Laser-induced Damage Process Research Articles

Femtosecond laser induced damaging inside fused silica detected by a single-pulse ultrafast measurement system.

The dynamics of damage inside the fused silica induced by a femtosecond laser has been characterized by a single-pulse ultrafast measurement system that is built upon the pump-probe mechanism. Our investigation disclosed the quantitative relationship between the size of the damaged area and the pulse energy. The dynamic measurement experiments showed that the radial size of the damaged area increased rapidly from 0 to 21 µm within ∼10 ps before stabilizing at 21 µm with the pulse energy of 1.1 mJ, which follows the rule of Boltzmann function. Moreover, we demonstrated that the structure inside the damaged area kept changing for about 200 ps before the formation of a double-void structure. The developed system alongside the proposed analysis method is expected to be of great importance in understanding the dynamics of laser-induced damage process in laser micromachining.

Three-temperature modeling of laser-induced damage process in silicon

Laser excitation in silicon from femto- to pico-second time scales is studied. We assume the three-temperature model which describes the dynamics of the distinct quasi-temperatures for electrons, holes, and lattice. Numerical results for damage threshold reproduce the experimental results not only quantitatively, but qualitatively as well, showing dependence on laser pulse duration. Comparison with experimental data suggests that electron emission and thermal melting are both responsible for damage in silicon. We found that electron–phonon relaxation time has a significant effect on pulse duration dependence of electron emission.

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.

Millisecond laser-induced damage process of (001) silicon wafer

The damage process of (001) silicon wafer subjected to millisecond Gaussian laser irradiation is investigated. The laser pulse width is 1 ms, and the laser energy density ranges from 18 to 32 J / cm2. The damage mechanism is discussed by combining real-time experimental observation with numerical analysis based on a dislocation model. The first damage is dislocation multiplication, which is induced by the long irradiation time of a millisecond laser. The subsequent melting process begins at slip lines, not the spot center as is usually assumed. Fracture also occurs outside the spot center. Precise temperature in the cooling period cannot be obtained by simulation due to complicated dependence of thermal conductivity on dislocation, which may be the most urgent problem in further studies.

Role of each step in the combined treatment of reactive ion etching and dynamic chemical etching for improving the laser-induced damage resistance of fused silica.

We investigate the role of each step in the combined treatment of reactive ion etching (RIE) and dynamic chemical etching (DCE) for improving the laser-induced damage resistance of fused silica optics. We employ various surface analytical methods to identify the possible damage precursors on fused silica surfaces treated with different processes (RIE, DCE, and their combination). The results show that RIE-induced defects, including F contamination, broken Si-O bonds, luminescence defects (i.e., NBOHCs and ODCs), and material densification, are potential factors that limit the improvement of laser-induced damage resistance of the optics. Although being capable of eliminating the above factors, the DCE treatment can achieve rough optical surface with masses of exposed scratches and pits which might serve as reservoirs of the deposits such as inorganic salts, thus limiting the further improvement in damage resistance of fused silica. The study guides us to a deep understanding of the laser-induced damage process in achieving fused silica optics with enhanced resistance to laser-induced damage by the combined treatment of RIE and DCE.

Different material modifications in laser-induced damage of optical films in air and vacuum environments

A combination study of material modifications for the catastrophic laser-induced damage by 12 ns, 1064 nm laser pulse irradiation was investigated in air and vacuum environments. The samples were observed by optical microscope, scanning electron microscopy, laser micro-Raman spectroscopy, photoluminescence spectrum; moreover 2D numerical calculations were performed. According to our results, vacuum pumping induced higher laser field intensities, accompanying with more absorption of laser energy in the laser-induced damage process. All of these resulted in the material melting and subsequent re-solidification in the laser-damaged core in vacuum. While, in the air conditions, plasma expanding due to material heating and associated thermal expansion induced a large stress field, which caused the material deformation and formation of delamination and crack in the laser-damage region.

Simulation of the nanosecond-pulse laser damage of KDP surface by the smoothed particle hydrodynamics method.

We present a simulation method to reproduce the damage crater formation and particle ejection phenomena observed in the laser-induced surface damage process of potassium dihydrogen phosphate (KDP) crystals. Based on the smoothed particle hydrodynamics method, which is commonly used for solving shock and blast problems, equivalent explosion simulation models of the laser-induced damage process have been established. Moreover, laser damage experiments combined with time-resolved techniques are performed on KDP surfaces to investigate the impact of laser fluences on the shockwave propagation and the particle ejection speed. We find that the simulation models can predict the laser-induced damage behaviors of the KDP crystal, which verifies the validity of the proposed method.

Quasi-particle calculations on electronic and optical properties of the peroxy linkage and neutral oxygen vacancy defects in amorphous silica

