manufacturing. Here we investigate the damage of black glass in a high laser fluence system, which refers to high fluence laser beams in fusion class laser systems. Results show a significant dependence of damage parameters on the laser pulse duration at 355 nm before and after surface treatment by SiO2 films, which include the laser-induced damage threshold (LIDT), morphology and depth. The simulation indicates that the LIDT of SiO2 films on black glass is improved obviously, which is 14.1 J/cm2, while that of normal black glass is 10.2 J/cm2. The presence of impurities will aggravate the damage of the thin films and pollutes the laser system. LIDT of SiO2 films containing impurities is only 3.6 J/cm2. The experiments show that the average LIDT of black glass is 9.54 J/cm2 while that of thin films with impurities is no more than 4.12 J/cm2. These salient results provide a new concept for the protection of absorbing stray light for ultra-clean manufacturing.

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.

Influence of coating thickness on laser-induced damage characteristics of anti-reflection coatings irradiated by 1064 nm nanosecond laser pulses.

The influence of coating thickness on laser-induced damage (LID) characteristics of anti-reflection (AR) coatings irradiated by 1064 nm nanosecond laser pulses was investigated. Two HfO2/SiO2 AR coatings with different physical thicknesses, 0.7 and 2.7 μm, were prepared and tested. To study the effect of coating thickness on a laser-induced damage threshold (LIDT) in isolation, electric field intensities (EFIs) at the substrate-coating interface were kept the same by using proper AR designs. Moreover, 2 nm artificial gold particles with a density of 10 mm-2 were implanted into the substrate-coating interface to achieve reliable experimental results. An optical microscope (OM) and a scanning electron microscope (SEM) were used for an online LIDT test and offline LID morphology observation, respectively. The typical LID morphology of thicker AR coatings was flat bottom craters with diameters of 20-50 μm, which can be easily observed by an online OM. For thinner AR coatings, hemispherical craters with diameters down to 1 μm were found as typical LID morphology by a SEM. However, these tiny craters could not be observed by an online OM. Moreover, such tiny craters did not grow with subsequent pulses, so they did not degrade the functional laser damage resistance of the thin AR coatings. When identified with an online OM, the LIDT of thinner AR coatings is found to be about two times higher than the thicker ones, and large delamination was mainly found as the LID morphology of AR coatings with high fluence. When observed with a SEM, the LIDT of thin AR coatings with tiny craters was over 60% lower than the LIDT of thick AR coatings, which agrees with the model that less energy is required to form smaller LID craters of thinner coatings.

Ultraviolet Laser-induced ignition of RDX single crystal.

The RDX single crystals are ignited by ultraviolet laser (355 nm, 6.4 ns) pulses. The laser-induced damage morphology consisted of two distinct regions: a core region of layered fracture and a peripheral region of stripped material surrounding the core. As laser fluence increases, the area of the whole crack region increases all the way, while both the area and depth of the core region increase firstly, and then stay stable over the laser fluence of 12 J/cm2. The experimental details indicate the dynamics during laser ignition process. Plasma fireball of high temperature and pressure occurs firstly, followed by the micro-explosions on the (210) surface, and finally shock waves propagate through the materials to further strip materials outside and yield in-depth cracks in larger surrounding region. The plasma fireball evolves from isotropic to anisotropic under higher laser fluence resulting in the damage expansion only in lateral direction while maintaining the fixed depth. The primary insights into the interaction dynamics between laser and energetic materials can help developing the superior laser ignition technique.

Damage morphology and mechanism in ablation cutting of thin glass sheets with picosecond pulsed lasers

We experimentally investigated the morphology and mechanism of laser-induced damage in the ablation cutting of thin glass sheets with picosecond pulsed lasers and we compared the experimental results to our models. After several passes of laser ablation, we observed two different kinds of damage morphologies on the cross-section of the cut channel. They are distinguished to be the damage region caused by high-density free-electrons and the heat-affected zone due to the heat accumulation, respectively. Furthermore, micro-cracks can be observed on the top surface of the workpiece near the cut edge. The nano-cracks could be generated by high energy free-electrons but opened and developed to be visible micro-cracks by thermal stress generated in the heat-affected zone. The crack length was proportional to the volume of heat-affected zone. Heat-affected-zone and visible-cracks free conditions of glass cutting were achieved by controlling the repetition rate and spatial overlap of laser pulses.

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.

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.

Wavelength Dependence of Laser-Induced Bulk Damage Morphology in KDP Crystal: Determination of the Damage Formation Mechanism

Wet etch process is applied to expose the bulk damage sites in KDP crystals to the surface for the examination by scanning electron microscopy (SEM) and optical microscopy. The damage sites induced by 1064 nm laser consist of three distinct regions: a core, an outer region of modified material, and some oriented cracks. Laser irradiated with 355 nm results in an increase of damage density, a decrease of core diameter and, rarely, occurrence of the crack. Wavelength dependence of the damage feature suggests that a repulsive force exists among the adjacent plasmas, which prevents further expansion of plasma and decreases the size of plasma. The deposited energy absorbed by the smaller plasma may not be able to generate the crack.

外加偏置电场下类金刚石薄膜激光损伤形貌

外加偏置电场下类金刚石薄膜激光损伤形貌

Influence of plasma treatment on laser-induced damage characters of HfO_2 thin films at 355nm

HfO(2) thin films were deposited by e-beam evaporation, and were post-treated with plasma under different flow rate ratios of argon to oxygen. By measuring the surface defect density, weak absorption, laser-induced damage threshold (LIDT) and damage morphology, the influence of the flow rate ratio of argon to oxygen on the laser-induced damage characters of HfO(2) thin films were analyzed. The experimental results show that plasma treatment is effective in reducing the surface defect density of thin films. Compared with the as-grown sample, the absorption reduction is obvious after plasma treatment when argon and oxygen flow rate ratio is 5:25, but the absorption increases gradually with the continued increase of argon and oxygen flow rate ratio. LIDT measurements in 1-on-1 mode demonstrate that plasma treatment is not effective in improving LIDT of the samples at 355 nm. Damage morphologies reveal that the LIDT is dominated by nanoscale absorbing defects in subsurface layers, which agrees well with our numerical simulation result based on a spherical absorber model.

