Laser Pulses Of Different Durations Research Articles

Conformation of von Willebrand factor in shear flow revealed with stroboscopic single-molecule imaging

Abstractvon Willebrand factor (VWF) is a multimeric blood protein that acts as a mechanical probe, responding to changes in flow to initiate platelet plug formation. Previously, our laboratory tests had shown that using single-molecule imaging that shear stress can extend surface-tethered VWF, but paradoxically, we found that the required shear stress was higher than reported for free-in-flow VWF, an observation inconsistent with basic physical principles. To resolve this inconsistency critical to VWF’s molecular mechanism, we measured free-VWF extension in shear flow using pulsed laser stroboscopic imaging of single molecules. Here, laser pulses of different durations are used to capture multiple images of the same molecule within each frame, enabling accurate length measurements in the presence of motion blur. At high shear stresses, we observed a mean shift in VWF extension of <200 nm, much shorter than the multiple-micron extensions previously reported with no evidence for the predicted sharp globule-stretch conformational transition. Modeling VWF with a Brownian dynamics simulation, our results were consistent with VWF behaving as an uncollapsed polymer rather than the theorized compact ball. The muted response of free VWF to high shear rates implies that the tension experienced by free VWF in physiological shear flow is lower than indicated by previous reports and that tethering to platelets or the vessel wall is required to mechanically activate VWF adhesive function for primary hemostasis.

In English

This paper presents a theory that allows one to calculate the energy absorbed by spheroidal metal nanoparticles when irradiated by ultrashort laser pulses of different duration in the region of surface plasmon resonance. Simple analytical expressions are obtained to calculate the absorption energy dependent on the frequency of carrier laser wave, on the pulse duration, and on the ratio between the axes of the ellipsoids. It is shown that at the frequency of the carrier (laser) wave, which coincides with that of the surface plasmon, the maximum absorption is observed for spherical nanoparticles. As the carrier frequency deviates from the surface plasmon one, two maxima appear in the absorption spectrum, dependent on the ratio of spheroidal axes: one corresponds to the elongated particles and the other to the flattened ones.

Induced stress arising in crystalline silicon under exposure to ultra-short laser pulses of different duration in air and water

The silicon surface was modified in a single-pulse mode with femto-picosecond laser pulses (0.3 and 10 ps) in the near-IR range (1030 nm) during ablation in air and water. The resulting structures were studied using Raman microscopy. In the course of the study, it was found that nanocrystallites with a size of 7-8 nm appear at the crater boundary. Local mechanical stresses were found in the center of the crater, the sign of which depends on the applied energy density. The highest local compressive stresses arise in water in the subfilamentation mode at maximum energy densities. Keywords: silicon, Raman spectroscopy, ultrashort pulses, single-pulse ablation in air and liquid, local stresses.

Observation of stimulated thermal scattering of light upon nonsteady-state interaction between a laser pulse and a medium

The induced thermal scattering of light has been experimentally investigated for short laser pulses of different durations. It is shown that the observed anomalously large anti-Stokes shift (Ω ≫ 1/(2πτ), where τ is the thermal grating relaxation time) in the reflected signal depends on the pump pulse duration.

Laser Spectroscopic Sensors for the Development of Anthropomorphic Robot Sensitivity.

The development of underwater robotics sensitivity, which is based on the sensors of laser spectroscopy methods, have been discussed. The ways to improve Laser Induced Fluorescence (LIF) and Laser Induced Breakdown Spectroscopy (LIBS) methods were investigated in order to develop and create laser sensitivity for underwater robotics. A brief overview is done in the article, where LIF and LIBS spectroscopy in underwater robotics are used as spectroscopy sensors in order to investigate underwater environments by means of underwater vehicles. Limit of Detection (LoD) of oil and oil product solutions in the seawater have been detected by means of nanosecond and femtosecond spectroscopy LIF. All results, which had been received by laser pulses of different duration, were compared. The same experiments have been provided in order to measure concentrations of elements in the seawater and solutions by the LIBS method. It was discovered that the LoD of a group of elements was reduced when the femtosecond LIBS was used. Anthropomorphic complexes were under discussion in order to adopt laser spectroscopy sensors for underwater environments. The submersible module, which was constructed to investigate and examine laser spectroscopy sensors, has been described.

