Laser Pulses Of Nanosecond Duration Research Articles

Laser-driven quasi-static B-fields for magnetized high-energy-density experiments

We present measurements of magnetic fields generated in laser-driven coil targets irradiated by laser pulses of nanosecond duration, 1.053 μm wavelength, 500 J energy, and ∼1015 W/cm2 intensity, at the LULI2000 facility. Using two perpendicular probing axes, proton deflectometry is used to characterize the coil current and static charge at different times. Results reveal various deflection features that can be unambiguously linked to a looping quasi-steady current of well-understood polarity or to a static charging of the coil surface. Measured currents are broadly consistent with predictions from a laser-driven diode-current source and lumped circuit model, supporting the quasi-steady assessment of the discharges. Peak magnetic fields of ∼50 T at the center of 500-μm-diameter coils, obtained at the moderate laser intensity, open up the use of such laser-driven coil targets at facilities worldwide to study numerous phenomena in magnetized high-energy-density plasmas, and its potential applications.

Signal domain adaptation network for limited-view optoacoustic tomography

Optoacoustic (OA) imaging is based on optical excitation of biological tissues with nanosecond-duration laser pulses and detection of ultrasound (US) waves generated by thermoelastic expansion following light absorption. The image quality and fidelity of OA images critically depend on the extent of tomographic coverage provided by the US detector arrays. However, full tomographic coverage is not always possible due to experimental constraints. One major challenge concerns an efficient integration between OA and pulse-echo US measurements using the same transducer array. A common approach toward the hybridization consists in using standard linear transducer arrays, which readily results in arc-type artifacts and distorted shapes in OA images due to the limited angular coverage. Deep learning methods have been proposed to mitigate limited-view artifacts in OA reconstructions by mapping artifactual to artifact-free (ground truth) images. However, acquisition of ground truth data with full angular coverage is not always possible, particularly when using handheld probes in a clinical setting. Deep learning methods operating in the image domain are then commonly based on networks trained on simulated data. This approach is yet incapable of transferring the learned features between two domains, which results in poor performance on experimental data. Here, we propose a signal domain adaptation network (SDAN) consisting of i) a domain adaptation network to reduce the domain gap between simulated and experimental signals and ii) a sides prediction network to complement the missing signals in limited-view OA datasets acquired from a human forearm by means of a handheld linear transducer array. The proposed method showed improved performance in reducing limited-view artifacts without the need for ground truth signals from full tomographic acquisitions.

Nonlinear optical properties and carrier recombination lifetime of GaN.

Nonlinear optical properties of a selection of gallium nitride samples have been measured using picosecond and nanosecond duration laser pulses at 532nm. The values of the two-photon absorption coefficient, free carrier absorption cross section, and free carrier refraction cross section are determined along with the recombination lifetime of photogenerated carriers. The effect of hot isostatic pressing on these properties in samples with linear absorption at the band edge due to defects is explored.

Direct measurement of optical properties of glacier ice using a photon-counting diffuse LiDAR

AbstractThe production of meltwater from glacier ice, which is exposed at the margins of land ice during the summer, is responsible for a large proportion of glacier mass loss. The rate of meltwater production from glacier ice is especially sensitive to its physical structure and chemical composition which combine to determine the albedo of glacier ice. However, the optical properties of near-surface glacier ice are not well known since most prior work has focused on laboratory-grown ice or deep cores. Here, we demonstrate a measurement technique based on diffuse propagation of nanosecond-duration laser pulses in near-surface glacier ice that enables the independent measurement of the scattering and absorption coefficients, allowing for a complete description of the processes governing radiative transfer. We employ a photon-counting detector to overcome the high losses associated with diffuse optics. The instrument is highly portable and rugged, making it optimally suited for deployment in remote regions. A set of measurements taken on Crook and Collier Glaciers, Oregon, serves as a demonstration of the technique. These measurements provide insight into both physical structure and composition of near-surface glacier ice and open new avenues for the analysis of light-absorbing impurities and remote sensing of the cryosphere.

10 kHz laser-induced schliere anemometry for velocity, Mach number, and static temperature measurements in supersonic flows.

