Optical Breakdown Threshold Research Articles

Model of the Effects of Femtosecond Laser Pulse Energy on the Effective Z-Position of the Resulting Cut after Laser-Induced Optical Breakdown

Presented in this study is a validated semiempirical model of the effects of femtosecond laser pulse energy on the effective z-position of the resulting cut after laser-induced optical breakdown. Different energy levels (for the same spot spacings, i.e., different doses) running from 1.07 × (~60 nJ) to 3.8 × (~215 nJ) of the laser-induced optical breakdown (LIOB) threshold (~56 nJ) were used to perform flat cuts within flat poly(methyl methacrylate) (PMMA) samples at the same nominal focus position (depth within the material). The z-locations of the effective cut and the anterior surface of the PMMA were confocally determined. The difference (PMMA_surface—Cut_plane) was used as a surrogate for the effective shift in the z-position of the cut. A consistent and continuous shift towards less deep cuts was observed for increasing pulse energies. The z-shift spreads by up to 22 µm thinner cuts for the largest pulse energy (3.8× LIOBTh, ~215 nJ). Higher pulse energies seem to significantly reduce the cutting depth. The results for PMMA are in good agreement with the moving LIOB model.

Narrow intensity range optical anisotropy in air induced by a femtosecond laser breakdown.

In this paper, we study the optical anisotropy induced by femtosecond laser radiation in air during an optical breakdown. Using a transverse pump-probe technique, we demonstrate that this anisotropy appears in a narrow range of pump intensities, which are close to the optical breakdown threshold in air and lead to a phase shift of probe radiation, polarized collinear to the pump. The intensity range where an induced intense anisotropy occurs makes it possible to estimate the magnitude of the 5th-order Kerr nonlinear refractive index component in air.

Influence of antireflection Si coatings on the damage threshold of fused silica upon irradiation with mid-IR femtosecond laser pulses.

Recent progress in the development of high-power mid-IR laser sources and the exciting laser driven physical phenomena associated with the irradiation of solids via ultrashort laser pulses in that spectral region are aimed to potentially create novel capabilities for material processing. In particular, the investigation of the underlying physical processes and the evaluation of the optical breakdown threshold (OBT) following irradiation of bulk dielectric materials with mid-IR femtosecond (fs) pulses have been recently presented. In this Letter, we will explore the conditions that generate sufficient carrier excitation levels which lead to damage upon irradiation of a dielectric material (SiO2) coated with antireflection (AR) semiconducting films (Si) of variable thickness with fs pulses. Simulation results demonstrate that the reflectivity and transmissivity of the Si/SiO2 are thickness dependent which can be employed to modulate the damage threshold of the substrate. The study is to provide innovative routes for selecting material sizes that can be used for antireflection coatings and applications in the mid-IR region.

Transient electron temperature and density changes in water breakdown induced by femtosecond laser pulses

When a femtosecond laser pulse travels through water, optical breakdown occurs when the laser intensity exceeds a certain threshold. This photoionization (PI) process and the resulting laser-induced plasma can strongly influence the laser ablation of substances, including transparent biological tissue, etc. However, numerical models of the initial evolution of the laser plasma and its early electron temperature remain inadequate. Here, a transient ionization rate equation involving the solvation process is proposed for calculating the free-electron number density (FED) generated during the interaction of the femtosecond laser with the water surface, along with the temporal evolution of the electron temperature in the focusing region during the entire femtosecond laser pulse utilizing an improved temperature model. The predictions of the proposed model are in better agreement with the experimentally determined breakdown thresholds. The results indicate that the existence of solvated electrons has a considerable influence on the determination of the optical breakdown threshold in water, which may also be one of the reasons why traditional numerical models consistently fail to forecast the actual breakdown threshold.

