Dependence Of Damage Threshold Research Articles

3D nanopolymerization and damage threshold dependence on laser wavelength and pulse duration

Abstract The dependence of the polymerization and optical damage thresholds in multi-photon polymerization (MPP) lithography was studied using a broadly-tunable laser system with group delay dispersion (GDD) control. The order of non-linearity and the light–matter interaction mechanisms were investigated using the resolution bridges method for non-photosensitized SZ2080TM and photosensitized SZ2080TM + IRG369 prepolymers. Energy deposition, voxel dimension growth, and the size of the dynamic fabrication window (DFW) were measured in the 700–1300 nm wavelength range at three different pulse durations measured at the sample – 100, 200 and 300 fs. Polymerization was observed at all wavelengths and pulse durations without significant differences in the achieved minimal spatial dimension ( < 300 ${< } 300$ nm). This was achieved despite the broad range of excitation wavelengths used which spanned two- and three-photon absorption bands, and the differences in the absorption spectra of the prepolymers. The lateral and longitudinal voxel growth dynamics revealed an abrupt change in the power dependence of polymerization and a significant variation of the DFW – from 1 at 1250 nm to 29 at 700 nm. This result can be interpreted as a consequence of a change in the instantaneous refractive index and a lowering of the polymerization but not the damage threshold. The optimization of energy delivery to the material by a wavelength-tunable laser source with pulse duration control was experimentally validated. These findings are uncovering the complexity of polymerization mechanisms and are useful in further development of MPP technology.

Ultrafast multipulse damage threshold of femtosecond high reflectors.

Laser-induced damage threshold is a fundamental figure of merit of femtosecond optical components used in large-scale laser systems. We tested a series of ultrafast mirrors featuring high band-gap dielectric materials as well as improved design and coating techniques. In a broad range of the damage test pulse train involving between 10 and 100,000 pulses (40fs, 800nm), pure dielectric high reflectors exhibit around 1.5 J/cm2 and hybrid Ag-multilayer mirrors can exhibit well above 1.2 J/cm2 damage threshold. In addition, a reference antireflection coating exceeded 2 J/cm2. Damage threshold dependence on the number of pulses was similar for all optics involved.

Retinal thermal damage threshold dependence on exposure duration for the transitional near-infrared laser radiation at 1319 nm.

The retinal damage effects induced by transitional near-infrared (NIR) lasers have been investigated for years. However, the damage threshold dependence on exposure duration has not been revealed. In this paper, the in-vivo retinal damage ED50 thresholds were determined in chinchilla grey rabbits for 1319 nm laser radiation for exposure durations from 0.1 s to 10 s. The incident corneal irradiance diameter was fixed at 5 mm. The ED50 thresholds given in terms of the total intraocular energy (TIE) for exposure durations of 0.1, 1 and 10 s were 1.36, 6.33 and 28.6 J respectively. The ED50 thresholds were correlated by a power law equation, ED50 = 6.31t0.66 [J] where t is time [s], with correlation coefficient R = 0.9999. There exists a sufficient safety margin (factor of 28~60) between the human ED50 thresholds derived from the rabbit and the maximum permissible exposure (MPE) values in the current laser safety standards.

Corneal thermal damage threshold dependence on the exposure duration for near-infrared laser radiation at 1319 nm.

The corneal damage effects induced by 1319-nm transitional near-infrared laser have been inves- tigated for years. However, the damage threshold dependence on exposure duration has not been revealed. The in vivo corneal damage thresholds (ED50s) were determined in New Zealand rabbits for 1319-nm laser radiation for exposure durations from 75 ms to 10 s. An additional corneal ED50 was determined at 1338 nm for a 5-ms exposure. The incident corneal irradiance diameter was fixed at 2 mm for all exposure conditions to avoid the influence of spot size on threshold. The ED50s given in terms of the corneal radiant exposure for exposure dura- tions of 5 ms, 75 ms, 0.35 s, 2 s, and 10 s were 39.4, 51.5, 87.2, 156.3, and 311.1 J∕cm 2 , respectively. The 39.4 J∕cm 2 was derived from the ED50 for 1338 nm (27.0 J∕cm 2 ). The ED50s for exposure durations of 75 ms to 10 s were correlated by a power law equation, ED50 ¼ 128.9t 0.36 in J∕cm 2 , where t was the input in the unit of second, with correlation coefficient (R) of 0.997. Enough safe margins existed between the ED50s and the maxi- mum permitted exposures from current laser safety standard. © 2016 Society of Photo-Optical Instrumentation Engineers (SPIE)

