Action Of Laser Pulses Research Articles

Analytical modeling of thermal processes during femtosecond laser pulse action on metallic nanoparticle

The analytical approach has been proposed for the modeling of the thermal processes during the action of femtosecond laser pulse on metallic (gold) nanoparticle with allowance for the temperature dependences of the electron heat capacity and the electron-phonon coupling factor of gold based on thermal excitation of electron density. This leads to novel dependence of the electron-phonon equal temperature and the coupling factor on the maximal electron temperature. Whole process of femtosecond nanoparticle action has been divided into three stages which include firstly the heating of electron system by pulse, secondly the electron-phonon energy exchange and the establishment of the equal temperature of electrons with lattice, and thirdly the nanoparticle cooling with the corresponding analytical description of each stage. The satisfactory agreement with the computer results for femtosecond pulses validates of used analytical approach. This model can be used for the estimation of laser and nanoparticle parameters in the fields of the applications of femtosecond laser pulses.

Studying the Influence of Nanosecond Pulsed Laser Action on the Structure of Submicrocrystalline Titanium

Studying the Influence of Nanosecond Pulsed Laser Action on the Structure of Submicrocrystalline Titanium

Acceleration of electrons in hollow glass capillaries under the action of intense picosecond laser pulses

The generation of relativistic electron beams in hollow glass capillaries is investigated using a picosecond laser system. A plasma is formed in a capillary due to the ablation of its inner walls by a specially formed pre-pulse. Electron beams with an opening angle of ∼40 mrad, exponential energy spectrum with maximum energy about 20 MeV, and a total charge of ∼0.2 nC are experimentally detected.

Analysis of the Effect of Nd:YAG Laser Irradiation on Soft Tissues of the Oral Cavity in Different Modes in an In Vivo Experiment

The development of laser medicine has led to its use in dentistry further to improve existing treatment methods, including surgical techniques. The variety of lasers allows them to be used for procedures on the soft and bone tissues of the oral cavity as well as on the tissues of the teeth. The short duration of laser pulse action on tissues, selective action on pathological tissues in a sterile surgical field, and activation of local and humoral immunity of the oral cavity provides an increase in the regeneration potential of tissues of the postoperative area, which contributes to the shortening of wound process phases, favorable course of the postoperative period, and shortening of the healing time. Our article presents the experience of using the Nd:YAG laser in different modes in replicating the effect of curettage of periodontal pockets in an experiment on laboratory animals. According to the study results, there was a difference in the healing time of soft tissues after their exposure to several modes of the Nd:YAG laser, which makes it possible to recommend each of them for individual clinical cases.

Radiation signal accompanying the Schwinger effect

The properties of the forced oscillations of electron-positron plasma (EPP) generated from vacuum under the action of a short laser pulse are considered. Calculating the density of the conduction and polarization currents within the quantum kinetic approach, we demonstrate the presence of plasma oscillations at the frequency of the external field and its odd harmonics. It is expected that radiation generated by these plasma oscillations can be observed outside the interaction region, for example, outside the focal spot of two counterpropagating laser beams, and can serve as an indicator of the Schwinger mechanism of the EPP creation from vacuum.

Optical and Electrophysical Properties of Thin Zinc Oxide Films Doped with Manganese Oxide and Obtained by Laser Deposition

Nanostructured thin films on a silicon substrate were obtained on a ceramic of zinc oxide doped with manganese oxide by high-frequency periodic pulsed laser action with f ~ 10–15 kHz, wavelength λ = 1.064 μm at power density q = 150 MW/cm2 in a vacuum chamber with p = 2.7 Pa. The surface morphology and the elemental composition of the obtained films were studied using atomic force microscopy, scanning electron microscopy, and X-ray spectral microanalysis. Features of the transmission spectra in the visible, near, and middle IR regions were determined. The electrophysical properties of the ZnO + 2% MnO2/Si heterostructure were analyzed.

Action of Excimer Laser Pulses on Light-Emitting InGaAs/GaAs Structures with a (Ga,Mn)As-Layer

Action of Excimer Laser Pulses on Light-Emitting InGaAs/GaAs Structures with a (Ga,Mn)As-Layer

Pulsed laser modification of layered B-C and mixed BCx films on sapphire substrate

