Laser-induced Optical Breakdown Research Articles

Laser processing of dielectrics using spatiotemporally tuned ultrashort pulses

The authors report on the theoretical and experimental studies of laser-induced optical breakdown on the surface of fused silica to elucidate the influence of time delay and spatial separation between two ultrashort pulses on the position and size of the modification. Carriers involved in the damage formation including free electrons in the conduction band and self-trapped excitons (STEs) are investigated. The relationship between damage morphology and time delay shows that the seeding carriers (free electrons and STEs) generated from the first pulse are found to play a significant role for the second pulse—which is temporally and spatially separated from the first pulse—in creating the critical electron density needed for an optical breakdown. Consequently, processing outcomes, such as accuracy (position of the hole) and resolution (size of the hole), depend on the interplay of various laser-induced physics that can be tailored for specific goals. As a demonstration, laser lithography with resolution below the diffraction limit is achieved by exploiting multipulse induced physics. This work is a step toward repeatable laser processing of dielectrics beyond the diffraction limit and provides insights into ultrafast laser-matter interaction under the condition of an extremely high pulse repetition rate.

Photoaging Reversibility in Asian Patients With Melasma Treated Using Picosecond Lasers With a Diffractive Lens Array: A 1-Year Prospective Observational Cohort Study.

Picosecond lasers appear to be effective and safe in treating pigmentation and photoaging disorders through laser-induced optical breakdown. To analyze the feasibility of photorejuvenation using picosecond lasers with diffractive lens array (DLA) in patients with melasma. Ten Asian (N = 10) women with melasma and Fitzpatrick skin Type IV were enrolled and treated using 755-nm picosecond alexandrite lasers with DLA. All individuals were assessed before treatment, and at 12, 20 weeks, and 1 year by post-hoc test on melasma area and severity index (MASI) and with VISIA Complexion Analysis System using percentile rank for measurement. The median participant age was 46.5 years. The average MASI continually and significantly (p < .05) decreased until the 1-year follow-up, with the photoaging characteristics, such as wrinkles and red areas improving simultaneously (p < .05). Spots, texture, pores, ultraviolet (UV) spots, brown spots, and porphyrins exhibited alleviation, but this improvement relapsed by the 1-year follow-up. No postinflammatory hyperpigmentation or hypopigmentation occurred. In patients with melasma, picosecond laser treatment with DLA may alleviate pigmentation disorder and the related photoaging characteristics (e.g., wrinkled skin and increased vascularity), and the effects may be maintained for a long time. Nevertheless, post-treatment clinical visits every 3 to 6 months are recommended.

Hydrodynamic ejection caused by laser-induced optical breakdown

A focused laser can cause local optical breakdown of a gas, which leads to rapid deposition of energy into a high-temperature plasma kernel that expands and induces a complex flow. For some conditions, hot gas is rapidly ejected along the laser axis up to distances several times the kernel size, with a particularly curious feature: relatively small changes in, for example, initial pressure can cause the direction of this ejection to reverse. Detailed axisymmetric simulations of a model energy kernel in an inert gas provide a hydrodynamic description of this phenomenon, reproducing key observations in corresponding experiments, including the vortex-ring-like features that constitute the ejection. These simulations are analysed to show how changes in the early-time kernel can lead to ejection or its reversal via alteration in the relative strength and position of the vorticity produced. A corresponding semi-infinite geometry is used to isolate two mechanisms: vorticity production by the generated shock and by baroclinic torque at the kernel boundary. Dependence on the initial kernel asymmetry is quantified, as it ultimately determines whether the vorticity, upon its subsequent evolution, develops into the ring-like structure that ejects. Even simple elongation of the energy kernel alone can reverse the direction.

Some problems of modeling laser-induced filaments

Experimental data of formation laser-induced filaments in various media (potassium chloride, water, air and silicon carbide) are represented. These phenomena are analyzed as processes of Nonlinear and Relaxed Optics. Problems of modeling the creation the volume laser-induced filaments are investigated. Comparative analysis of plasma, nonlinear optical, diffractive and interference phenomena (including diffractive stratification), shocking processes (including Cherenkov radiation) and physical-chemical processes (including cascade model of excitation the proper chemical bonds in the regime of saturation the excitation), methods and models are represented and discussed. The optical breakdown for various matters has various natures: from shock ionization of gas to disruption of all chemical bonds for solid in the region of interaction light and matter or from nonequilibrium radiated processes in gas and liquid to irreversible phase transformations in solid. For diffraction stratification the modified models of Rayleygh rings was used. We show that this model allow to explain the experimental data for silicon carbide more effectively as Lugovoy-Prokhorov theory of moving foci. Modified Niels and Aage Bohrs models (microscopic) and Golub model (macroscopic) of Cherenkov radiation were used for the explanation of generation continuous radiation. Diffraction stratification shows the surface conic nature of Cherenkov radiation. It was show that physical-chemical method of estimations of corresponding processes is more general as electromagnetic (Kerr media) and one allow explain basic terms of resulting chain process with united point of view. Modified I. Frank model of interference the Cherenkov radiation was used for the explanation laser-induced optical breakdown in silicon carbide. Modified Rayleygh model and methods of continuum mechanics was created and used for the estimation sizes and form of observing nanovoids of silicon carbide. In whole the represented models allow to explain the corresponding chain more fuller and really as other models because one take into account of nonlinear optical transformation of primary laser radiation.