The propagation loss of ultrashort laser pulses in water clouds

When a high-power laser beam is incident on water clouds, often along with optical breakdown occurs. In that case, the droplet becomes highly opaque and strongly absorbs incident laser light. In this study, we construct a transient coupling model to elucidate the evolution of the light field and the distribution of plasma in the interaction between laser and cloud droplet in detail, and further quantify the nonlinear absorption of droplet under the action of high-power laser. The results showed that linear dependence existed between the proportion of absorbed energy by the droplet and the incident laser wavelength at the same laser energy. Such a relationship, combined with existing scattering theory, can be used to evaluate the propagation distance of the ultrashort laser in water clouds and the specific light attenuation under different parameters. In the same type of water cloud, the laser propagation distance decreases with increasing laser intensity, thereby confirming the availability of non-linear absorption on the laser propagation. Furthermore, a smaller pulse duration typically corresponds to a larger peak power, which results in stronger non-linear absorption and a shorter propagation distance in water clouds. On the other hand, a comparison of laser propagation distances in different types of water clouds reveals that scattering effects dominate in dense water clouds, with stronger propagation losses to the incident laser. Finally, the light attenuation in different water clouds is given accordingly. This study opens up new perspectives for the assessment of laser propagation loss in different water clouds and the propagation distance.

Methods of stabilization of halogenated hydrocarbons during automated physico-chemical cleaning of metal-optics

The results of the study of the possibility of maintaining the physicochemical properties of purified halogenated solvents in a stable state are summarized. Particular attention is paid to the possibility of stabilizing solvents during multiple use. The novelty is to justify the choice of the most effective stable azeotrope to remove contamination. A kinetic analysis of the physicochemical cleaning of metal optics from contaminants by azeotropes was carried out. Diagnostics of the optical breakdown threshold made it possible to find more efficient azeotropes. The results of a study of the possibility of maintaining stable properties of purified halogenated solvents are presented. Ways to increase the stability by obtaining azeotropes of high purity with an inhibitor in the distillation purification of contaminated solvents were investigated and proposed. On the example of metal optics, the fundamental idea of the physicochemical process of metal cleaning as an interconnected process of interaction between the energy characteristics of the washing mixture and the energy characteristics of the surface layer is substantiated.

Optical AR Coatings of the Mid-IR Band for ZnGeP2 Single Crystals Based on ZnS and Oxide Aluminum

In this work, the parameters of antireflection interference coatings based on alternating layers of ZnS/Al2O3 on the laser-induced damage threshold (LIDT) of ZGP crystals under the action of Ho:YAG laser radiation at a wavelength of 2.097 μm were determined. The coating deposition was carried out using the ion-beam sputtering method. The LIDT of the sample with a coating based on alternating layers ZnS and Al2O3 was equal to WoE = 3.45 J/cm2, and the LIDT of the uncoated sample was equal to WoE = 2.23 J/cm2. An increase in the optical breakdown threshold by ~55% was observed after the deposition of an AR coating based on ZnS and Al2O3 materials. An assumption was made about the absence of local fluctuations in the composition and mechanical stresses in the case of the coated sample, namely that this leads to good adhesion of the multilayer coating to the polished surface of the crystal, and as a result to an increase in the optical breakdown threshold as compared to the uncoated sample due to closure of the dangling chemical bonds and bulk defects emerging on the polished surface.

Backward SRS suppression of picosecond pulses in water upon moving the pump beam waist from the water volume through the surface

We report, for the first time to our knowledge, suppression of backward stimulated Raman scattering (BSRS) of picosecond pulses (57 ps, 532 nm) due to the development of optical breakdown in the surface (0 – 3 mm) water layer with a shift of the beam waist (lens focal length of 83 mm) to the water – air interface without changing the pump pulse energy (∼1.3 – 1.5 mJ). In this case, SRS generation in forward direction is observed even in the presence of breakdown. When the focal plane coincides with the surface, the BSRS generation is restored without optical breakdown, despite an increase in the pump radiation intensity due to a decrease in the beam diameter. It is significant that the optical breakdown threshold in the water volume was unattainable even with an increase in the pump pulse energy by more than an order of magnitude – up to 16 mJ. The mechanism of self-consistent summation of nonlinear optical processes, such as electrostriction, beam self-focusing, beam phase conjugation, and BSRS pulse compression is discussed.