A new, to the best of our knowledge, technique for measuring velocity and Mach number in freestream flow is discussed and demonstrated. The technique, laser-induced schliere anemometry, uses a laser to write a laser-induced schliere in the flow, which can then be imaged using high-speed schlieren imaging. Here, we use a laser-induced plasma from the focusing of nanosecond-duration laser pulses from a pulse burst laser to write the disturbance. The resulting localized index of refraction gradient left from the plasma is tracked well beyond the plasma emission lifetime using schlieren imaging, and velocity is found from tracking or through a simple correlation analysis. The blast wave is also used to independently determine the Mach number via the Mach cone effect, which provides information about the mean static temperature. This technique shows great potential for use in characterizing freestream flow in supersonic facilities and is demonstrated here in a Mach 2 blowdown facility and a Mach 4 Ludwieg tube.

Effect of Neodymium Laser Pulses of Nanosecond Duration on Brown Coal

Effect of Neodymium Laser Pulses of Nanosecond Duration on Brown Coal

Modelling laser ultrasound waveforms: The effect of varying pulse duration and material properties.

Optical generation of ultrasound using nanosecond duration laser pulses has generated great interest both in industrial and biomedical applications. The availability of portable laser devices using semiconductor technology and optical fibres, as well as numerous source material types based on nanocomposites, has proliferated the applications of laser ultrasound. The nanocomposites can be deposited on the tip of optical fibres as well as planar hard and soft backing materials using various fabrication techniques, making devices suitable for a variety of applications. The ability to choose the acoustic material properties and the laser pulse duration gives considerable control over the ultrasound output. Here, an analytical time-domain solution is derived for the acoustic pressure waveform generated by a planar optical ultrasound source consisting of an optically absorbing layer on a backing. It is shown that by varying the optical attenuation coefficient, the thickness of the absorbing layer, the acoustic properties of the materials, and the laser pulse duration, a wide variety of pulse shapes and trains can be generated. It is shown that a source with a reflecting backing can generate pulses with higher amplitude than a source with an acoustically-matched backing in the same circumstances when stress-confinement has not been satisfied.

Features of microrelief formation during laser treatment of Cu-Cr-Zr alloy surface for diffusion welding

The effect of UV laser pulses of nanosecond duration on surface of Cu-Cr-Zr alloy samples was investigated. Noticeable changes in the morphology of the surface layer after irradiation were found at pre-threshold values of the energy density of ∼ 0.2 – 0.8 J/cm2. Changes are traces of thermoplastic deformation resulting from influence of a concentrated energy flow and appears in irregular elevation of the irradiated surface area of the sample by up to 1 μm and in some cases – more. A developed metal surface, deformation of the crystal structure of the surface layer can support the diffusion process during diffusion welding. For energy densities of the order of 1 J/cm2 or more, optical breakdown with the formation of a crater on the metal surface occurred. The mechanical impulse of a laser plasma when exposed to a metal surface prevents the thermomechanical expansion of the material, and the effect of raising the surface of the samples was not observed.

Improvement of high-speed steel cutting tool production technology using the method of nanostructure generation by laser ablation

The article reveals a problem of improving the production technology of cutting tools made of high-speed steels by the method of generating nanostructures on their surface with the help of laser ablation. The spiked microstructures obtained on the surface of the instrument are coated with nanostructures. Experimental studies to modify the surfaces of cutting tools with laser ablation method are performed in air and in liquids. As a result of the analysis of the experimental samples surface morphology we can conclude that the modification of the surface of cutting tools made of P6M5 high-speed steel with laser pulses of nanosecond duration can be used in air and in liquids as a method of increasing the resistance characteristics of a cutting tool.

Influence of nanosecond laser irradiation on the structure and conductivity of BCx films

Thin-film precursors of BCx were formed by pulsed laser codeposition of boron and carbon. Targets made of pressed boron and carbon powders with an equal element content (B/C = 1/1) and an increased carbon content (B/C = 1/3) were used. The films were deposited on sapphire substrates at elevated temperature (700°C) which determined the initial properties of the precursor BCx films. Irradiation of the films was carried out by laser pulses of nanosecond duration with varying intensity. The films obtained by laser annealing of BCx (Q-BCx) were studied by scanning electron microscopy and micro-Raman spectroscopy. Irradiation under optimal conditions made allowed to realize pulsed melting of the films and partial preservation of their continuity on the substrate. The local structure of Q-BCx films and the nature of the changes in their electrophysical properties depended on the composition of the precursor films and the laser irradiation regimes.