A computational model for nanosecond pulse laser-plasma interactions

A multi-physics numerical model for laser-induced optical breakdown and laser-plasma interaction in a non-equilibrium gas is presented, accounting for: production of priming electrons via multi-photon ionization, energy absorption, cascade ionization, induced hydrodynamic response, and shock formation and propagation. The gas is governed by the Navier-Stokes equations, with non-equilibrium effects taken into account by means of a two-temperature model. The space-time dependence of the laser beam is modeled with a flux-tube formulation for the Radiative Transfer Equation. The flow governing equations are discretized in space using a second-order finite volume method. The semi-discrete equations are marched in time using an implicit-explicit (IMEX) dual time-stepping strategy, where diffusion and chemistry are solved implicitly, whereas convection is explicit. Application to a 20 ns long 50 mJ pulse laser-induced breakdown in quiescent O2 shows the advantages of this temporal discretization during and just after the laser pulse, while a less-expensive symmetric Strang splitting (with implicit chemistry) is sufficient for the post-breakdown gas dynamics after ≃ 0.1 μs. The integrated model is shown to reproduce key features of corresponding experiments.

Plasmon enhanced low frequency stimulated Raman scattering in water due to optical breakdown in gas nanobubbles

In the current study, we evaluated the effect of presence of gas nanobubbles in water samples on nonstationary stimulated Raman scattering (SRS) of laser pulses and laser-induced optical breakdown. The SRS excitation threshold was higher and the optical breakdown threshold was lower in nanobubble-containing water compared to nanobubble-free water. The spectrum of the SRS signal from nanobubble-free water samples was found to differ from that of nanobubble-containing water samples. The SRS spectrum of water without nanobubbles contained only a narrow peak with a maximum near 3410 cm−1, which can be associated with stretching vibrations of water molecules. However, in water with nanobubbles an additional SRS signal in the frequency range of 500–1000 cm−1 with a maximum at 700 cm−1 appeared at certain pump intensities. The intensity of this signal significantly exceeded that with the maximum at 3410 cm−1. To interpret these results, we put forward a hypothesis that the detected effect is related to the huge enhancement caused by the generation of a resonance surface plasmon inside a gas nanobubble during optical breakdown. Under these circumstances, the frequency of plasmon resonance coincides with the Stokes shift frequency, and the plasmon is excited due to the beating of the pump wave and the Stokes wave. An expression for the plasmon resonance frequency as a function of the nanobubble radius was obtained.

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.

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.

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.

Physical Origin of Early Failure for Contaminated Optics

Laser-Induced optical breakdown often occurs unexpectedly at optical intensities far lower than those predicted by ultra-short pulse laser experiments, and is usually attributed to contamination. To determine the physical mechanism, optical coatings were contaminated with carbon and steel microparticles and stressed using a 17 kW continuous-wave laser. Breakdown occurred at intensity levels many orders of magnitude lower than expected in clean, pristine materials. Damage thresholds were found to strongly follow the bandgap energy of the film. A thermal model incorporating the particle absorption, interface heat transfer, and free carrier absorption was developed, and it explains the observed data, indicating that surface contamination heated by the laser thermally generates free carriers in the films. The observed bandgap dependence is in direct contrast to the behavior observed for clean samples under continuous wave and long-pulse illumination, and, unexpectedly, has similarities to ultra-short pulse breakdown for clean samples, albeit with a substantially different physical mechanism.

Measurement of Electron Density from Stark-Broadened Spectral Lines Appearing in Silver Nanomaterial Plasma

This work communicates results from optical emission spectroscopy following laser-induced optical breakdown at or near nanomaterial. Selected atomic lines of silver are evaluated for a consistent determination of electron density. Comparisons are presented with Balmer series hydrogen results. Measurements free of self-absorption effects are of particular interest. For several silver lines, asymmetries are observed in the recorded line profiles. Electron densities of interest range from 0.5 to 3 × 1017 cm−3 for five nanosecond Q-switched Nd:YAG radiation at wavelengths of 1064 nm, 532 nm, and 355 nm and for selected silver emission lines including 328.06 nm, 338.28 nm, 768.7 nm, and 827.3 nm and the hydrogen alpha Balmer series line at 656.3 nm. Line asymmetries are presented for the 328.06-nm and 338.28-nm Ag I lines that are measured following generation of the plasma due to multiple photon absorption. This work explores electron density variations for different irradiance levels and reports spectral line asymmetry of resonance lines for different laser fluence levels.