Thermophysical processes initiated by inert-matrix-hosted nanoparticles heated by laser pulses of different durations

In the present study, a model for the heating of inert-matrix-hosted metal nanoparticles with laser radiation taking into account the melting processes is examined. The calculations were performed using the characteristics of gold and pentaerythritol tetranitrate materials. The kinetic dependences of the temperature and molten-layer thickness on nanoparticle surface were calculated. The main non-dimensional governing parameters of the model were identified. An expression for the maximum thickness of molten layer was obtained. The results can be used in predicting the stability of nonlinear-optics devices with hosted gold nanoparticles, in raising the efficiency of hyperthermia cancer therapy, and in optimizing the optical detonators.

Numerical Investigation of the Plasma Formation in Distilled Water by Nd-YAG Laser Pulses of Different Duration

Water breakdown studies by Nd-YAG laser pulses of duration 100 fs, 30 ps and 6 ns at wavelength 1064 nm are pre- formed to investigate the physical mechanisms which couple the laser energy into the medium. Calculations are carried out applying a modified kinetic model of water breakdown previously developed by Kennedy (1995) to investigate the correlation between threshold intensity of breakdown and laser pulse length. The modifications considered the introduction of diffusion and recombination loss processes which might take place under the experimental conditions applied in these calculations. The validity of the model is tested by comparing the calculated threshold intensities and the experimentally measured ones where good agreement is shown. The study of the time evolution of the electron density clarifies the correlation between the pulse length and dominant ionization mechanism. The analysis of the spatial distribution of the electron density along the radial and axial distances of the focal spot showed that the size of the formed plasma increases with the increase of the pulse length. On the other hand, studies of self-focusing effect illustrated that under the investigated experimental conditions the effect of this process has an effective contribution only at laser pulses of the order of femtosecond scale when the laser beam is focused by a lens of a focal length ≥8.0 cm. This result in turns assures that using femtosecond pulses in ophthalmic microsurgery could be a safe tool from the retinal damage.

Initiation of a reactive material by a radiation beam absorbed by optical heterogeneities of the material

The heating dynamics and thermal ignition of a condensed transparent material by a radiation pulse absorbed by optical micro-heterogeneities present in the material were studied numerically and approximately analytically over a wide range of pulse durations. For quasistationary heating conditions of micro-heterogeneities, an analytical dependence of the radiation power density critical for ignition on the formal kinetic parameters of the chemical process and the radius of the particle absorbing the radiation was obtained. The critical energy characteristics of ignition are compared with experimental data on the initiation threshold of lead azide by laser pulses of different duration.

Time resolving imaging spectroscopy applied to the analysis of plasmas generated by pulsed lasers

Start your abstract here... Time resolved imaging spectroscopy were used to study the spatial and temporal evolution of LIBS (Laser Induced Breakdown Spectroscopy) plasmas generated in Cu substrates by laser pulses of different duration. Long laser pulses (microsecond) and short laser pulses (nanosecond) as well as multipulse emission were used for excitation. Analysis was made by using an imaging spectrometer with time resolved detection. Results show that the use of long laser excitation pulses produce emission spectra with the same signal to noise ratio, but with lower resolution than those produced with shorter ones. The different species generated in LIBS experiments as neutral or single ionized have a different spatial distribution inside the plasma. We demonstrated that using spatial discrimination procedures is possible to obtain spectra with the same signal to noise ratio than those obtained with a gating detector. In this case an appreciable advantage in cost reduction is obtained by replacing the gating detector by a cheap screen.

Terahertz time-domain-spectroscopy system based on femtosecond Yb:fiber laser and GaBiAs photoconducting components

We describe a terahertz time-domain-spectroscopy system that is based on photoconductive components fabricated from low-temperature-grown GaBiAs epitaxial layers and activated by femtosecond 1.03 μm pulses emitted by an Yb:fiber laser. Experiments performed with the laser pulses of different durations have evidenced that the spectral range of the system is limited by the photoexcited carrier lifetime. The optical to terahertz conversion efficiency of the system exceeds 10−4 with the usable spectral range of about 3 THz in the case of 170 fs excitation pulses.

