Optical Breakdown Research Articles

Femtosecond laser induced step-like structures inside transparent hydrogel due to laser induced threshold reduction.

In the area of laser material processing, versatile applications for cutting glasses and transparent polymers exist. However, parasitic effects such as the creation of step-like structures appear when laser cutting inside a transparent material. To date, these structures were only described empirically. This work establishes the physical and chemical mechanisms behind the observed effects and describes the influence of process and material parameters onto the creation of step-like structures in hydrogel, Dihydroxyethylmethacrylat (HEMA). By focusing laser pulses in HEMA, reduced pulse separation distance below 50 nm and rise in pulse energy enhances the creation of unintended step-like structures. Spatial resolved Raman-spectroscopy was used to measure the laser induced chemical modification, which results into a reduced breakdown threshold. The reduction in threshold influences the position of optical breakdown for the succeeding laser pulses and consequently leads to the step-like structures. Additionally, the experimental findings were supplemented with numerical simulations of the influence of reduced damage threshold onto the position of optical breakdown.In summary, chemical material change was defined as cause of the step-like structures. Furthermore, the parameters to avoid these structures were identified.

Frequency dependence of nonlinear absorption in fused silica measured with picosecond pulses at various focal lengths.

Nonlinear absorption is the key process to generate laser-induced in-volume modifications in transparent dielectrics such as waveguides or three-dimensional data matrix codes. We present a comprehensive parameter study about nonlinear absorption in fused silica using a picosecond laser at various focal lengths. Beginning at a focal length of 100 mm, we measure a strong frequency dependence of the saturation absorption. Reducing the focal length results in a decrease of the saturation absorption. After passing a threshold focal length, the saturation absorption increases drastically and the frequency dependence starts to decrease. At the final focal length of 6 mm we measure almost no frequency dependence. In order to explain our measurements, we used the theory of optical breakdown and filamentation. Nonlinear absorption measurement can become a promising tool for better process control during the generation of in-volume modifications in transparent dielectrics.

Ionization-field instability in the laser-induced breakdown of nanoporous dielectric

The theory of ionization-field instability in a continuous homogeneous medium is generalized to the heterogeneous nanoporous one with the purpose to clear and analyze the mechanism of nanograting formation during the optical breakdown process in a transparent dielectric (fused silica) containing multiple small inclusions (nanobubbles) with a lowered ionization threshold. Based on the Maxwell Garnett approximation extended to include the size effect caused by the finite ratio of the bubble sizes to the scale of unstable perturbations, we have obtained the equation system describing the spatiotemporal evolution of the plasma density, average field, and effective dielectric permittivity and have derived the characteristic equation connecting the temporal growth rate of these perturbations with their spatial period. Analysis of the roots of this equation shows that the unstable periodic perturbation structure having the maximal growth rate is close in character to the nanogratings observed experimentally (modulation in the direction of the pump wave polarization with the period approximately equal to the half-wavelength in the host material).

Efficacy and Safety Evaluation of Picosecond Alexandrite Laser with a Diffractive Lens Array for Treatment of Melasma in Asian Patients by VISIA Imaging System.

Objective: To evaluate the efficacy and safety of picosecond (ps) 755-nm alexandrite laser with a diffractive lens array (DLA) generating laser-induced optical breakdown, which may be beneficial for melasma treatment. Background: Melasma is notorious for difficult to treat with any modality setting. Recently, picosecond alexandrite laser with DLA seems promising for dealing with it without intolerable complications. Methods: Twenty (N = 20) Asian female melasma patients with Fitzpatrick skin type IV were recruited for 3 treatment sessions of picosecond 755-nm alexandrite laser with DLA at a 4- to 6-week interval. The pulse duration was 750 ps. An 8-mm spot size and the fluence of 0.4 J/cm2 was used over the target area with 2 passes per treatment area and around 2000-2500 passes in total. The repetition rate was 10 Hz. Melasma Area and Severity Index (MASI) score and VISIA® imaging system analysis were utilized for evaluation before treatment and 4 weeks after the completion of the third treatment session. The clinical improvement and adverse events were assessed by the physicians and patients, respectively. Results: The median age of the patients was 45 years (from 27 to 55 years). In the physicians' evaluation, 40% (n = 8) of patients showed good improvement and 40% (n = 8) of patients showed moderate improvement. The mean MASI score before and after laser therapy showed significant improvement from 9.0 ± 4.8 to 6.5 ± 3.7 (p < 0.001). VISIA analysis of the forehead presented significant improvement in spots (p = 0.007) and porphyrins (p = 0.032). Some patients experienced erythema (25%), pruritus (20%), and scaling (20%) but subsided within few days of using emollients and sunscreen. Only 5% (n = 1) of patients developed mild postinflammatory hyperpigmentation, which also subsided in 3 weeks. Conclusions: Three sessions of picosecond 755-nm alexandrite laser with a DLA were effective for melasma treatment in Asian patients with minimal side effects.