Photocarrier injection by two-photon excitation in rubrene single crystal

We performed photocarrier injection by two-photon excitation in rubrene crystal using laser pulses of nanosecond-duration at various wavelengths under an external electric field. Based on the excitation spectra, it is revealed that photocarriers are injected at excitation energies corresponding to the intrinsic absorption by two-photon process, in contrast to the midgap-states-mediated injection by one-photon process. This result means that the two-photon excitation method is useful to inject photocarriers free from the surface traps.

Annealing of Glassy and Ceramic Materials by Nanosecond Laser Pulses

The effect of pulsed laser radiation on glassy and ceramic materials is examined within the scope of the dynamic uncoupled problem of thermoelasticity for a half-space. Analytical relations expressing the thermal strength criteria for materials are obtained with three temporal shapes of laser pulses of nanosecond duration, making it possible to determine non-destructive regimes of annealing. The impact of the temporal shape of a laser pulse on the annealing regimes of materials is shown. It is found that laser pulses with a steep leading edge and a protracted trailing edge give thermoelastic stress reduction in the material.

Stimulated Emission and Optical Properties of Solid Solutions of Cu(In,Ga)Se2 Direct Band Gap Semiconductors

Stimulated emission, optical properties, and structural characteristics of non-irradiated and proton-irradiated Cu(In,Ga)Se2 thin films deposited on soda lime glass substrates using co-evaporation of elements in a multistage process were investigated. X-ray diffraction analysis, scanning electron microscopy, X-ray spectral analysis with energy dispersion, low-temperature photoluminescence, optical transmittance and reflectance were used to study the films. Stimulated emission at low temperatures of ~20 K was found in non-irradiated and proton-irradiated Cu(In,Ga)Se2 thin films upon excitation by laser pulses of nanosecond duration with a threshold power density of ~20 kW/cm2. It was shown that the appearance and parameters of the stimulated emission depend strongly on the concentration of ion-induced defects in Cu(In,Ga)Se2 thin films.

Controlled nanostructures formation on stainless steel by short laser pulses for products protection against falsification.

The coloration of stainless steel surface due to the formation of spatially periodic structures induced by laser pulses of nanosecond duration is demonstrated. The period of microstructures corresponds to the laser wavelength, and their orientation angle depends on the adjustment of laser polarization. The marking algorithm for the development of authentication patterns is presented. Such patterns provide several levels of protection against falsification (visual, colorimetric and structural) along with high recording speed and capability of automated reading.

Pump spectral linewidth influence on stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) and self‐termination behavior of SRS in liquids

The threshold, temporal behavior, and conversion efficiency of stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SBS) in three liquids (benzene, hexane, and dimethyl sulfoxide) and two crystals (calcite and barium nitrate) have been investigated under three largely different spectral linewidth conditions. Pumped with 532‐nm and nanosecond duration laser pulses of ≤ 0.01 cm−1 linewidth, only SBS can be generated in all tested liquids with a high nonlinear reflectivity. However when the pump spectral linewidth is ∼0.07 cm−1 or ∼0.8 cm−1, both SBS and SRS can be observed in benzene while only SRS can be generated in dimethyl sulfoxide; in all these cases SRS is the dominant contribution to the stimulated scattering but the efficiency values are drastically decreased due to the self‐termination behavior of SRS in liquids, which arises from the thermal self‐defocusing of both pump beam and SRS beam owing to Stokes‐shift related opto‐heating effect. In contrast, for SRS process in the two crystals, the thermal self‐defocusing influence is negligible benefitting from their much greater thermal conductivity, and a higher conversion efficiency of SRS generation can be retained under all three pump conditions.image