Theory for absorption of ultrashort laser pulses by spheroidal metallic nanoparticles

The theory for the electric and magnetic fields energy absorption by small metallic particles subjected to the irradiation by ultrashort laser pulses of different duration in the region of surface plasmon excitation is developed. For the particles of the oblate or prolate spheroidal shape there has been found the dependence of the absorbed energy on a number of factors, including a particle radius, a degree of the shape deviation from a spherical one, a pulse duration, the orientation of the magnetic field upon particle, the magnitude of carrier frequency, and the value of a shift of the carrier frequency of a laser ray from the frequency of the surface plasmon excitation in a spherical particle. An appreciable absorption grows at the length of free electron pass large compared to the particle size is established. The phenomenological and kinetic approach is compared each with other.

Femtosecond laser microstructuring in the bulk of diamond

In this paper we report the fabrication of graphitic microstructures in the bulk of diamond using 120-fs-laser pulses at 800 nm wavelength. Polished plates of single crystal diamond and optical quality polycrystalline CVD diamond were used as samples for 3D microstructuring. Under low fluence conditions and focusing a laser beam into the bulk of diamond plates, multipulse irradiation was found to result in the appearance and continuous growth of a laser-modified (graphitized) region from the focal plane towards the laser beam. Controlling the laser fluence and sample translation velocity (scanning beam velocity) allowed high-aspect-ratio ‘graphitic wires’ – microstructures of a few microns in diameter and several hundred micrometers in length – to be fabricated in the bulk of diamond. Physical processes responsible for the continuous growth of microscopic graphitic regions towards a laser beam are discussed. Results of comparative investigations of graphitic microstructures produced by laser pulses of different durations (120 fs and 300 ps) are presented to show the advantages of ultrashort laser pulses in 3D microstructuring of diamond.

Laser Ablation of Silver in Different Liquids: Optical and Nonlinear Optical Properties of Silver Nanoparticles

The optical, structural, and nonlinear optical properties of silver nanoparticles prepared using the method of laser ablation in various liquids at wavelengths of 397, 532, and 795 nm with laser pulses of different duration are studied. An analysis of the dimensional and spectral characteristics of the silver nanoparticles revealed a time dynamics of the nanoparticle size distribution in solutions. It is shown that thermal self-defocusing is observed for the case of nanosecond or shorter pulses generated with a high repetition rate. For picosecond and femtosecond pulses with a low repetition rate, the effects of self-focusing (γ = 3 × 10−13 cm2 W−1) and saturated absorption (β = −1.5 × 10−9 cm W−1) were observed in the solutions under study. The third-order nonlinear susceptibility of the silver nanoparticles was found to be 5 × 10−8 esu at a wavelength of 397 nm.

Dissociative ionization at high laser intensities: importance of resonances and relaxation for fragmentation

We investigated dissociative single and double ionization of the metal carbonyls Ni(CO)4, Fe(CO)5 and Cr(CO)6 in the gas phase by means of laser pulses of different durations (30–110 fs) and wavelengths (0.8 and 1.35 µm) at intensities of 2 × 1012–2 × 1014 W cm−2. The mass spectra show striking differences: for example, Fe(CO)5 strongly fragments at 0.8 µm but little at 1.35 µm, whereas for Ni(CO)4 fragmentation is higher at 1.35 µm than at 0.8 µm; chromium carbonyl shows little fragmentation at both wavelengths. In other cases, fragmentation first decreases and then increases again with intensity. These and other phenomena, also published ones, can readily be understood from long-known principles, namely resonances in the parent ions, sometimes also in the neutral molecules, in particular if relaxations are also taken into account. We emphasize that fragmentation and ionization are two separate processes. We also point out that in the process of dissociative ionization in intense laser radiation, one should generally consider intermediate states, even if there is no one-photon resonance.

Laser treatment of tribological DLC films

The friction and wear reduction in applications that allow only a minimal use of liquid lubricants is done with solid lubricant films or with protective coatings, such as diamond-like carbon (DLC). Further improvements are possible if the geometries of the contact surfaces are modified in a controlled way, as we have already demonstrated it for TiN and TiCN. In this work, the possibilities to generate patterned DLC coated low wear tribological surfaces by means of laser processing were investigated. In the first approach, a two step method was used: steel substrates were laser patterned and subsequently DLC films were deposited on them. The second considered approach was the laser processing of coated surfaces. DLC films were irradiated with laser pulses of different durations and energy densities (100 fs, 800 nm, <4 J/cm 2; 150 ns, 1064 nm, <10 J/cm 2) and the treated spots were examined using optical microscopy, SEM, AFM and Raman spectroscopy. The graphitisation of a-C:H films under both fs- and ns- regimes was shown as well as a film-peeling phenomenon during the ablation process. Microstructured and DLC coated surfaces obtained in the former approach were used for preliminary tribological tests (oscillation-friction-wear method). The results showed that the friction coefficient did not increase, as compared with the unstructured and DLC coated surfaces, and that the structure pores trapped the debris particles produced when the DLC film eventually broke.

