Improvement Of Laser-induced Damage Threshold Research Articles

Evolution of point defects in mechanical cracks of fused silica after CO2 laser melting

The traditional polishing method inevitably results in subsurface cracks in the fused silica, which seriously degrades their ultraviolet laser damage resistance. CO2 laser irradiation can melt these cracks and improve their laser induced damage threshold (LIDT). Photoluminescence spectrum and SEM-FIB were employed to investigate the changes in the material microstructure at the crack location with CO2 laser melting. The density of the oxygen-deficient centers of type II (ODC II) defects decreases, while the density of the non-bridging oxygen hole center (NBOHC) defects increases after high-temperature melting. The reason for this change is related to the dihydroxylation reaction and the participation of environmental oxygen in the defect type conversion. The reduction of ODC II defects is most likely the reason for the improvement of LIDT.

Femtosecond laser strengthening of electron-beam deposited SiO2 thin film on fused silica substrates

Femtosecond laser (@1030nm, 280fs) was applied to irradiate SiO2 thin films in the hope of improving damage resistance of films to nanosecond ultraviolet (UV) laser light (@355nm, 4ns). The optical and mechanical properties of laser strengthened SiO2 thin films were characterized and laser-induced damage threshold (LIDT) results show that femtosecond laser irradiation can significantly improve LIDT of the films to nanosecond UV laser light from 5.45J/cm2 to 28.96J/cm2 by a factor of 5. Fluorescence and Fourier Transform Infrared (FTIR) spectra indicate that the LIDT improvement is attributed to the decrease in hydroxyl group's(OH) density in SiO2 thin films resulting from femtosecond laser irradiation.

High laser fluence ITO coatings utilizing a Fabry-Perot thin film filter to reduce effective absorption.

Laser-induced damage threshold of transparent conductors, such as Indium Tin Oxide (ITO), is limited by their high optical absorption due to free carriers. However, the effective absorption of a transparent conductor thin film can be reduced by an order of magnitude, without changing the electrical characteristics of the film, when placed in a low electric field section of a multilayer coating. A Fabry-Perot thin film interference filter has both high transmittance and low electric field positions, so it is an ideal thin film structure for this application. Although Fabry-Perot interference filters are not known as particularly high laser-induced damage resistant coatings due to their resonant characteristics, a laser-induced damage threshold (LIDT) improvement of up to 8× was observed with this technique compared to single layer ITO coatings fabricated using either radio frequency magnetron sputtering or electron-beam deposition. Additionally, an approximately 4× LIDT improvement for a Fabry-Perot interference filter has been observed by the addition of ITO into the multilayer structure.

Laser-induced fluorescence and its effect on the damage resistance of fluoride-containing phosphate-based glasses

A series of multi-component fluoride-containing phosphate-based glasses prepared in a reducing atmosphere showed improved resistance to high-energy ultraviolet (UV) laser-induced damage and strong laser-induced fluorescence (LIF) within the glass bulk. The UV optical absorption, photoluminescence, and fluorescence decay properties of these glasses were investigated to explore the defect-related LIF mechanism and its underlying effect on the glasses’ laser-induced damage threshold (LIDT). Seven laser wavelengths ranging from 253 nm to 532 nm were used to excite the LIF, and two characteristic fluorescence bands peaking at approximately 414 nm and 780–800 nm occurred in all three types of glasses. The LIF band at 414 nm was attributed to PO3-EC defects in the second harmonic frequency (2ω) absorptive glass and third harmonic frequency (3ω) transparent glass, but Fe2+ ions in the fundamental frequency (1ω) absorptive glass. A later fluorescence band at 780–800 nm occurred due to POHC defects in the 2ω absorptive and 3ω transparent glasses and Fe3+ ions in the 1ω absorptive glass. A detailed study on the dynamic decay processes of two additional dominant fluorescence peaks at 450 nm and 780 nm under 266 nm excitation revealed the potential effect of LIF on LIDT improvement. The relatively longer LIF lifetime, higher LIF intensity, and larger LIF peak area corresponded with and contributed to a higher LIDT, especially in the 3ω transparent glass with a low UV absorption coefficient. This study provides strong evidence for the prior hypothesis between strong LIF and LIDT.