The effect of pulsed laser annealing (PLA, i.e., action of nanosecond laser pulses in air atmosphere) on surface morphology, structure, chemical state, and electrical properties of thin films consisting of boron and carbon atoms was studied. The nanolayered В/С and mixed ВСx~3 thin film precursors (with a thickness ~110–140 nm) were created on sapphire substrates by using the pulsed laser deposition. Scanning electron microscopy and Raman scattering measurements indicate that the process of stratification of the multilayer B/C films dominated during PLA of the films. The outer layers were removed, and closer to the substrate, the B and C layers were preserved, and they were not mixed. For the mixed ВСх films deposited at elevated temperatures, the PLA treatment enhances ordering of initially amorphous film structure. At the stage of melting of these films, oxygen (from surrounded air) penetrated inside the top layer of the film leading to the formation of a multiphase structure from g-BCx and B-doped GO/rGO after solidification. Studies of the chemical state of elements in the irradiated film have shown that О atoms, which penetrated the under-surface layers of the film, facilitated the formation of new chemical bonds in the B–C–O system. The concentration of O atoms in the depth of the film could reach 8%. The preferential oxidation of boron in a homogeneous mixture of B–C–O atoms indicated that intercalation of carbon matrix could be due to the incorporation of not only O atoms, but also B–O molecules. The laser irradiated ВСх films have a relatively low resistivity (~1.6 mΩ·cm) and semi-metallic dependence on temperature in the range 4.2–300 К. The spatial distribution of conductivity zones after PLA has an irregular structure, and its pattern looks like sand dunes, indicating that the PLA treatment leads to mixing zones of high with those of lower conductivity. The contact-tip resistivity inside the areas of high conductivity of the ВСx/Al2O3 samples may be significantly lower than that of pure graphite.

Electron thermal conductivity of nickel and aluminum in solid and liquid phases in two-temperature states

Analytical expressions for the electron thermal conductivity of nickel and aluminum are presented. The thermal conductivity depends on the electronic and ionic temperatures, density and phase of the substance (solid or liquid). The expressions obtained can be used in hydrodynamic calculations of the ablation of these metals or multilayer targets made from them. We consider the case when ablation is caused by the action of ultrashort laser pulses.

Compression-induced fracture in silicon dioxide as a mechanism of ultra fast plasma propagation under the action of intense laser pulse

In this paper a set of models is considered to describe the mechanism of ultra fast plasma propagation in silicon dioxide waveguide under the action of intense laser pulse. In particular, the paths via which the energy is absorbed are analyzed. For the first time, the absorption in the region ahead of the primary plasma front related to the medium fracture by the shock wave with magnitude greater than 30kbar is taken into account on the base of recently obtained experimental data. And it is clearly shown that this effect defines the propagation of the absorption wave with a speed ~3km/s that was observed experimentally and did not get an explanation yet. It is also shown that the competition between the energy absorption behind the shock wave and momentum losses due to transverse expansion defines the oscillations in the speed of absorption wave. The spatial period of oscillations depends unequivocally on the absorption length in the fractured region as well as on the fragments size. Obtained results can provide an interpretation of the unique experimental data on high-speed fracture of optical fibers under the action of intense laser pulse.

Extreme matter compression caused by radiation cooling effect in gigabar shock wave driven by laser-accelerated fast electrons

Heating a solid material with laser-accelerated fast electrons is a particularly useful method for generating a plane powerful shock wave with a pressure of several hundred or even thousands of Mbar in the laboratory. Behind the front of such a powerful shock wave, dense plasma is heated to a temperature of several keV. Then, a high rate of radiation energy loss occurs even in low-Z plasmas. In this paper, the strong compression of matter due to radiation cooling in a Gbar shock wave driven by fast electrons is studied using both computational and theoretical approaches. It is shown that the effect of radiation cooling leads to compression of matter in the peripheral region of the shock wave to a density several times greater than the density at its front. Heating a solid material by a petawatt flux of laser-accelerated fast electrons offers the opportunity to surpass the gigabar pressure level of plane shock waves generated by the impact of laser-accelerated pellets. Higher pressures of about 100 Gbar can be achieved under laboratory conditions only when a spherical target is imploded under the action of a terawatt laser pulse.

Analysis of gas bubble formation on light-absorbing microinclusion in liquid during laser irradiation: experimental and theoretical investigation

A simplified theoretical model that describes the formation of a gas bubble on a light-absorbing microinclusion in liquid during laser irradiation is presented. The experimental results of irradiation of agglomerates of carbon black nanoparticles with nanosecond laser pulses in distilled water and aqueous solution of glycerol are obtained. The satisfactory correspondence between the experimental and theoretical results is achieved at low values of the microinclusion light absorptivity and can be associated with additional light scattering caused both by the formation of the gas bubble during the laser pulse action and by the phenomenon of critical opalescence in the superheated liquid. With the increase in the laser fluence against the background of the formation of the small-sized gas bubbles, the large-sized gas bubbles appear, which qualitatively agrees with the dependence of gas bubble size on the laser fluence. In case of multipulse laser irradiation, heat accumulation contributes to more uniform distribution of the gas bubbles’ size. The investigated regularities can be used for optimization of the laser parameters in biomedical and other laser applications.