Factors influencing pricking pain threshold using a CO2 laser

The purpose of this study was the evaluation of the best experimental conditions in healthy subjects for the measurement of the minimal thermal energy density Et which induced pricking pain on the volar surface of the left forearm by means of CO2 laser pulses. Et was measured on a well-defined area, using laser pulses of different durations and constant power P. The dependence of Et on the stimulus power P, the size A of the radiated area and the surface temperature Te were explored. In the first part of the study, these relations were obtained using a computer program, from the calculated spatio-temporal distribution of the skin temperature during, and following, a laser pulse which caused pricking pain. The second part studied a set of subsequent measurements carried out on a group of five healthy trained subjects and agreed only in part with the calculated data. We found that the measurement error on Et was less than 10% with P between 1.5 and 3 W, and A between 0.15 and 0.25 cm2, respectively. The influence of sensitization and adaptation phenomena on the measured data was also explored. We also show a rhythmic annual change of Te and Et.

Mechanical damage in transparent solids caused by laser pulses of different durations

A mechanical damage of transparent solids caused by local laser-produced heating is considered. Applicability of thermoelasticity equations to the analysis of the mechanical stage of laser damage is substantiated for both short and ultrashort pulses. Damage criteria have been obtained for the crack-formation mechanism. On the basis of these criteria, the conditions for the transition from the crack-formation damage to defect generation and ablation have been clarified. The analysis can be applied to both intrinsic (impact and multiphoton ionization) and extrinsic (initiated by absorbing inclusions) absorption mechanisms of laser radiation.

State of the development of X-ray lasers and applications at LSAI

Obtaining an X-ray laser emission from plasmas, created and driven by an intense IR laser, has been pursued at the Laboratoire de Spectroscopie Atomique et Ionique (LSAI) for several years. At present, we operate various types of X-ray lasers driven by IR laser pulses of different durations (600 ps, 100 ps, and 600 ps/1 ps). A review of different techniques used at the LSAI to produce a strongly amplified emission using the collisional excitation pumping is presented. In the second part of this paper, to illustrate the potential of the X-ray lasers for applications, we present the main results obtained with an X-ray laser emitting at 21.2 nm in a study of surface defects of a niobium cathode, induced by strong electrical fields. We also describe a novel imaging interferometry device using an X-ray laser as a source and designed as a tool for high-resolution diagnostic of dense plasmas.

Effect of the plasma density scale-length on harmonic generation in laser-plasma interaction

The conversion efficiency of harmonics emitted by laser driven plasma was investigated via PIC (Particle In Cell) simulation. We calculated the efficiencies of harmonics generated in plasma with different scale- lengths irradiated by p-polarized, intense (but still non-relativistic) laser pulses of different durations. We found that the plasma scale-length plays an important role in harmonic generation, but this effect is quite different for different harmonic order, different pulse duration or different plasma density.

Some Aspects of the Inverse Problem of Determination of Cr^(4+) :YAG Absorption Cross Sections Using Experimental Data of Transmission

Mathematical modelling of nonlinear transmission of Cr4+:YAG crystals excited by Nd:YAG laser pulses of different duration are reported. Numerical simulation of transmission using a five-level scheme which included finite excitation lifetimes in Cr4+:YAG, focusing and diffraction of transversely nonhomogeneous pump radiation allowed to specify more precisely the limits of applicability of the four-level model. Critical analysis of typical simplifications commonly used for solving an inverse problem of determination of absorption cross sections is undertaken. It is shown that some of these assumptions that seem plausible enough might lead to considerable deviations of the determined absorption cross sections from their “true” values. A technique for determination of confidence intervals in case of the nonlinear regression is emphasized.