Broadband leaky Lamb waves excited by optical breakdown in water.

Optical breakdown of water is used as a sound source to excite a broadband set of leaky Lamb waves in submerged aluminum plates. The source is shown to simultaneously excite guided modes spanning 0.1-5 MHz in frequency and 0-0.8 mm-1 in wavenumber. The measured response overlaps well with dispersion curves for Lamb waves in the plates, revealing strong coupling to both symmetric and antisymmetric modes. The strongest responses arise when a mode's phase velocity approximately equals the plate's compressional wave velocity. These results are shown to arise from an interplay of the sensing geometry, guided wave speeds, and signal processing. Finally, implications for non-contact sensing are discussed.

Kinetic Control of [AuCl4]- Photochemical Reduction and Gold Nanoparticle Size with Hydroxyl Radical Scavengers.

Laser-induced photochemical reduction of aqueous [AuCl4]- is a green synthesis approach requiring no chemical reducing agents or stabilizers; but size control over the resulting gold nanoparticles remains a challenge. Under optical breakdown conditions producing hydrated electrons (eaq-) and hydroxyl radicals (OH•) through decomposition of water, [AuCl4]- reduction kinetics follow an autocatalytic rate law, which is governed by rate constants: nucleation rate k1, dependent on eaq-; and growth rate k2, dependent on the OH• recombination product, H2O2. In this work, we add the hydroxyl radical scavengers isopropyl alcohol and sodium acetate to limit H2O2 formation. Higher scavenger concentrations both lowered k2 values and produced smaller gold nanoparticles with Gaussian size distributions and remarkably narrow mass-weighted size distributions. With sufficiently high scavenger concentrations, the mean nanoparticle size could be tuned from 3.8 to 6.1 nm with polydispersity indices below 0.08. Both the higher surface area-normalized catalytic activity of the gold nanoparticles synthesized in the presence of scavengers, and FTIR measurements, indicate no capping ligands on the nanoparticle surfaces. These results demonstrate that the size distributions of "naked" gold nanoparticles produced by photochemical [AuCl4]- reduction can be effectively tuned by controlling the reaction kinetics.

Shock wave emission and cavitation bubble dynamics by femtosecond optical breakdown in polymer solutions.

Shock wave emission and the dynamics of micrometer-sized cavitation bubbles generated after femtosecond optical breakdown in viscoelastic fluids were investigated experimentally using high-speed photography with 5 million frames/s and acoustic measurements. The viscoelastic fluids consisted in a 0.5% polyacrylamide solution, with a strong elastic component, and a 0.5% carboxymethylcelullose, with a weak elastic component. Breakdown in water and both polymer solutions generated a purely compressive pressure wave. The maximum amplitude and the duration of the breakdown shock wave as well as the maximum bubble radius were not affected by the polymer additives. The notable influence of the polymer additives was found during the collapse phase of the bubble and is manifested by a prolongation of the oscillation time of the bubble and a reduction of the maximum pressure of the shock wave emitted during bubble collapse. A sizeable attenuation of the bubble collapse was found in the elastic polyacrylamide solution. The present results are consistent with an interpretation which invokes the effect of enhanced levels of uniaxial extensional viscosity on the collapse of micrometer-sized cavitation bubbles. The consequences for cavitation in blood are also discussed.