Fused silica contamination layer removal using magnetic field‐assisted finishing

AbstractFused silica optics used in lasing systems requires a high laser‐induced damage resistance. Processes typically used to polish fused silica lenses induce subsurface and surface damage that collect ceria abrasive, creating a layer of contamination. The contamination can be a precursor to laser damage during use. A preliminary study showed the feasibility of magnetic field‐assisted finishing (MAF) for polishing fused silica and suggested possible beneficial effects of the MAF‐polished surface on the laser‐induced damage threshold (LIDT). This paper proposes a method to examine the fundamental polishing characteristics of MAF for fused silica. Using the proposed method, this paper explores the material removal characteristics of the MAF process and improves the understanding of the MAF polishing mechanism. The 45% improvement of LIDT shows the efficacy of MAF for removing the contamination layer of fused silica surfaces with minimal changes in the surface roughness.

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.

Detailed near-surface nanoscale damage precursor measurement and characterization of fused silica optics assisted by ion beam etching.

Near-surface nanoscale damage precursor generated from the fabrication process has great influence on laser-induced damage threshold improvement of fused silica. In this work, high-resolution transmission electron microscopy (HRTEM) is used to characterize the arrangement of material particles near surface. The nanoscale defects in the Beilby layer could be clearly distinguished. And we find ion beam etching (IBE) has little effect on the arrangement of material particles. This microscopic phenomenon makes IBE a promising technique for the detection of nanoscale near-surface damage precursors. To further investigate the nanoscale near-surface damage after chemical mechanical polishing, a trench is generated by ion sputtering to contain the nature and characteristics of nanoscale precursors in different depths. The evolutions of chemical structure defects and nanoparticles are measured and their laser-induced absorption performance are tested. The results show that there is a nanoscale defect layer (~360nm) beneath the Beilby layer. A model for nanoscale defect layer of fused silica after CMP is offered. In the model, the quantitative density of nanoparticles falls exponentially with increasing the depth and the contents of ODC and NBOHC decreases linearly, respectively. Research results can be a reference on characterizing nanoscale defects near surface and conducting post-processing technologies to improve the laser damage resistance property of fused silica.

Effect of annealing on the damage threshold and optical properties of HfO2/Ta2O5/SiO2 high-reflection film

The effect of thermal annealing on the optical properties, microstructure, and laser-induced damage threshold (LIDT) of HfO2/Ta2O5/SiO2 HR films has been investigated. The transmission spectra shift to a short wavelength and the X-ray diffraction peaks of monoclinic structure HfO2 are enhanced after thermal annealing. The calculated results of the m(−111) diffraction peak show that the HfO2 grain size is increased, which is conducive to increasing the thermal conductivity. Thermal annealing also reduces the laser absorption of high-reflection films. The improvement of thermal conductivity and the decrease of laser absorption both contribute to the improvement of LIDT. The experimental results show that the highest LIDT of 22.4 J/cm2 is obtained at 300°C annealing temperature. With the further increase of annealing temperature, the damage changes from thermal stress damage to thermal explosion damage, resulting in the decrease of LIDT.

Laser Induced Damage Threshold of Ta2O5 and Ta2O5/SiO2 Films at 532 and 1064 nm

Laser induced damage threshold (LIDT) of conventional electron beam deposited Ta2O5 and SiO2 thin films on BK-7 substrate are studied to see the effect of multilayer boundary and related effect for their combination as high and low index material films when number of layers increases as well as improvement on upper two non-quarter wave thickness layers. Reflection spectra of these samples show a relatively small decrease in reflection in non-quarter wave layer design and Grazing Incidence X-ray Diffraction (GIXRD) patterns show that deposited films are amorphous, having high (4.2 eV) band gap, sufficient high refractive index and very low extinction coefficient or low absorption which are factors favourable for high laser induced damage threshold. LIDT measured for single layer at 1064 nm is higher than that of multilayers and vice-versa at 532 nm wavelength. However, improvement of LIDT with non- quarter wave outer layers is more at 532 nm than 1064 nm.

Investigation of laser-induced damage threshold improvement mechanism during ion beam sputtering of fused silica

In this paper, the influence of ion beam sputtering (IBS) process on laser damage resistance as a function of sputtering depth and typical defects which associated with laser damage performance was investigated. Damage test results reveal that the damage resistance of HF etched surface can be further enhanced about 30% by appropriate IBS removal depth (less than ~1000nm). Within this removal depth, the IBS process can remove the redeposited reaction products during HF acid etching process, improve surface quality and reduce chemical structure defects concentration. However, further ion sputtering often results in a decrease rather than an increase of damage threshold with enhanced surface densification and increased chemical structure defects of ODC and NBOHC which generated from sputtering damage. Moreover, the sputtered surface will accelerate the chemical reaction of surface atoms with water molecules. Thus the newly obtained hydroxylation layer rich in highly absorptive products can result in the decrease of laser damage resistance. The study reveals the improvement mechanism by IBS process, and provides both technical guidance and theoretical basis for the optimization of the post-process of fused silica.