Optical breakdown of multilayer thin-films induced by ultrashort pulses at MHz repetition rates

Multilayer coatings composed of TiO(2), Ta(2)O(5), HfO(2), or Al(2)O(3) as high-index materials and SiO(2) as low-index material were investigated for laser-induced damage using 1 ps, 5 µJ pulses generated by a mode-locked Yb:YAG thin-disk oscillator operating at a wavelength of 1030 nm and repetition rate of 11.5 MHz. Previously reported linear band gap dependence of damage threshold at kHz repetition rates was confirmed also for the MHz regime. Additionally, we studied the effect of electric field distribution inside of the layer stack. We did not observe any significant influence of thermal effects on the laser-induced damage threshold in this regime.

Theory of laser-induced inclusion-initiated damage in optical materials

We present the results of theoretical studies of the laser-induced damage in transparent solids containing absorbing inclusions. The investigation is based on the inclusion-initiated thermal explosion model. Key aspects of the model are considered: thermal instability initiated by the inclusions; a mechanism of photoionization of a surrounding layer of a host material by a thermal ultraviolet-radiation of laser-heated inclusions; the thermal instability kinetics, and an associated pulse-width dependence of the laser-induced damage threshold. Also, statistical features of the damage, caused by random spatial distribution of the inclusions in the materials, and a final stage of the damage process-a mechanical stress-produced crack formation-are analyzed. A comparison of the theoretical results, related to the pulse-width dependence of the damage threshold, with experimental data for some typical optical materials in a wide pulse-width range is presented.

Application of Ductile Fracture Criterion in Hot Forging Damage of Mn18Cr18N Steel

In this paper, the hot forging behavior of Mn18Cr18N steel is studied based on the viewpoint of meso-damage. According to high temperature tensile stress - strain curve of Mn18Cr18N steel under different temperatures, the relationships of void initiation strain and fracture strain with deformation temperature are obtained. The temperature dependence of damage threshold is obtained at 950°C~1200°C for Mn18Cr18N steel by Cockcroft & Latham criterion, That is, Ci,f value increases monotonously with temperature increasing. The corresponding deformation amounts of void initiation and fracture appearing are predicted accurately by combining Cockcroft & Latham criterion with finite element analysis during hot forging of Mn18Cr18N steel, which is in good agreement with the actual production. Under these experimental conditions, once upsetting deformation amount should be controlled in 50% based on hot upsetting test and finite element simulation. These provide theoretical instruction for preventing forging cracking of Mn18Cr18N steel.

Mechanism initiated by nanoabsorber for UV nanosecond-pulse-driven damage of dielectric coatings

A model of plasma formation for UV nanosecond pulse-laser interaction with SiO(2) thin film based on nanoabsorber is proposed. The formalism considered the temperature dependence of band gap. The numerical results show that during the process of nanosecond pulsed-laser interaction with SiO(2) films, foreign inclusion absorbing a fraction of incident radiation heats the surrounding host material through heat conduction causing the decrease of the band gap and making the initial transparent matrix into an absorptive medium around the inclusion. During the remainder pulse, the abosorbing volume of the host material is effectively growed and lead to the formation of the damage craters. We investigated the experimental damage craters and compared with theoretical prediction. The pulselength dependence of damage threshold was also investigated.

Growth and properties of Ln3+ doped PbMeO4 (Me = W, Mo; Ln = Yb, Nd) single crystals

The single crystals of pure, Ln-doped PbMeO4 (Me=Mo, W; Ln=Nd, Yb) and PbMo1−xWxO4 (x=0.2, 0.5, 0.8) solid solutions were grown by the CZ method. The dependence of damage threshold upon activator concentration for pure and doped lead molybdate and tungstate crystals was investigated and discussed The laser pulse radiation damage morphology of PbMoO4:Ln, PbWO4:Ln and PbMo1−xWxO4 crystals was studied.