Femtosecond laser micromachining in ophthalmic hydrogels: spectroscopic study of materials effects

Femtosecond laser micromachining relies on tightly focused, ultrashort laser pulses to locally modify material properties through nonlinear absorption, and is finding applications in the field of vision correction. Here we study the material effects of femtosecond laser micromachining in hydrogels to understand the mechanisms of laser-induced refractive index (RI) changes. Single layer dense line patterns were inscribed successively into the middle of hydrogels using a 405 nm femtosecond laser. A maximum phase change of ∼1.2 waves could be obtained at 543 nm with only 60 mW beam intensity at the focal volume. The phase change profile, fitted to a photochemical scaling model from earlier study, indicated that the micromachining process was mainly dominated by two-photon absorption. A confocal micro-Raman system was custom-designed to quantify the structural changes in written regions, especially the local water content. Change in the local water content exhibited three distinctive stages as a function of beam intensity. Below the optical breakdown threshold, a significant increase of local water content within the written layer was observed, while all Raman signatures remained the same. We posited that the negative RI changes were likely due to the generation of free radicals, followed by water permeation in the modified volume. This increase of local water content, likely presented as free water, was further confirmed by thermogravimetric analysis. Fourier-transform infrared spectroscopy was also used to gain insights into the chemical changes in the depolymerization stage.

Evaluation of laser-induced plasma ablation focusing on the difference in pulse duration.

A pulsed laser cause vaporization of tissue by plasma if a laser can provide high-density energy within a very short pulse duration. Such phenomena are called laser-induced plasma ablation. The influence of the laser-induced plasma ablation for tissue is unclear because the ablation mechanism is differing regardless of two lasers provide almost the same power density. The two kinds of lasers' vaporization mechanism (Nanosecond laser output could cause an optical breakdown in the air depending on power density and pulse duration of the laser and Femtosecond laser output could cause a breakdown only on solids surface since pulse irradiation time is shorter than energy transfer time) are evaluated by using thermal damage and destruction of tissue. The experimental results show that nanosecond laser caused vaporization without thermal damage and destruction at the tissue approximant 300 μm away from the ablation area. The pulsed laser which has high power density and longer pulse duration than energy transfer time is suitable for plasma ablation not depending on thermal process.

Time-resolved plasma spectroscopy of diatomic molecular cyanide

In this work, measurements of cyanide (CN) molecular spectra following laser-induced optical breakdown are communicated. The recorded time-resolved data are Abel inverted to determine the spatial distributions of the maximum CN signals and to evaluate molecular excitation temperatures. The experiments are conducted in a 1:1 CO2:N2 gas mixture contained in a cell kept at atmospheric pressure, and the micro-plasma is generated using 150 mJ, 6 ns Q-switched Nd:YAG laser radiation. The optical emissions from the micro-plasma are dispersed using a 0.64 m spectrometer and are recorded along the wavelength and slit dimensions using an intensified charge-coupled device.

Measurements of Gaseous Hydrogen–Nitrogen Laser-Plasma

This work communicates laser-plasma experiments in a gaseous mixture of hydrogen and nitrogen. Time-resolved spectroscopy measures the first four Balmer-series hydrogen lines together with selected neutral and ionized nitrogen lines. Optical breakdown plasma is generated in a 1:1 hydrogen:nitrogen mixture at ambient temperature and 0.27-atm pressure. Time-resolved spectroscopy records emitted radiation with spatial resolution along the slit height for the H α , H β , H γ , and H δ lines. For 13 selected time delays from 0.25 μ s to 3.25 μ s and 0.025 μ s gate-widths, micro-plasma diagnostics is evaluated. Of interest are the peak separation and width of H δ and width of H γ for electron densities in the range of 0.1 to 1.0 × 10 17 cm - 3 , and comparisons with H β and H α diagnostics. Integral inversions interrogate spatial distributions of the plasma expansion. Applications include laboratory and stellar astrophysics plasma diagnosis.

A simple and efficient centrifugation filtration method for bacterial concentration and isolation prior to testing liquid specimens with laser-induced breakdown spectroscopy

An inexpensive filtration device was designed and constructed to rapidly concentrate bacteria in a liquid suspension on the surface of a disposable filter medium while at the same time separating the bacterial cells from larger contaminants in the suspension on the basis of their size. The device consists of a two-stage insert that is held rigidly in a standard tube during bacterial suspension centrifugation. The filters can be easily removed from the insert for subsequent testing with laser-induced breakdown spectroscopy in a process that takes only 3 min. Filter media of 0.45 μm pore size was found to capture approximately 90–95% of the cells in suspension. A limit of detection of 90,000 cells per laser shot was calculated by constructing a calibration curve from multiple suspensions of varying concentration. Deposition of the bacterial lawn across the surface of the 9.5 mm diameter filters was found to be uniform to within ±20% of the mean as determined by the total measured optical breakdown emission. Use of 5 μm pore size filter media in the first stage of the insert was found to remove close to 100% of a 12 μm grain size tungsten contamination from the suspension while removing 10% of the bacterial cells. This mounting protocol provides a very convenient method for sample preparation that makes use of common techniques, apparatus, and procedures that would be familiar to clinicians or microbiological pathology laboratory personnel.