Recently, fused silica has been used to prepare the optical windows in the inertial confinement fusion (ICF) equipment. Challenge of application of fused silica is due to the defect-related optical absorption which is considered as the main mechanism of laser-induced damage process. However, due to structural complexity, calculation of the defect-related absorption from the first principles is only limited to small clusters, and a full treatment using the state of art GW and Bathe-Salpeter equation (BSE) method is still lacking.In this work, density functional theory calculations are performed to study the defect structure of the peroxy linkage (POL) and the neutral oxygen vacancy (NOV) defects in amorphous silica. Firstly, well relaxed structure is generated by using a combination of the bond switching Monte Carlo technique and the DFT-based structure optimization. Secondly, the defect structures are generated and studied in both the ground singlet (S0) and the first excited triplet (T1) states. Finally, the electronic and optical properties of the considered structures are studied by applying the self-consistent quasi-particle GW (sc-QPGW) and the BSE methods in Tamm-Dankoff approximation.In the ground state S0, the POL defect is found to be stable and shares a similar local structure to the H2O2 molecule. However, in T1 state, the POL defect breaks into a pair of E' center ( - Si ) and peroxy oxygen radial ( O-O-Si-). For the NOV defect, the optimized Si-Si bond length in the ground state is 2.51 with a variation of 0.1 due to the structural disorder. In comparison to the ground state, the optimized Si-Si bond length in T1 state increases to 3.56 .The scGW/BSE calculation on the defect free structure predicts a quasi particle band gap of 10.1 eV and an optical band gap of 8.0 eV, which are consistent well with the available experimental results. For the POL defect, the scGW/BSE calculation reveals a weak exciton peak at 6.3 eV. Below 6.3 eV, no new exciton peak is found, implying that the experimentally suggested 3.8 eV peak could not be attributed to the POL defect. Calculations of the NOV defect gives a strong and highly polarized optical absorption peak at 7.4 eV which is close to the previous experimental result at 7.6 eV. The structural relaxation induced by NOV also contributes to another absorption peak at 7.8 eV.

Experimental demonstration of laser damage caused by interface coupling effects of substrate surface and coating layers.

The laser damage resistance of the coatings for high-power laser systems depends greatly on the surface quality of the substrate. In our work, experimental approaches were employed to understand the interface coupling effect of the substrate surface and coatings on the laser resistance of the coatings. A 1064nm anti-reflection (AR) coating was deposited by an e-beam coater onto fused silica with and without micro-scale pits (structural defects). The micro-scale pits were precisely fabricated by femtosecond laser processing to prevent the emergence of subsurface cracks. Different deposition temperatures were characterized in order to verify the intensity of the interface coupling effect of the substrate and coating layers. Our experimental results indicate that impurities that are introduced in the finishing process, shifted to the substrate surface, and aggregated during the heating process, play a much more crucial role than structural defects (length: ∼7 μm; width: ∼3 μm; depth: ∼0.8 μm) in the laser-induced damage process. By effectively reducing the intensity of the interface coupling effect, the e-beam AR coatings, whose laser-induced damage resistance was closed to the bare substrate, was prepared.

Thermodynamic damage mechanism of transparent films caused by a low-power laser

A new model for analyzing the laser-induced damage process is provided. In many damage pits, the melted residue can been found. This is evidence of the phase change of materials. Therefore the phase change of materials is incorporated into the mechanical damage mechanism of films. Three sequential stages are discussed: no phase change, liquid phase change, and gas phase change. To study the damage mechanism and process, two kinds of stress have been considered: thermal stress and deformation stress. The former is caused by the temperature gradient and the latter is caused by high-pressure drive deformation. The theory described can determine the size of the damage pit.

High-resolution photothermal microscope: a sensitive tool for the detection of isolated absorbing defects in optical coatings

The photothermal deflection technique allows us to highlight the presence of inhomogeneities of absorption in optical components. This nondestructive tool is of great interest to the study of the role of contaminants, inclusions, and impurities in the laser-induced damage process. We show that the detection of nanometer-sized isolated absorbing defects requires the development of an adapted photothermal setup with high detectivity and high spatial resolution. Thus it is essential to improve the resolving power up to its theoretical limit.

Investigation of blast waves generated by laser induced damage processes

Spherical blast waves generated by laser induced damage processes were studied in the weak shock region using wideband microphone detection. Shock wavefront propagation was examined and a comparison with the predictions of the point explosion model was made. Based on the agreement between the theory and experiment, blast wave energy and energy conversion efficiency were determined.

Laser Damage in Optical Materials

R M Wood 1986 Bristol: Adam Hilger xv + 131 pp price £19.50 (IOP members' price £15.60) ISBN 0 85274 792 6 A useful review of laser-induced damage processes in optical materials, this is very much concerned with the practicalities, drawing on the author's experience in the field over many years and on the proceedings of various Boulder laser damage symposia. By far the most important feature of the book is the unequalled and invaluable compilation of damage thresholds in appendices 1–10.

Highly nonlinear interactions in n-type InSb with a picosecond CO<inf>2</inf>laser

The electron-hole plasma generation mechanism in n-type InSb was studied with picosecond CO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> laser pulses. It was found that at very high intensities the electron ionization can be adequately described by a tunneling mechanism. The relevance of the present study to the understanding of laser-matter interaction at high intensities and the laser-induced damage process in semiconductors are discussed.

Surface and bulk laser-damage statistics and the identification of intrinsic breakdown processes

The experimental evidence presented confirms the existence of a statistical nature in the laser-induced intrinsic damage process both in the bulk and on the surfaces of transparent materials. A damage process determined by an electron-avalanche breakdown with statistical starting properties was found to be consistent with the data. The importance of irradiating only a small volume of the material in order to avoid studying inclusion-induced laser damage is discussed. In addition, a new procedure was developed to permit rapid identification of the occurrence of damage due to an intrinsic process.

Laser-Induced Damage Probability at 106 μm and 069 μm

The first directly comparable measurements of the laser-induced surface damage process at both ruby and Nd:YAG laser wavelengths are reported. The most striking feature of the data is that all the materials studied are harder to damage at 0.69 microm than at 1.06 microm. The probabilistic nature of the laser-induced damage process at 1.06 microm was explored further by measuring the distribution of breakdown starting times with an image-converter streak camera. The observed distribution is described by the compound probability that breakdown occurs at a particular time, given that it has not occurred before that time. In addition, several connections between the probabilistic and thresholdlike interpretations of laser-induced damage are discussed. It is shown that these points of view are not totally incompatible.