Mechanisms of femtosecond laser-induced damage in magnesium fluoride

We studied the single-shot damage in magnesium fluoride irradiated by 800 nm femtosecond (fs) laser. The dependence of damage thresholds on the laser pulse durations from 60 to 750 fs was measured. The pump–probe measurements were carried out to investigate the time-resolved electronic excitation processes. A coupled dynamic model was applied to study the microprocesses in the interaction between fs laser and magnesium fluoride. The results indicate that both multiphoton ionization and avalanche ionization play important roles in the femtosecond laser-induced damage in MgF 2.

Mechanism of femtosecond laser-induced damage in magnesium fluoride

Single-shot damage morphology of magnesium fluoride induced by femtosecond laser pulse was studied with scanning electron microscope (SEM). The laser pulses used were 800 nm central wavelength. By the linear relationship between the area of damage spot and the logarithm of the laser energy, we measured the dependence of damage threshold on the pulse duration ranging from 55 femtosecond (fs) to 750 fs. We have calculated the two-photon absorption in the conduction band and improved the multiple-equation model (Phys. Rev. Lett. 92 187401, 2004). The experimental results are well explained by our model.

超短パルスレーザによる材料加工現象 １ 金属を対象とした改良型分子動力学法の開発

Recently, short-pulse lasers have been applied to microprocessing in the field of various industries. It becomes more difficult to experimentally observe the microprocessing phenomena as pulse width becomes short. Numerical analysis with a continuum model has a limit in elucidation of such phenomena, therefore, computer simulations at the atomic or molecular level must be important. The authors have carried out the molecular dynamics simulation for the first time in the field of laser materials processing and have been making fusion and evaporation phenomena clear at the atomic level. In this paper, a modified molecular dynamics method for metal was developed, in which calculation of the molecular dynamics is carried out compensating the heat conduction by free electrons at each time step. Its validity was confirmed by calculating thermal conductivity and thermal diffusivity. Pulse width dependence of damage threshold was also investigated.

超短パルスレーザによるマイクロ加工の分子動力学シミュレーション

Recently, short-pulse lasers have been applied to microprocessing in the field of various industries. It becomes more difficult to experimentally observe the microprocessing phenomena as pulse width becomes short. Numerical analysis with a continuum model has a limit in elucidation of such phenomena, therefore, computer simulations at the atomic or molecular level must be important. The authors have carried out the molecular dynamics simulation of laser materials processing. In this paper, a modified molecular dynamics method for metal developed by the authors, in which calculation of the molecular dynamics is carried out compensating the heat conduction by free electrons at each time step, was applied to elucidate the laser microprocessing phenomena. Pulse width dependence of damage threshold, evaporation process, velocity distribution of evaporation particles and temperature profile in the material were investigated.

Damage threshold dependence on electron–phonon coupling in Au and Ni films

Multi-shot damage thresholds of thin Au and Ni films in the thickness range from 20 nm to 1500 nm for Au and 10 nm to 100 nm for Ni induced by 200-fs pulses at 400 nm have been investigated by light scattering. A dramatic difference in the dependence of damage threshold on film thickness for Au and Ni is observed. Both metals show a linear thickness dependence of the absorbed fluence at the damage threshold for thin films and a saturation as the film thickness increases, but the onset of saturation differs for both metals by one order of magnitude (50 nm for Ni and 500 nm for Au). This behavior can be explained by the different electron–phonon coupling in both metals. By applying the two-temperature model, we show that the energy transport after absorption of a fs-laser pulse is mainly determined by diffusion of hot electrons during the first picoseconds. This is in contrast to the case of ns-laser pulses, where the energy transport is governed by the heat diffusion in the lattice. The thermal diffusion lengths for Au and Ni are similar, whereas the electron diffusion length depends on the strength of the electron–phonon coupling, which differs by two order of magnitude for Au and Ni. This fact is responsible for the different thickness dependence of the ablation threshold for fs-pulses.