Gas dynamics of laser ignition in butane-air combustible mixtures

The advantages of laser ignition compared to traditional electric spark are in optical breakdown threshold, being inversely proportional to pressure up to hundreds of bars; and in the possibility to reduce harmful emissions (NOx, CH, etc.) at fuel lean mixtures ignition. Laser-induced combustion waves are known to appear faster and are more stable than when induced by electric spark, so can also be used for ramjets, rocket thrusters, and gas turbines. Laser ignition occurs as a result of gas breakdown, formation of plasma and shock waves leading to the combustion core formation. For butane-air mixtures (p∼1÷3 bar) of different compositions (φ∼0.8÷1.4), the dynamics of the combustion core and shock wave propagation induced by laser ignition (1064 nm, 12 ns) with 200 mJ pulses was studied. Using schlieren photography, propagation velocities of shock wave front and combustion core boundary were measured within 0.2÷4 µs after exposure.

Optical breakdown threshold of silver halide polycrystalline structures acted on by the pulsed radiation of a Ho:YAG laser

The optical breakdown thresholds are determined for 100-μm-thick polycrystalline plates made from AgBr0.99I0.01, Ag0.95Tl0.05Br0.95I0.05, and AgBr0.75Cl0.25 when they are irradiated by a Ho:YAG laser at 2.096 μm (pulse-repetition rate 10 kHz and pulse width 25 ns). The dependences of the transmittances of these samples on the mean power of the laser radiation are obtained. It is shown that the optical breakdown threshold depends on the transmittance of the material. The mean optical breakdown threshold of the samples lies in the region from 0.89±0.09 to 1.13±0.09 J/cm2.

Nano-particle enhancement of diagnosis with Laser-Induced plasma spectroscopy

This work discusses nano-particle augmentation of signals in laser-induced plasma spectroscopy. Radiation from a Nd: YAG laser device generates optical breakdown at nano-structured target materials. Extensive scientific investigations of different types of nanomaterial explore dependencies on nanoparticle size, laser wavelength and fluence, and time delay to obtain stronger signals from nano-material than from corresponding solid bulk matter. Modelling of the measurements interprets the occurrence of enhanced detectability of atomic species. Opportunities present themselves for biological and spectrochemical synthesis.

On the role of the pre-ionization mechanism in the optical breakdown of molecular oxygen induced by CO2 laser: Numerical investigation

A numerical study is presented to investigate the threshold intensity dependence on the gas pressure in the breakdown of molecular oxygen induced by CO2 laser radiation with a wavelength of 10.591 μm and a pulse FWHM of 64 ns [Camacho et al., J. Phys. D: Appl. Phys. 41, 105201 (2008)]. This experiment allowed for a new method of providing an adequate density of the seed electrons required to ignite the breakdown mechanism. The investigations are based on a modification of a previously developed model [Gamal and Omar, Radiat. Phys. Chem. 62(5), 361–370 (2001)], which solves a differential equation and designates the time evolution of the electron energy distribution numerically and a set of rate equations that describe the change of the excited state population. The model considered inverse bremsstrahlung absorption as the main electron energy gain process leading to oxygen breakdown. As an interesting finding, in comparing the calculated and measured thresholds as a function of gas pressure, computations could precisely reveal the densities of the created seed electrons at each value of the tested pressures. The unsystematic variation of these densities with the gas pressure clarified the origin of the slight oscillations observed in the measured thresholds. Analyzing the electron energy distribution function and its parameters in three gas pressure regions could determine the correlation between the gas pressure and the electron gain and loss processes responsible for oxygen breakdown. This analysis explained the violation from the simple p−1 law observed experimentally in the relation between threshold intensity and gas pressure.