Interferometric analysis of sub-nanosecond laser-induced optical breakdown dynamics in the bulk of fused-silica glass.

Dynamics of laser-induced optical breakdown in the bulk of fused-silica glass irradiated by a sub-nanosecond laser pulse at a wavelength of 790 nm with a fluence of 522 J/cm2 was studied by the femtosecond time-resolved complex interferometry in Nomarski arrangement utilising a Fresnel bi-prism. Evolution of the plasma channel and the development of the free electron density were in focus of the investigation. The measured ultimate length of the plasma channel was equal to 30 μm and almost doubled the length estimated within the moving breakdown model. The history of the transient electron density distribution in the plasma was reconstructed from the phase shift maps using the inverse Abel transform and it revealed further deviation from this model. The core of the plasma channel exhibited at the last stages of the development a considerable level of the electron density up to 2.4×1020 cm-3. The signature of the pre-breakdown phase has been identified as radiation caused by ionization-released electrons interacting with ions and has been demonstrated in solids for the first time in this way. Origin of the discrepancy between the theoretical prediction of the moving breakdown model and the measured values of the channel length is discussed as well.

Optical breakdown as a broadband underwater acoustic source

Current underwater transducers provide little support for remote measurement of material properties. Such measurements are often limited by high impedance contrast, and target inhomogeneities that arise from biofouling and corrosion. In the presence of these limitations, a sensing system would benefit from broadband, spatially-localized target excitation. The high bandwidth would allow a return signal to encode broader information content about the target, while spatial localization would provide stronger target excitation and higher spatial resolution for measurements. Although these source properties are not achievable with traditional transducer designs, they can be produced through laser-induced optical breakdown in water. During optical breakdown, focused laser light excites a localized plasma, which expands and produces a mechanical shock wave. The shock properties are set by both the medium hydrodynamics and the laser parameters, allowing the acoustic source to be optically tuned. In laboratory experiments, breakdown from a 4 mJ/ 4ns laser pulse emits an exponential pressure wave with a decay constant under 250 ns and peak pressure near 600 kPa at 2 cm range. Here, I discuss the physics and fundamental limitations of acoustic transduction via underwater optical breakdown. Experimental results illustrate how the acoustic source properties can be tuned through optical control.

Time-resolved optical probing of nanosecond laser-induced breakdown plasma in polymethyl methacrylate (PMMA)

In this paper, laser-induced optical breakdown in a transparent dielectric was studied by pump–probe beam method. The breakdown was induced by Nd:YAG nanosecond laser beam in polymethyl methacrylate (PMMA). The main laser beam was separated by a splitter and used as probe beam. An appropriate optics used to direct the probe beam passing through the breakdown region perpendicularly to the pump laser beam. Using fast photodiodes and oscilloscope, the transmitted/ reflected signals (associated with the probe/ pump beam) were monitored. Analyzing these signals can be used to describe the breakdown process. The results show that the dynamics of transmissivity and reflectivity of the produced plasma depend on the pump laser intensity.

Short-pulsed laser for the treatment of tattoos, pigmented lesions, scars and rejuvenation.

This review describes the use of picosecond lasers for the treatment of tattoos, pigmented lesions, scars, and their use in rejuvenation. These devices have delivered enhanced efficacy for the treatment of tattoos and pigmented lesions when compared to the older 40-50 nanosecond devices. The fractional delivery with the picosecond devices have opened up a new method of rejuvenation for photodamaged skin and the treatment of scars. The delivery of these high-energy short pulses have created zones of injury in the skin referred to as areas of laser-induced optical breakdown. These areas of damage appear to produce cytokines and chemokines which result in epidermal and dermal repair and remodeling. The dual use of these devices with the flat and the fractional optics have made these devices useful in many ways that have been unanticipated.

Preparation and micropatterning of gold nanoparticles by femtosecond laser-induced optical breakdown

Abstract We fabricated gold nanoparticles (NPs) via a femtosecond (fs) laser-induced cavitation bubbling. By introducing intense fs laser pulses into HAuCl 4 aqueous solution containing 1-propanol, optical breakdown and the following cavitation bubble generation occurred at the focus through multiphoton absorption of water, resulting in the formation of gold nanoparticles (Au NPs). Intermediate species such as radicals and solvated electrons produced upon the bubbling contribute to gold ion reduction, while direct ionization of Au-chloride complex, photothermal decomposition and direct optical decomposition are excluded as possible mechanisms. Utilizing the fs laser-induced bubbling we demonstrate in-situ micropatterning of Au NPs on a substrate from the same HAuCl 4 aqueous solution containing 1-propanol.