Analytical optimization of the laser induced refractive index change (LIRIC) process: maximizing LIRIC without reaching the damage threshold

Abstract A method to determine the optimum laser parameters for maximizing laser induced refractive index change (LIRIC) while avoiding exceeding the damage threshold for different materials with high water content (in particular, polymers such as hydrogels or the human cornea) is proposed. The model is based upon two previous independent models for LIRIC and for laser induced optical breakdown (LIOB) threshold combined in a simple manner. This work provides qualitative and quantitative estimates for the parameters leading to a maximum LIRIC effect below the threshold of LIOB.

A simple strategy for increasing optical waveguide performance using spherical aberration

Femtosecond Laser Writing (FLW) has some advantages over competing techniques; it is a direct, highly flexible, and maskless process that can inscribe waveguides in almost any type of transparent material. FLW has proven to produce photonic integrated circuits with innovative design in a deterministic way for several applications, including quantum photonics and microfluidics. Although direct laser writing of waveguides has been extensively studied, there is still an intense search for methods to produce waveguides with lower losses and higher optical quality, because improvements in this sense will impact photonic research and technology. In this work, straight waveguides were directly inscribed into Eagle XG glass samples in a single process step at fabrication depths up to 370 µm. The propagation loss and the mode profile for single-mode waveguides were obtained. The laser pulse energy threshold that triggers the optical breakdown as a function of the depth was measured and used to study its relationship with the resulting waveguide optical properties. A trend between the waveguide losses and the observed pulse energy threshold for optical breakdown was found, which is connected with the microscope objective spherical aberration. It can be useful for finding the best conditions for producing low-loss waveguides. Compared to the waveguides written with the corrected pulses, propagation losses up to 2.5 times smaller were observed in our experiments when applying the strategy presented here. Therefore, by taking advantage of the inherent spherical aberration, the FLW technique can be made capable of much more at even higher intensities than previously possible.

Heat Treatment of the Surface of the ChS57 Alloy with Powerful Nanosecond Ultraviolet Laser Pulses

The threshold of optical breakdown of the nickel alloy ChS57 (Inconel) was measured at a wavelength of 0.355 μm with a laser pulse duration of 10 ns. Heat treatment of ChS57 above pulse energy density threshold (1 - 2.5 J/cm2) occurred mainly in the ablative mode with almost no melting. The elemental composition of the surface layer did not change at an irradiation in a fixed spot. When a laser beam moves along the surface of the sample at a speed of 1 mm / s and at pulse energy density of about 0.02 J/cm2, oxygen was detected in the elemental composition (3 – 4 wt. %). However, the proportions of the elemental composition of the alloy remained virtually unchanged. Heat treatment under threshold at pulse energy density ≥ 0.25 J/cm2 revealed a rise of the surface layer with traces of high-temperature plastic deformation in the form of slippage on grain boundaries and crystallographic slip.

Synergistic effect of picosecond optical and nanosecond electrical pulses on dielectric breakdown in aqueous solutions

The combined effect of short (picosecond) optical and (nanosecond) electrical pulses on dielectric breakdown is investigated both theoretically and experimentally. It was demonstrated that nanosecond electrical pulses (nsEPs), being applied simultaneously with picosecond optical pulses, reduce the threshold for optical breakdown. Experimental results are discussed with respect to an extended model for opto-electrical-induced breakdown. The newly unveiled effect is expected to play a significant role in spatially confined electroporation and further advances in laser-ablation-based processes while also allowing for measurements of ambipolar diffusion constants.

Electric-field-induced second-harmonic generation using high-intensity femtosecond laser pulses over the observable optical breakdown threshold.

We investigated the performance of electric-field-induced second-harmonic generation (E-FISHG) by spectroscopic measurement using high-intensity femtosecond laser pulses. The second-harmonic intensity increased quadratically versus the applied electric field, as expected from the theory, up to 15 kV/cm with the laser energy up to 2.5 mJ, which is ∼5 times higher than the observable optical breakdown threshold. In addition, when the laser energy was 2.8 mJ, ∼80 times signal intensity at 0.23 mJ was obtained. These results suggest that the electric-field measurement by E-FISHG with high-intensity second harmonics is expected by using high-intensity laser pulses above the observable optical breakdown threshold. Spectroscopic measurement shows no E-FISHG of white light generated by self-phase modulation in laser-induced filament.