Speckle patterning of a pumping laser light as a limiting factor for stimulated fluorescence emission in dense random media.

The features of fluorescence emission in a dye-doped dense multiple scattered medium under pulsed laser pumping are considered in terms of confined excitation in small zones associated with laser speckles occurring in a pumped medium. The results of numerical modeling of the fluorescence emission kinetics are compared to the experimental data obtained using the rhodamine 6G-doped layers of the densely packed TiO2 (anatase) particles pumped at 532 nm by 10 ns laser pulses. The intensity of pump radiation during the action of laser pulses was varied from 1·105 W/cm2 to 5·107 W/cm2. In the recovery of the ratios of stimulated to a spontaneous emission, the spectra of the stimulated component were fitted using the spectral function derived by R. Dicke. In the framework of the considered concept, saturation of the ratio of the stimulated to a spontaneous emission and linear growth of an integrated fluorescence output with a practically unchangeable half-width of the emission spectra at high pump intensities are interpreted.

Modeling of shock-wave processes in aluminum under the action of a short laser pulse

A hydrodynamic model of shock-wave processes in a material under the action of a short high-intensity laser pulse is considered. The simulation is carried out for the case of an aluminum target 90 μm thick, irradiated by a laser pulse with a duration of 70 ps and a maximum intensity of 14.7 TW/cm2. In the corresponding laboratory experiment, on the rear side of the target after irradiation, a spall of a part of the material is recorded at a depth of 10±1 μm. Calculation of the time dependence of the pressure and density of aluminum in the spall plane makes it possible to determine the tensile strength of aluminum at a high strain rate.

Study of the kinetic dependences of the glow and determination of the threshold characteristics of ignition of coals with different stages of metamorphism under the action of microsecond laser pulses

In this work, the study of the kinetic dependences of the glow and determination of the threshold characteristics of the ignition of coal of grades long-flame, semi-lean caking, weakly caking, lean and anthracite under the action of microsecond laser pulses was carried out. For the aforementioned coals, three successive stages of ignition were found with characteristic Hcr thresholds. The first stage of ignition is associated with the heating of the surface of coal particles and the ignition of microprotrusions; the duration of the glow practically repeats the duration of the laser pulse. The second stage of ignition is associated with the release and ignition of volatiles in a millisecond time interval; the third-with the ignition of a non-volatile residue, the glow duration reaches 120 ms. With an increase in the degree of coalification, almost constant values of the first ignition threshold Hcr(1) are found. For the studied coals, with an increase in the degree of coalification, a decrease in the second threshold Hcr<2) and an increase in the third ignition threshold Hcr (3)are observed.

Фото- и механолюминесцентные свойства люминофора SrAl2O4 : (Eu2+, Dy3+) и использование его для визуализации динамики деформаций материалов

The work investigated the deformations of materials arising from the action of high-power laser pulses. Composite mechanoluminescent materials based on a polymer and a phosphor powder were used to visualize and record the dynamics of the development of deformations. Mechanoluminescent materials were applied to the surface of the materials under study. It is shown that the spatial distribution of the glow intensity of the mechanoluminescent layer and the rate of its change make it possible to judge the magnitude and rate of deformation of materials when they are exposed to laser pulses. The kinetics and photoluminescence spectrum of composite materials were studied under the combined action of short-wave and long-wave laser radiation. It is shown that the photoluminescence excited by a long-wavelength laser is associated with the activation of the populated levels of the traps.

Laser-induced modification of the scleral collagen framework for changing its hydraulic permeability

An action of the IR laser pulses with a wavelength of 1.56 μm, moderate intensity, and various durations on a collagen framework structure and sclera tissue permeability is considered. It is shown that the temperature thresholds of collagen denaturation in tissues under the laser irradiation are substantially lower as compared to those under conventional homogeneous heating in a calorimeter oven. A tissue matrix modified at temperatures below the denaturation threshold is observed in images obtained by nonlinear optical microscopy in the regime of second harmonic generation. The modification has specific features under long-duration (with a duration longer than 1 s) and short-duration (10 – 30 ms) action: separation of a fibre bundle into groups with distorted passage in the first case and bundle splitting to chaotically directed fibres in the second case. The character of the tissue change determines the coefficient of organic substance diffusion into a tissue material. After matrix amorphisation (collagen denaturation) or distortion of the fibre group arrangement in the collagen bundle, the diffusion coefficient falls by 10 % – 40 %, whereas after bundle splitting it increases by 25 % – 50 % as compared to the value of (3 ± 0.1) × 10−6 cm s−1, which is specific for intact tissue. The results obtained make us to assume that the specific disordering of the tissue collagen framework under the short-duration action may be used for increasing the permeability of the scleral tissue.