Photoacoustic studies of laser damage in oxide thin films

Laser damage thresholds of ZrO2, TiO2 and HfO2 films of optical thickness λ(λ = 248 nm) evaporated on quartz glass substrates were investigated with the photoacoustic probe beam deflection technique in a 1-on-1 irradiation mode for a range of fluences from below to above the thin-film laser damage threshold. In addition, irradiated spots were investigated systematically by a video imaging difference technique. It is demonstrated that the photoacoustic technique allows a more sensitive and precise determination than optical inspection of the onset of damage. An exponential dependence of damage thresholds on the apparent band gap of the respective thin-film material, as determined by optical absorption spectroscopy, was found. Results for amorphous and polycrystalline films of BK7 glass and SQ1 quartz substrates are compared and the influence of a SiO2 protective coating on the laser-damage threshold is investigated.

Excimer laser angioplasty: Tissue ablation, arterial response, and fiber optic delivery

The results of recent work directed towards the application of excimer lasers to angioplasty are presented. Several laser-tissue interactions are examined, including the effect of the XeCl laser optical pulse duration (7-300 ns) on the threshold fluence for ablation of arterial wall and on the quality of the cut in human postmortem artery, the potential for altering the ablation threshold fluence of arterial plaque in swine by treatment with hematoporphyrin derivative, and the healing response of swine arterial wall to surgical irradiation with an argon laser and a pulsed XeCl laser. The dependence of damage thresholds and transmission properties of selected commercial fused silica fibers on the laser pulsewidth and wavelength are determined.

Dependence of Laser-Induced Damage Threshold of Anti-Reflection Coatings on Substrate Surface Roughness

The geometrical shapes of damage sites in 5-layer SiO2/ZrO2 anti-reflection (AR) coatings generated by 1 µm laser irradiation have been accurately determined. The damage can be classified into two types: deep damage and shallow damage, the former having a lower threshold. The dependence of the damage threshold of deep damage on the substrate surface roughness is given. It is suggested that the presence of absorption centers embedded in the pores or scratches on the surface of the substrate is responsible for determining the damage threshold. Possible approaches that will lead to stronger AR coatings are proposed.

Angular dependence of the damage threshold of thin metal films

An experimental investigation was made of the angular dependence of the laser damage threshold of thin metal films. It was found that in the case of the p polarization of the incident radiation there was a considerable reduction in the damage threshold of silver, copper, and aluminum films when the angles of incidence corresponded to efficient excitation of surface polaritons.

Implications of the corneal temperature range in the prediction of laser thermal damage.

Corneal temperatures of the rhesus monkeys have been measured under conditions that may exist during laser experiments. The minimum and maximum temperatures found for all experimental conditions were 29.54 degrees C and 39.16 degrees C, respectively, a range of 9.62 degrees C. A computer model of thermal damage due to laser irradiation was used to determine the effect varying initial temperature could have on corneal damage thresholds (ED50's). The range of 9.62 degrees C found in monkeys for all experimental conditions corresponded to a 39% difference in threshold power. The dependence of damage thresholds on initial temperature could be an important factor to consider when basing laser safety standards on damage threshold data.

Laser induced damage in optical materials: 7th ASTM symposium

The Seventh ERDA-ASTM-ONR-NBS Symposium on Laser Induced Damage in Optical Materials was held at the National Bureau of Standards in Boulder, Colorado, on 29-31 July 1975. These Symposia are held as part of the activities in ASTM Subcommittee II on Lasers and Laser Materials, which is charged with the responsibilities of formulating standards and test procedures for laser materials, components, and devices. The Chairman of Subcommittee II is Haynes Lee, of Owens-Illinois, Inc. Co-chairmen for the Damage Symposia are Arthur Guenther of the Air Force Weapons Laboratory and Alexander J. Glass of Law-rence Livermore Laboratory. Over 150 attendees at the Symposium heard forty-five papers on topics relating fabrication procedures to laser induced damage in optical materials; on metal mirrors; in ir window materials; the multipulse, wavelength, and pulse length dependence of damage thresholds; damage in dielectric films and at exposed surfaces; as well as theoretical discussions on avalanche ionization and multiphoton processes of importance at shorter wavelengths. Of particular importance were the scaling relations developed from several parametric studies relating fundamental properties (refractive index, surface roughness etc.) to the damage threshold. This year many of the extrinsic influences tending to reduce a materials damage resistance were isolated such that measures of their egregious nature could be quantified. Much still needs to be accomplished to improve processing and fabrication procedures to allow a measurable approach to a materials intrinsic strength to be demonstrated.