Interaction Of Short Laser Pulses Research Articles

Modeling of Short-Pulse Laser Interactions with Monolithic and Porous Silicon Targets with an Atomistic-Continuum Approach.

The acquisition of reliable knowledge about the mechanism of short laser pulse interactions with semiconductor materials is an important step for high-tech technologies towards the development of new electronic devices, the functionalization of material surfaces with predesigned optical properties, and the manufacturing of nanorobots (such as nanoparticles) for bio-medical applications. The laser-induced nanostructuring of semiconductors, however, is a complex phenomenon with several interplaying processes occurring on a wide spatial and temporal scale. In this work, we apply the atomistic-continuum approach for modeling the interaction of an fs-laser pulse with a semiconductor target, using monolithic crystalline silicon (c-Si) and porous silicon (Si). This model addresses the kinetics of non-equilibrium laser-induced phase transitions with atomic resolution via molecular dynamics, whereas the effect of the laser-generated free carriers (electron-hole pairs) is accounted for via the dynamics of their density and temperature. The combined model was applied to study the microscopic mechanism of phase transitions during the laser-induced melting and ablation of monolithic crystalline (c-Si) and porous Si targets in a vacuum. The melting thresholds for the monolithic and porous targets were found to be 0.32 J/cm2 and 0.29 J/cm2, respectively. The limited heat conduction mechanism and the absence of internal stress accumulation were found to be involved in the processes responsible for the lowering of the melting threshold in the porous target. The results of this modeling were validated by comparing the melting thresholds obtained in the simulations to the experimental values. A difference in the mechanisms of ablation of the c-Si and porous Si targets was considered. Based on the simulation results, a prediction regarding the mechanism of the laser-assisted production of Si nanoparticles with the desired properties is drawn.

Betatron Radiation and Bremsstrahlung in the Interaction of Intense Laser Pulse with Solid Target

X-ray generation via synchrotron radiation and bremsstrahlung in the interaction of short laser pulses with a solid target is of much current interest owing to its numerous applications. The efficiency of laser to X-ray energy conversion is thus a crucial factor. We found that the energy conversion efficiency of synchrotron radiation and bremsstrahlung is mainly governed by the ratio of the laser pulse width to the preplasma width, which is in turn governed by the laser profile, intensity, and spot size. Synchrotron radiation dominates when the ratio is less than unity, otherwise bremsstrahlung dominates. The type of radiation can thus be controlled by tailoring the laser parameters.

Расчет разлета мишени для задач оптимизации лазерного ускорения ионов

At present, a significant part of experiments on the interaction of high-power short laser pulses with solid targets is faced with the problem of target modification under the action of a nanosecond prepulse. In this paper, a series of hydrodynamic calculations of target irradiation with a nanosecond laser pulse is performed, which describes the plasma density profiles arising during the target expansion as a function of the parameters of the laser prepulse/additional pulse. The results obtained can make it possible to improve the efficiency of ion acceleration in the interaction of short laser pulses with a profiled plasma target.

Experimental verification of TNSA protons and deuterons in the multi-picosecond moderate intensity regime

Ion acceleration from high intensity short pulse laser interactions is of great interest due to a number of applications, and there has been significant work carried out with laser energies up to a few 100 J with 10's of femtosecond to 1 ps pulse durations. Here, we report results from an experiment at the OMEGA EP laser, where laser energy and pulse length were varied from 100 to 1250 J and 0.7–30 ps, respectively, in the moderate (2×1017–2×1018 W/cm2) laser intensity regime. Ions and electrons were simultaneously measured from disk targets made of CH and CD by a Thomson parabola and a magnetic spectrometer, respectively. Measurements showed that the electron temperature, Te (MeV), has a dependence on the laser energy, EL (J), and pulse duration, τL (ps), and its empirical scaling was found to be 0.015×EL0.90τL−0.48. The maximum proton and deuteron energies are linearly dependent on the electron temperature, (5.60 ± 0.26)×Te and (3.17 ± 0.18)×Te, respectively. A significant increase in proton numbers with the laser energy was also observed. The increase in the maximum proton energy and proton count with higher energy longer duration pulses presented in this article shows that such laser conditions have a great advantage for applications, such as the proton radiograph, in the moderate laser intensity regime.

Isochoric heating of solid-density plasmas beyond keV temperature by fast thermal diffusion with relativistic picosecond laser light.

The interaction of relativistic short-pulse lasers with matter produces fast electrons with over megaampere currents, which supposedly heats a solid target isochorically and forms a hot dense plasma. In a picosecond timescale, however, thermal diffusion from hot preformed plasma turns out to be the dominant process of isochoric heating. We describe a heating process, fast thermal diffusion, launched from the preformed plasma heated resistively by the fast electron current. We demonstrate the fast thermal diffusion in the keV range in a solid density plasma by a series of one-dimensional particle-in-cell simulations. A theoretical model of the fast thermal diffusion is developed and we derive the diffusion speed as a function of the laser amplitude and target density. Under continuous laser irradiation, the diffusion front propagates at a constant speed in uniform plasma. Our model can provide a guideline for fast isochoric heating using future kilojoule petawatt lasers.