Laser-induced damage of 355 nm high-reflective mirror caused by nanoscale defect

Al2O3/SiO2 multilayer high-reflective (HR) mirrors at 355 nm were prepared by electron beam evaporation, and post-irradiated with Ar/O mixture plasma. The surface defect density, reflective spectra, and laser-induced damage characteristics were measured using optical microscopy, spectrophotometry, a damage testing system, and scanning electron microscopy (SEM), respectively. The results indicated that moderate-time of irradiation enhanced the laser-induced damage threshold (LIDT) of the mirror, but prolonged irradiation produced surface defects, resulting in LIDT degradation. LIDT of the mirrors initially increased and subsequently decreased with the plasma processing time. SEM damage morphologies of the mirrors revealed that nanoscale absorbing defects in sub-layers was one of the key factors limiting the improvement of LIDT in 355 nm HR mirror.

Investigation of surface damage precursor evolutions and laser-induced damage threshold improvement mechanism during Ion beam etching of fused silica.

Surface damage precursor evolution has great influence on laser-induced damage threshold improvement of fused silica surface during Ion beam etching. In this work, a series of ion sputtering experiment are carried out to obtain the evolutions of damage precursors (dot-form microstructures, Polishing-Induced Contamination, Hertz scratches, and roughness). Based on ion sputtering theory, surface damage precursor evolutions are analyzed. The results show that the dot-form microstructures will appear during ion beam etching. But as the ion beam etching depth goes up, the dot-form microstructures can be mitigated. And ion-beam etching can broaden and passivate the Hertz scratches without increasing roughness value. A super-smooth surface (0.238nm RMS) can be obtained finally. The relative content of Fe and Ce impurities both significantly reduce after ion beam etching. The laser-induced damage threshold of fused silica is improved by 34% after ion beam etching for 800nm. Research results can be a reference on using ion beam etching process technology to improve laser-induced damage threshold of fused silica optics.

Annealing effects on microstructure and laser-induced damage threshold of HfO2/SiO2 multilayer mirrors.

HfO2/SiO2 periodic multilayer high reflection mirrors have been prepared by a reactive electron-beam evaporation technique. The deposited mirrors were annealed in the temperature range from 300°C to 500°C. The effects of annealing on optical, microstructural, and laser-induced damage characteristics of the mirrors have been investigated. The high reflection band of the mirror shifts toward a shorter wavelength with increasing annealing temperature. As-deposited and annealed mirrors show polycrystalline structure with a monoclinic phase of HfO2. Crystalinity and grain size increase upon annealing. The laser-induced damage threshold (LIDT) has been assessed using a 532 nm pulsed laser at a pulse width of 7 ns. The LIDT value of the multilayer mirror increases from 44.1 J/cm2 to 77.6 J/cm2 with annealing up to 400°C. The improvement of LIDT with annealing is explained through oxygen vacancy defects as well as grain-size-dependent thermal conductivity. Finally, the observed laser damage morphology, such as circular scalds and ablated multilayer stacks with terrace structure, are analyzed.

Modeling the effect of nanosecond laser conditioning on the femtosecond laser-induced damage of optical films.

The effect of nanosecond laser conditioning on the femtosecond laser-induced damage behaviors of Al2O3, HfO2, SiO2 single layers and Al2O3/SiO2 high reflectors (HR) are explored. During femtosecond laser damage test, negative effects on enhancing the femtosecond laser-induced damage threshold (LIDT) of optical films after the nanosecond laser conditioning is found, which is opposite to the LIDT improvement in the nanosecond range. To explain the mechanism after nanosecond laser conditioning, a theoretical model including multiphoton ionization (MPI), avalanche ionization (AI) and decays of electrons with one defect state is built to simulate the evolution of electron density in the conduction band. A permanent mid-gap defect state resulting from the process of laser conditioning is introduced in our model, which is found to contribute seed electrons to conduction band and hence accelerate the final breakdown. Both the experimental result and theoretical calculation agree very well with each other.

The Optimization of Termination in Combined Technique of Magnetorheological Finishing and HF Etching for Laser-Induced Damage Threshold High-Efficiency Improving of Fused Silica

The developing high-power laser systems are requiring higher laser-induced damage threshold (LIDT) and fabrication efficiency of fused silica optical elements. To solve these problems, MRF polishing and HF etching have been combined utilized wiping and passivating structural defects as well as removing impurities. Furthermore, the LIDT improvement is dependent greatly on the corporation of processes. But the LIDT improving mechanism is partly ambiguous yet, that may lead to random or experimental parameter choice and ultimately generate unsatisfied results. Consequently, this paper focuses on the termination mechanism of the process. Atom Force Microscope (AFM) measurement, finite difference time-domain (FDTD) simulation and LIDT test will be utilized to analysis and validate the optimization theoretically and practically. Finally, in one side, the LIDT value of optimized-terminated sample is 16.7J/cm2, which is about the same level (even 3.7% higher) as that of the over-etched one. In another side, the etching process time could be shorted by 32% using the optimized method.