Влияние технологии постростовой обработки на порог оптического пробоя монокристалла ZnGeP2

The threshold of optical breakdown of a ZnGeP2 single crystal manufactured by LOK LLC, Russia, was determined, which was W0d =1.8 J / cm2, and the threshold of optical breakdown of a crystal manufactured by Harbin Institute of Technology, China, was also measured, which was W0d =2.1 J/cm2 (at a wavelength of 2,097 microns of laser radiation and a pulse repetition frequency of 10 kHz with a pulse duration of 35 ns).The effect of post-processing of ZnGeP2 single crystals (polishing of working surfaces )is investigated, application of antireflection interference coatings) to the threshold of optical breakdown of the surface of these crystals. It is established that the presence of silicon conglomerates in the interference antireflection coating leads to a decrease in the optical breakdown threshold.

Bubble dynamics of laser-induced cavitation in plasmonic gold nanorod solutions and the relative effect of surface tension and viscosity

Laser-induced cavitation (LIC) bubbles and the shockwaves they form upon collapse are destructive to nearby solid boundaries, making them of interest for biomedical and industrial applications. Furthermore, the LIC bubbles provide spatial control that can be tuned by the bubble size, collapse time and shockwave intensity. The inclusion of plasmonic nanoparticles, such as gold nanoparticles (GNP) in the liquids where LIC bubbles are formed, can further enhance the absorption of light, allowing for bubble formation at lower laser energies. However, the effect of the physical properties of such liquids on LIC bubble dynamics remains unknown. In this study, the dynamics of LIC bubbles in water–ethanol, water-glycerol, and water-GNP solutions were investigated by simultaneous high-speed shadowgraphy and spatial transmittance modulation. The first set of experiments demonstrated that LIC bubbles induced in the GNP solutions led to more efficient cavitation formation with lower fluence compared to solutions without GNPs, thereby producing higher-intensity pressure waves. A second set of experiments was conducted to determine the surface tension of GNP solutions at room temperature and was found to be 70.62 mN/m. With this information, and the corresponding values reported in the literature for ethanol and glycerol, we aimed at discerning the role of surface tension and viscosity on the dynamics of LIC bubbles, apart from the enhanced optical absorption of the GNP solutions. We observed that the optical breakdown threshold for plasma formation was reduced by 18% in GNP solutions as compared to DI water and 10.4% compared to ethanol, and the intensity of initial shockwaves in the GNP solutions was much higher than those in DI water. This enhanced intensity of shockwaves in GNP solutions compared to DI water opens a new avenue for the enhancement of cancer cell treatment and anti-bacterial applications in the biomedical field and the enhancement of the laser ablation technique in the industrial setting.

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.

Pulsed Laser-Induced Nucleation of Sodium Chlorate at High Energy Densities

We report on a study of laser-induced nucleation (LIN) of sodium chlorate in supersaturated aqueous solutions using focused nanosecond laser pulses at high energy densities (420 kJ cm–2). On irradiation with a single laser pulse, optical breakdown was observed in the form of a luminous plasma, and numerous microbubbles were produced. On the basis of the observations, we estimate the energy threshold for optical breakdown in the solutions to be 70 J cm–2. Remarkably, even at high energy densities, single laser pulses produced on average only one or two crystals. The mean number of crystals obtained was 1.5 (532 nm) and 1.8 (1064 nm) per sample (3 cm3). The effect of left circularly polarized (LCP) and right circularly polarized (RCP) light on the nucleation of dextrorotatory (d) versus levorotatory (l) enantiomorphs of cubic (phase I) sodium chlorate crystals was investigated. No significant correlation between the helicity of circular polarization and the chirality of enantiomorph was observed. The result...

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.

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.

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.