A Simple Solver for the Two-Fluid Plasma Model Based on PseudoSpectral Time-Domain Algorithm

We present a solver of 3D two-fluid plasma model for the simulation of short-pulse laser interactions with plasma. This solver resolves the equations of the two-fluid plasma model with ideal gas closure. We also include the Bhatnagar-Gross-Krook collision model. Our solver is based on PseudoSpectral Time-Domain (PSTD) method to solve Maxwell's curl equations. We use a Strang splitting to integrate Euler equations with source term: while Euler equations are solved with a composite scheme mixing Lax-Friedrichs and Lax-Wendroff schemes, the source term is integrated with a fourth-order Runge-Kutta scheme. This two-fluid plasma model solver is simple to implement because it only relies on finite difference schemes and Fast Fourier Transforms. It does not require spatially staggered grids. The solver was tested against several well-known problems of plasma physics. Numerical simulations gave results in excellent agreement with analytical solutions or with previous results from the literature.

Terahertz radiation generation from short-pulse laser interaction with electron-hole plasmas

In semiconductors, electrons and their corresponding holes exist as free carriers that are created in pairs in inter-band transitions. At very high carrier densities, the carrier-carrier collisions dominate over carrier-lattice collisions and these carriers begin to behave collectively to form a plasma. In this study, we present phase-matched terahertz (THz) radiation generation from laser interaction with electron-hole plasmas. Two collinear laser pulses of finite spot size propagate in an electron-hole plasma to generate THz radiation. The lasers beat and exert a ponderomotive force on electrons and holes. The carriers oscillate with oscillatory velocity at the laser beat frequency. The laser beat wave interacts with the plasma density perturbation and produces a nonlinear transverse current with nonzero curl. This nonlinear current drives coherent THz radiation. Spacial density modulation of hole density is considered to satisfy the phase-matching condition for resonant interaction. The systematic analysis of this mechanism shows an efficient and tunable source of THz radiation, which varies with various parameters such as the density modulation, laser intensity and other laser parameters.

Electromagnetic pulse emission from target holders during short-pulse laser interactions

For the first time, a global model of electromagnetic pulse (EMP) emission connects charge separation in the laser target to quantitative measurements of the electromagnetic field. We present a frequency-domain dipole antenna model which predicts the quantity of charge accumulated in a laser target as well as the EMP amplitude and frequency. The model is validated against measurements from several high-intensity laser facilities, providing insight into target physics and informing the design of next-generation ultra-intense laser facilities. EMP amplitude is proportional to the total charge accumulated on the target, but we demonstrate that it is not directly affected by target charging time (and therefore the laser pulse duration), provided the charging time is shorter than the antenna characteristic time. We propose two independent methods for estimating the charging time based on the laser pulse duration. We also investigate the impact of target holder geometry on EMPs using cylindrical, conical, and helical holders.

Development of a predictive capability of short-pulse laser-driven broadband x-ray radiography

High intensity, short-pulse laser interaction with a solid metal target produces broadband hard x-rays potentially for various applications of x-ray radiography. Here experimental benchmarking of numerical modelling for short-pulse laser-driven broadband x-ray radiography is presented. Angular dependent x-ray spectra are first calculated with a hybrid particle-in-cell code, Large Scale Plasma (LSP), using fast electron parameters inferred from an analysis of measured bremsstrahlung signals. Subsequently, a calculated x-ray spectrum in the direction of radiography is used in photon transport calculations using a Monte Carlo code, Particle and Heavy Ion Transport code System (PHITS), to simulate a radiographic image including a modelled 3D test object, an x-ray attenuation filter and an image plate detector. Simulated radiographic images are compared with measurements obtained in an experiment using a 50-TW Leopard short-pulse laser at the University of Nevada Reno. Results show that simulations reproduce the experimental images well for three different attenuation filters (plastic, aluminium, and brass), while 1D transmission profiles for the plastic and aluminium filters are quantitatively in good agreement. The modelling approach established in this work could be used as a predictive tool to simulate radiographic images of complex 3D solid objects at any arbitrary angular position or to optimize experimental components such as the source spectrum, x-ray attenuation filters and a detector type depending on a radiographic object without carrying out radiographic experiments.

Solving a System of Differential Equations Containing a Diffusion Equation with Nonlinear Terms on the Example of Laser Heating in Silicon

We present a finite-difference integration algorithm for solution of a system of differential equations containing a diffusion equation with nonlinear terms. The approach is based on Crank–Nicolson method with predictor–corrector algorithm and provides high stability and precision. Using a specific example of short-pulse laser interaction with semiconductors, we give a detailed description of the method and apply it to the solution of the corresponding system of differential equations, one of which is a nonlinear diffusion equation. The calculated dynamics of the energy density and the number density of photoexcited free carriers upon the absorption of laser energy are presented for the irradiated thin silicon film. The energy conservation within 0.2 % has been achieved for the time step 10 8 times larger than that in case of the explicit scheme, for the chosen numerical setup. The implemented Fortran source code is available in the Supplementary Materials. We also present a few examples of successful application of the method demonstrating its benefits for the theoretical studies of laser–matter interaction problems. Finally, possible extension to 2 and 3 dimensions is discussed.