Combined technique of elastic magnetorheological finishing and HF etching for high-efficiency improving of the laser-induced damage threshold of fused silica optics

The inadequate laser-induced damage threshold (LIDT) of optical elements limits the future development of high-power laser systems. With the aim of raising the LIDT, the elastic passivating treatment mechanism and parameter optimization of a combined magnetorheological finishing (MRF) and HF etching process are investigated. The relationships among the width/depth ratio of defects and parameters of the passivating treatment process (MRF and HF etching), relative intensity (RI), and LIDT of fused silica (FS) optics are revealed through a set of simulations and experiments. For high-efficiency improvement of LIDT, in an elastic passivating treatment process, scratches or other defects need not be wiped off entirely, but only passivated or enlarged to an acceptable profile. This combined process can be applied in polishing high-power-laser-irradiated components with high efficiency, low damage, and high LIDT. A 100 mm×100 mm×10 mm FS optic window is treated, and the width/depth ratio rises from 3 to 11, RI decreases from 4 to 1.2, and LIDT is improved from 7.8 to 17.8 J/cm2 after 385 min of MRF elastic polishing and 60 min of HF etching. Comparing this defect-carrying sample to the defect-free one, the MRF polishing time is shortened, obviously, from 1100 to 385 min, and the LIDT is merely decreased from 19.4 to 17.8 J/cm2. Due to the optimized technique, the fabricating time was shortened by a factor of 2.6, while the LIDT decreased merely 8.2%.

Influence of composition and seed dimension on the structure and laser damage of nodular defects in HfO2/SiO2 high reflectors

Seeds are the sources for forming nodular defects that largely limit the improvement of laser-induced damage threshold of 1ω laser mirrors in the nanosecond pulse regime. To shed more light of the composition and sizes of seeds on the associated structure of nodular defects and laser damage sensitivity, nodular defects were generated in 1064 nm HfO(2)/SiO(2) high reflectors with different sizes of absorbing Au and nonabsorbing SiO(2) nanoparticles located on the surfaces of substrates. The width dimensions, inner structures, and damage morphologies of nodular defects were characterized by an atomic force microscope, a field emission scanning electron microscope, and a focused ion beam. It was found that the composition and size both influenced the structure and the laser damage of nodular defects. The width of nodules from SiO(2) seeds were larger than that formed by the same size of Au seed. A nodule grown from a small seed generally tends to have a continuous and stable boundary. The ejection fluences of nodules generated from different size absorbing Au and nonabsorbing SiO(2) seeds were totally different. The results were interpreted from the aspects of absorption cross sections of seeds and mechanical stability of nodular structures.

Influence of overcoat on laser-induced damage threshold of 532 and 800 nm TiO 2/SiO 2 high reflectors

TiO 2/SiO 2 high reflectors with and without a SiO 2 overcoat are deposited by electron-beam evaporation. The film properties are characterized by visible spectrometry measures, structure analysis, roughness and laser-induced damage threshold (LIDT) tests, surface defects and damage morphology observation. The effects of overcoats on LIDT at 532 nm, 8 ns and 800 nm, 220 ps laser pulses are investigated. The relations between film structure, roughness, surface defects, electric field and LIDT are discussed. It is found that overcoats can increase the LIDT at these two laser wavelengths. The reduction of peak temperature, the low defects density and roughness, the low intrinsic absorption of SiO 2 and its amorphous structure are the main reasons for LIDT improvement by overcoats.

Optical performance and laser induced damage threshold improvement of diffraction gratings used as compressors in ultra high intensity lasers

This paper studies gratings engraved in a multilayer dielectric stack for ultra high intensity laser compressors. A metal layer is inserted between the substrate and the dielectric stack to reduce the number of dielectric bilayers and thus the mechanical stress within the stack. A code taking account the fluctuation range of the geometrical parameters during the fabrication process is used to numerically optimize the mirror stack and study different groove profiles to increase the reflected efficiency and the laser induced damage threshold. It is evidenced that of all the profiles leading to good diffraction performances, those with the greatest groove depth and width values result in the smallest enhancement of the electric field square inside the grating with a decrease by a factor close to 2.5.