Machine learning-assisted early ignition prediction in a complex flow

Machine learning methods are used to improve the efficiency by which turbulence-resolved simulations predict whether a hydrogen jet in air crossflow will successfully ignite. The flush-mounted jet issues perpendicularly from the wall of a low-speed wind tunnel into a turbulent boundary layer wherein a laser-induced optical breakdown (LIB) hotspot is deposited. A detailed hydrogen chemical mechanism is used to model the radicals and any subsequent chemical reactions. A dielectric-barrier discharge actuator generates body forces and hydrogen radicals near the jet orifice. We focus on the success or not of the ignition based on LIB location. A challenge is that definitive determination of this requires long simulations, up to 440 µs after the LIB deposition. This is particularly expensive since multiple simulations are required to find the threshold. To reduce the computational effort, three short-time (91 µs) criteria are proposed, evaluated, and compared: a constructed criterion based on detailed observations of radicals near the stoichiometric surface and two machine learning approaches, each trained on 38 realizations. The constructed criterion provides a low-cost estimate of the ignition boundary that is unambiguous in 45 of the 50 training and test trials, so only 10% of them would need to be simulated longer. The trained neural networks correctly predict outcomes in all cases evaluated, with the more automated procedure—a convolutional neural network (CNN) trained on two-dimensional images—providing the most definitive outcome prediction. From the CNN, a sensitivity analysis is used to determine which kernel features from the two-dimensional input data, as defined by intermediate-layer network weights, are important for identifying ignition.

Destructive mechanisms in laser induced forward transfer

Laser Induced Forward Transfer (LIFT) is an additive direct-writing technique, in which a piece of material (ink) is transferred from a donor to a receiver surface, utilizing a laser impulse. In practice, the process of jet formation can suffer from irreproducibility. We identify two possible destructive mechanisms due to multiple optical breakdowns (originating from imperfections of the optical system) and rarefaction waves (originating from impurities), both with harmful consequences caused by cavitation. Based on experiments in a model system that allows for visualization and numerical simulations employing the boundary integral method, we reveal the underlying fluid dynamics of both mechanisms. Finally, to overcome the irreproducibility, we provide recommendations for the industrial use of LIFT.

Condensation of laser‐produced gold plasma during expansion and cooling in a water environment

Physical processes involved in laser ablation in liquid (LAL) are studied using a gold target irradiated through transparent water. During and after irradiation, the heated material from the surface of a target produces a plume that expands into liquid‐forming nanoparticles (NPs). The LAL method of NP production is ecologically much cleaner than others. A better understanding of the processes associated with complicated hydrodynamic phenomena leading to LAL is important for controlled manufacturing. We consider laser pulses with different durations τL covering fifth orders of magnitudes ranging from 0.1 ps to 0.5 ns and large absorbed fluences Fabs near optical breakdown of liquid. It is shown that the trajectory of the contact boundary with liquid at the middle and late stages after passing the maximum intensity of the longest pulse is rather similar for very different pulse durations if energies Fabs are comparable. We trace how hot (in a few eV range) dense gold plasma expands, cools down, intersects a saturation curve, and condenses into NPs appearing first inside the water‐gold diffusively mixed intermediate layer where gold vapour has the lowest temperature. Later, the pressure around the gold‐water contact drops down below the critical pressure for water. As a result, the nanoparticles find themselves in gaseous water bubble where density of water gradually decreases to 10−4 − 10−5 g/cm3 at maximum bubble expansion.

A novel method of triggering fiber fuse inside glass by optical breakdown and glass drilling as its application

Glass is a light transmissive material at room temperature. Meanwhile, glass absorbs light under high temperature. The authors have researched processing bulk glass by utilizing this characteristic. A phenomenon called fiber fuse is usually recognized as an unwelcome phenomenon that occurs in an optical fiber, making it unworkable. However, the authors previously developed a glass processing method that purposely induces the fiber fuse phenomenon in bulk glass for drilling purposes by laser irradiation of a metal foil placed on the surface of the glass. This paper introduces an alternate, novel method to trigger fiber fuse inside bulk glass without requiring the metal foil, in which optical breakdown by pulsed laser irradiation induces the fiber fuse. As a preliminary step, the optical breakdown process with pulsed laser was determined. High-speed camera observation clarified that a breakdown occurred in the vicinity of the focal point and that the generated emission disappeared within 100 μs. In the case in which both pulsed and CW lasers were applied, the emission induced by the pulsed laser did not disappear but rather extended toward the light source. In addition, numerical analyses revealed that CW laser irradiation in the high-temperature region induced by the pulsed laser caused a temperature increase in the region. The high-temperature region extended toward the light source. In addition, blind-hole drilling has been demonstrated. Although the metal foil method can be used for glass drilling, applying the novel method provides two advantages: metal foil is not required and a shorter modified area remains at the bottom of the hole.