Investigation of the Structure of Hard Alloys That Have Undergone Bulk Pulsed Laser Hardening Using X-Ray Structural Analysis

The main principles of volumetric pulsed laser hardening method are presented in the article. The results of experiments based on tensometry with further processing using the LabVIEW system are shown. Obtained results confirm the appearance of the mechanical shock wave in materials due to the action of a single high-energy laser pulse. The received dependences were confirmed for hard alloys with different chemical compositions. Using of X-ray structural analysis allowed to establish that various hard alloys processed by pulsed laser hardening characterized by the third kind (nanostructure) changing the structure due to the accumulation of structural defects (sizes of mosaic blocks, crystal microscopic distortions, density of dislocations). It has been established that the third kind changing the structure increases the main service mechanical properties of hard alloys (microhardness, abrasive wear resistance, flexing strength). In addition, it was found that the change in the hard alloys properties is directly proportional to the change in the nanostructure of materials.

Моделирование кристаллизации модифицированного наноразмерными частицами расплава при лазерной обработке поверхности металла

A 2D mathematical model is proposed for the modification of an iron-based alloy with refractory nanosized particles. Numerical simulation of the processes during the modification of the surface layer of the substrate metal using the energy of a laser pulse has been carried out. Within the framework of the proposed model, the processes of heating and melting of metal on a substrate covered with a layer of nanosized refractory particles penetrating into the molten metal, convective heat transfer in the melt, and solidification after the end of the pulse are considered. Metal melting is considered in the Stefan approximation, and when the melt is cooled, the model of heterogeneous nucleation and subsequent crystallization is used. The fluid flow is described by the Navier-Stokes equations in the Boussinesq approximation. The distribution of nanoparticles in the melt is modeled by moving markers. Based on the results of calculations, the mode of pulsed laser action is determined, in which a flow is formed for a homogeneous distribution of particles of the modifying substance in the presence of a surfactant in the metal. The volume of the solid phase formed around the nucleus determines the size of the grain structure in the solidified alloy. The liquidus temperature changes depending on the concentration of dissolved carbon in the melt. In the numerical simulation of the solidification of the surface layer of the metal, it was found that the conditions of nucleation and crystallization differ significantly in the volume of the melt. It is determined that the duration of nucleation in a supercooled melt is several tens of microseconds. The maximum number of crystallization centers occurs in areas where heat removal occurs most rapidly. With the growth of the solid phase in the melt and the release of the latent heat of crystallization, the value of supercooling decreases, the nucleation stops and the number of formed crystallization centers does not change further. The distribution of the dispersion of the crystal structure over the volume of the melted metal is estimated. It was found that as the melt cools, sequential-volume crystallization occurs.

Воздействие импульсов эксимерного лазера на светоизлучающие InGaAs/GaAs-структуры с (Ga, Mn)As-слоем

It was studied the possibility of laser annealing modification of the properties of the (Ga,Mn)As layer located on the surface of a quantum-well InGaAs/GaAs structure, while retaining its radiative properties. The structures with four InGaAs/GaAs quantum wells (indium content was varied from 0.08 to 0.25), located at different distances from the (Ga,Mn)As layer, were fabricated by combining the methods of MOCVD-hydride epitaxy and pulsed laser deposition. The LPX-200 excimer laser pulse energy density was varied from 200 to 360 mJ/cm2, and the depth of laser action was determined from the change in the photoluminescence spectra of the quantum wells. In describing the results obtained, a model of the laser annealing process was used, based on solving the problem of heat propagation in a one-dimensional GaAs system, taking into account the (Ga,Mn)As layer on the surface. Changes in the structural and galvanomagnetic properties of the samples as a result of laser irradiation were analyzed. It is shown that as a result of pulsed laser action at a laser energy density range of 250 - 300 mJ/cm2, it is possible to preserve the emissive properties of the active region (InGaAs/GaAs quantum well) located at a distance of 10 - 12 nm from the (Ga,Mn)As layer and modify ferromagnetic properties of the semiconductor (Ga,Mn)As, namely: to increase the temperature of the ferromagnet-paramagnetic phase transition to values of at least 120 K. The results obtained are promising for the development of technology for devices of spin optoelectronics.