Electron density and temperature diagnostics in laser-induced hydrogen plasma

Laser-induced optical breakdown is achieved by using Q-switched, Nd:YAG radiation focused into ultra-high-purity (UHP) hydrogen gas at a pressure of 1.08 ± 0.03 × 105 Pa inside a cell. The plasma emission spectra are dispersed by a Czerny-Turner type spectrometer and detected with an intensified charge-coupled device (ICCD). Stark-broadened hydrogen Balmer series Hα and Hβ line profiles are used as a spectroscopic tool for the determination of electron density and excitation temperature. Spatial variation of electron density and temperature at 0.40 µs are extracted from the recorded intensities of Hα and Hβ lines. Temporal variations of electron density and excitation temperature are also presented for the time delay range of 0.15 µs to 1.4 µs.

Adaptive optics assisted and optical coherence tomography guided fs-laser system for ophthalmic surgery in the posterior eye.

While fs-lasers are clinically established for surgery in the anterior eye, their use in the posterior eye is impeded by aberrations and focus position errors. We implemented a laboratory system to investigate whether fs-laser surgery in the posterior eye is made more feasible by aberration correction and tomographic image guidance. Aberration correction is obtained by adaptive optics (AO) and the image guidance is accomplished by optical coherence tomography (OCT). System characteristic measurements and cutting experiments were performed inside an eye model. By aberration correction, wavefront errors were reduced from 270 nm root-mean-square (rms) to 64 nm rms, ignoring Zernike terms for tilts and focus. The Strehl ratio of the assigned point spread function is improved from 0.11 to 0.78. The threshold pulse energy of laser-induced optical breakdown in water is lowered from about 3.0 to about 1.3???J measured at the eye model entrance. After laser cutting of a synthetic foil placed 300???m in front of porcine retinal tissue with the corrected system, postoperative three-dimensional OCT imaging showed no lesions in the tissue. Our results corroborate that AO and OCT will be two essential assistive components for possible clinical systems for fs-laser–based surgery in the posterior eye.

Laser-induced optical breakdown spectroscopy of polymer materials based on evaluation of molecular emission bands

Laser-induced breakdown spectroscopy (LIBS) for composition analysis of polymer materials results in optical spectra containing atomic and ionic emission lines as well as molecular emission bands. In the present work, the molecular bands are analyzed to obtain spectroscopic information about the plasma state in an effort to quantify the content of different elements in the polymers. Polyethylene (PE) and a rubber material from tire production are investigated employing 157nmF2 laser and 532nm Nd:YAG laser ablation in nitrogen and argon gas background or in air. The optical detection reaches from ultraviolet (UV) over the visible (VIS) to the near infrared (NIR) spectral range. In the UV/VIS range, intense molecular emissions, C2 Swan and CN violet bands, are measured with an Echelle spectrometer equipped with an intensified CCD camera. The measured molecular emission spectra can be fitted by vibrational-rotational transitions by open access programs and data sets with good agreement between measured and fitted spectra. The fits allow determining vibrational-rotational temperatures. A comparison to electronic temperatures Te derived earlier from atomic carbon vacuum-UV (VUV) emission lines show differences, which can be related to different locations of the atomic and molecular species in the expanding plasma plume. In the NIR spectral region, we also observe the CN red bands with a conventional CDD Czerny Turner spectrometer. The emission of the three strong atomic sulfur lines between 920 and 925nm is overlapped by these bands. Fitting of the CN red bands allows a separation of both spectral contributions. This makes a quantitative evaluation of sulfur contents in the start material in the order of 1wt% feasible.

Influence of absorption induced thermal initiation pathway on irradiance threshold for laser induced breakdown.

We investigated the influence of thermal initiation pathway on the irradiance threshold for laser induced breakdown in transparent, absorbing and scattering phantoms. We observed a transition from laser-induced optical breakdown to laser-induced thermal breakdown as the absorption coefficient of the medium is increased. We found that the irradiance threshold after correction for the path length dependent absorption and scattering losses in the medium is lower due to the thermal pathway for the generation of seed electrons compared to the laser-induced optical breakdown. Furthermore, irradiance threshold gradually decreases with the increase in the absorption properties of the medium. Creating breakdown with lower irradiance threshold that is specific at the target chromophore can provide intrinsic target selectivity and improve safety and efficacy of skin treatment methods that use laser induced breakdown.