Broadband acoustic properties of a murine skull

It has been well recognized that the presence of a skull imposes harsh restrictions on the use of ultrasound and optoacoustic techniques in the study, treatment and modulation of the brain function. We propose a rigorous modeling and experimental methodology for estimating the insertion loss and the elastic constants of the skull over a wide range of frequencies and incidence angles. A point-source-like excitation of ultrawideband acoustic radiation was induced via the absorption of nanosecond duration laser pulses by a 20 μm diameter microsphere. The acoustic waves transmitted through the skull are recorded by a broadband, spherically focused ultrasound transducer. A coregistered pulse-echo ultrasound scan is subsequently performed to provide accurate skull geometry to be fed into an acoustic transmission model represented in an angular spectrum domain. The modeling predictions were validated by measurements taken from a glass cover-slip and ex vivo adult mouse skulls. The flexible semi-analytical formulation of the model allows for seamless extension to other transducer geometries and diverse experimental scenarios involving broadband acoustic transmission through locally flat solid structures. It is anticipated that accurate quantification and modeling of the skull transmission effects would ultimately allow for skull aberration correction in a broad variety of applications employing transcranial detection or transmission of high frequency ultrasound.

Short and long-term phototoxicity in cells expressing genetic reporters under nanosecond laser exposure

Nanosecond-duration laser pulses are exploited in a plethora of therapeutic and diagnostic applications, such as optoacoustic imaging. However, phototoxicity effects of pulsed radiation in living cells, in particular those expressing genetic reporters, are not well understood. We established a three-dimensional fluorescent protein expressing cellular model in order to reliably investigate the extent and major exposure parameters responsible for both photobleaching and phototoxicity under pulsed laser exposure, unveiling a variety of possible effects on living cells, from reversible photobleaching to cytotoxicity and cell death. Significant losses of fluorescence levels were identified when exposing the cells to illumination conditions considered safe under common standards for skin exposure in diagnostic imaging applications. Thus, the use of photolabile fluorescent proteins and their in vivo exposure parameters have to be designed carefully for all applications using pulsed nanosecond radiation. In particular, loss of signal due to bleaching may significantly alter signals in longitudinal measurements, making data quantification challenging.

Localized Heating of Nanostructures by Coherent Laser Pulses

By performing rigorous numerical simulations, we show that it is possible to create a nanoscale temperature distribution in a collection of metallic nanoparticles with the aid of laser coherent control, without the need for nanofabrication. For coherent laser pulses of nanosecond duration and optimized geometrical configurations of the nanoparticles, the temperature distribution can last for up to thousands of nanoseconds due to the larger relaxation times of the various cooling mechanisms. Using optimized laser pulses obtained by an adaptive-control process, one can address locally the temperature of a given target–nanoparticle paving the way for nanoscale chemical-reaction control via laser fields.

Optical emission spectroscopy in pulsed laser deposition of silicon

The generation of a homogeneous plasma plume is necessary for the pulsed laser deposition of thin films. In this work, we investigate the effects of nanosecond-duration laser pulses to ablate polycrystalline Si targets in vacuum (<10−4 Pa) at room temperature. The laser wavelength covers the range from ultra-violet to infrared by using a KrF (248 nm, 25 ns) and a Nd:YAG (1064 nm, 532 nm, 355 nm, 5 ns) laser. The films were deposited at laser fluences from 1 to 6 J/cm2 and characterized by atomic force microscopy and spectroscopic ellipsometry. Time-integrated optical emission spectra were obtained for excited neutrals and ionized Si species in the plasma produced between 0.5 and 11 J/cm2. The relation between the ionized species and film properties were discussed.

A comparative study of the ionic keV X-ray line emission from plasma produced by the femtosecond, picosecond and nanosecond duration laser pulses

We report here an experimental study of the ionic keV X-ray line emission from mag- nesium plasma produced by laser pulses of three widely different pulse durations (FWHM) of 45 fs, 25 ps and 3 ns, at a constant laser fluence of ∼1.5 × 10 4 Jc m −2 . It is observed that the X-ray yield of the resonance lines from the higher ionization states such as H- and He-like ions decreases on decreasing the laser pulse duration, even though the peak laser intensities of 3.5 × 10 17 Wc m −2 for the 45 fs pulses and 6.2 × 10 14 Wc m −2 for the 25 ps pulses are much higher than 5 × 10 12 Wc m −2 for the 3 ns laser pulse. The results were explained in terms of the ioniza- tion equilibrium time for different ionization states in the heated plasma. The study can be useful to make optimum choice of the laser pulse duration to produce short pulse intense X-ray line emission from the plasma and to get the knowledge of the degree of ionization in the plasma.