Study on the morphology of laser induced damage in K9 glass by focused nanosecond pulse

The characteristics of laser induced damage morphology in the bulk of the K9 glass caused by focused nanosecond laser pulse is analyzed in detail. The damage shape looks like a spindle with a large front-end and its thickness gradually decreases on going to the rear end. According to the experimental results, the damaged range can be divided into four zones, namely the filamentary plasma channel, fractures, the fusion zone and the region of varying refractive index. The function of laser pulse energy deposition in space and the expansion pressure of shock wave are deduced, and the corresponding damage morphology is analyzed based on the spatial distribution of pressure. The theoretical analysis agrees with the experimental results.

Complex morphology of laser-induced bulk damage in K2H(2−x)DxPO4 crystals

We present a detailed study of the morphology of laser-induced bulk damage in K2H(2−x)DxPO4 crystals. We see three distinct regions of the internal damage sites: a rubble filled core, a shell that has probably been melted and compacted, and a larger outer region of slightly modified shocked material. The nature of these regions is important for understanding the impact of laser-induced damage on light scattering, which can be significant for high-power laser operation. We propose a simple model to explain the morphology and light scattering we observe.

Laser-induced damage morphology of high-reflective optical coatings

Two different kinds of 1064nm high-reflective (HR) coatings, with and without SiO2 protective layer, were prepared by electron beam evaporation. Three-dimensional damage morphology, caused by a Nd:YAG pulsed laser, was investigated for these HR coatings. Development of laser-induced damage on HR coatings was revealed by both temperature field calculation and discrete meso-element simulation. Theoretical results met experimental very well.

Laser Damage Properties of Optical Coatings with Nanoscale Layers Grown by Atomic Layer Deposition

The notable change in the structure and laser damage properties of coatings using periodically incorporated Al2O3 nanoscale layers grown by atomic layer deposition is demonstrated. Coatings with a nanoscale laminated structure had a higher transparency than pure TiO2 films grown at 400°C. The laser damage resistance of nanolaminate coatings is three times larger than that of pure TiO2 films. The morphology of laser-induced damage shows a significant reduction in the number of damage sites.

Roles of absorbing defects and structural defects in multilayer under single-shot and multi-shot laser radiation

Roles of absorbing defects and structural defects in multilayer under laser radiation were investigated. The HfO2/SiO2 dielectric mirrors for 355nm were prepared by conventional electron beam deposition. Two kinds of HfO2 with different purity were chosen as the high index material, and impurity content of the materials was accessed by glow discharge mass spectrometer (GDMS) and X-ray photoelectron spectroscopy (XPS). Laser damage testing was performed in both the “1-on-1” and “s-on-1” regime, using 355nm pulsed laser at a pulse length of 8ns. It was found that the single-shot laser-induced damage threshold (LIDT) is much higher than that of the multi-shot, and the absorbing defects hindered the improvement of laser damage resistivity greatly in the single-shot process, but in multi-shot mode the main factor influencing LIDT is accumulation of irreversible changes of structural defects. Optical microscopy and surface profiler were employed in mapping laser-induced damage morphology features after irradiation.

Laser-induced damage threshold of the antireflection coatings on quartz and sapphire under different laser modes

The laser-induced damage threshold (LIDT) and damage morphology of antireflection (AR) coatings on quartz and sapphire are investigated. A very interesting phenomena is found in the measurement. In the case of a single pulse laser, the LIDT of the AIR coatings on quartz is higher than that of sapphire. On the contrary, for a free-pulse laser, the LIDT of AIR coatings on sapphire is higher than that of quartz. (C) 2004 Society of Photo-Optical Instrumentation Engineers.

The morphology of pulsed laser-induced damage on the surface of a copper film mirror

A copper film mirror was irradiated using a pulsed Nd:YAG laser (1.06 μm,10 ns) through an aperture. After laser irradiation, the damaged spot was analyzed using an optical microscope and periodic damage rings were found at the laser irradiation spot, the average spacing of these rings being about 40– 50 μm . The damage ring could be observed in the periphery of the laser spot under lower laser energy densities. As the laser fluence increased, the periodic damage rings grew to cover the whole laser spot. Under higher laser energy density, the whole film was damaged and the rings disappeared. A Fresnel diffraction model is used to explain the appearance of these rings. The laser beam is diffracted by the aperture in the optic path, the laser intensity distribution at the copper film mirror becomes periodic rings and damage ripple pattern forms instantly with the laser ablation. The laser intensity distribution at the mirror surface is calculated using Collins formula, with which the period of the damage rings and other phenomena can be explained.

Laser damage studies of silicon surfaces using ultra-short laser pulses

Laser-induced damage morphology using femtosecond laser pulses on Si surfaces is reported. Damage morphology shows the ablation of material. A magnified view of the ablated portion shows a periodic surface structure in the form of ripples. The spacing of these ripples was between 0.5 and 2 μm and increased, on increasing the power density or number of pulses, and finally broke into parts, leaving well-ordered grains of approximate diameter 5 μm . Also for 100 or larger number of pulses, an amorphous ring in the periphery was formed. The diameter of this ring increased, on increasing either the laser fluence or the number of pulses. The formation of ripples has been explained with the help of the hypothesis of Boson condensation proposed by Van Vechten (Solid State Commun 39 (1981) 1285).