Numerical Investigation of Ultrashort Laser-Ablative Synthesis of Metal Nanoparticles in Liquids Using the Atomistic-Continuum Model.

We present a framework based on the atomistic continuum model, combining the Molecular Dynamics (MD) and Two Temperature Model (TTM) approaches, to characterize the growth of metal nanoparticles (NPs) under ultrashort laser ablation from a solid target in water ambient. The model is capable of addressing the kinetics of fast non-equilibrium laser-induced phase transition processes at atomic resolution, while in continuum it accounts for the effect of free carriers, playing a determinant role during short laser pulse interaction processes with metals. The results of our simulations clarify possible mechanisms, which can be responsible for the observed experimental data, including the presence of two populations of NPs, having a small (5–15 nm) and larger (tens of nm) mean size. The formed NPs are of importance for a variety of applications in energy, catalysis and healthcare.

Bremsstrahlung spectrum and photon dose from short-pulse high-intensity laser interaction on various metal targets

During the interaction of an intense picosecond laser pulse with a plasma created by a plastic foil ablated by a nanosecond laser pulse, relativistic electrons are produced. A metal solid target placed behind the foil allows converting these high-energy electrons into hard X-rays. The use of an ablated CH foil allows maximizing the conversion efficiency and thus the X-ray emission. In this study, the photon energy spectrum and dose are measured for different thicknesses of various metal targets such as tantalum. Numerical simulations including hydrodynamical radiative, particle-in-cell, and Monte Carlo codes are made to give comparison with the experimental data. These are also compared with that of a bremsstrahlung emission and photon dose model in which the energy loss by Ohmic heating arising from the return current driven by the background electrons of the conductive target is taken into account [A. Compant La Fontaine, Phys. Plasmas 25, 043301 (2018)]. The results obtained allow for benchmarks to test the accuracy of this model and to check that the dose is maximized for high-Z solid targets and thickness in the mm range in the relativistic interaction regime at ultrahigh laser intensity (>1018 W/cm2).

Laser-irradiated gold nanoparticles for breast cancer therapy

Current study involves laser-induced gold nanoparticles (GNPs) for breast cancer therapy. GNPs as a therapeutic agent using nanophotolysis approach are discussed. Coulomb explosion of GNPs is studied numerically using MATLAB. Our findings show that [Formula: see text] ions are generated for short pulse laser interaction of 8 ns and laser irradiation of 2 J/cm2 penetrates in the tumor depth of 3.30 cm. Cluster size of 98 nm produced [Formula: see text] nanobullets that target the breast tumor. Short pulse laser of 2.27 ns generates maximum shock front velocity of [Formula: see text] cm/s. Shock front of 20–146 [Formula: see text]m is produced for effectively damaging the breast cancer tissues. A total of 825 cells are damaged by [Formula: see text] ions. Current results are matched with the literature and are in good correlation. Hence, we conclude that laser-irradiated GNPs using nanophotolysis approach are useful for targeted therapy of breast tumor.

Interaction of short laser pulses with model contamination microparticles on a high reflector.

The response of model contamination particles located on the surface of a multilayer dielectric mirror when exposed to 1053nm laser pulses of 10ps or 0.6ps duration is investigated. Four different particle types were studied: stainless steel, borosilicate glass, polyethylene, and polytetrafluoroethylene, all having an average diameter of about 40μm. Irradiation with one laser pulse caused particles to eject from the surface with an onset fluence in the range 5× to 100×, depending on the particle type, below the particle-free, laser-induced damage threshold of the mirror. Morphological analysis showed, however, that the ejection process always generated ablation craters and/or secondary contamination, both of which can degrade the performance of the optic during subsequent pulses. Ejection and damage mechanisms are discussed for each particle type.

Thin target charging in short laser pulse interactions

A model of target charging in interaction with an intense short laser pulse is revised. It provides estimates of the current of energetic electrons escaping the target and the total accumulated electric charge as a function of the laser pulse characteristics and target thickness. Comparisons with experimental data show the model performance, in particular, in the case of targets thinner than the mean-free path of energetic electrons.

Optimization of electron acceleration by short laser pulses from low-density targets

Based on multidimensional particle-in-cell simulations of a short laser pulse interaction with a homogeneous planar target, we perform an optimization study to find the best design parameters for maximizing the number of high-energy electrons generated by a sub-petawatt class laser system for deep gamma radiography purposes. We find that a low-density target with an electron density of 10% of the critical density and a thickness of 240 μm irradiated by a 30 fs 4 J laser pulse can generate 7-nC electron bunches with a mean characteristic energy of 100 MeV.

Effect of pre-plasma on the ion acceleration by intense ultra-short laser pulses

AbstractIn the interaction of short-laser pulses with a solid density target, pre-plasma can play a major role in ion acceleration processes. So far, complete analysis of pre-plasma effect on the ion acceleration by ultra-short laser pulses in the radiation pressure acceleration (RPA) regime has been unknown. Then the effect of pre-plasma on the ion acceleration efficiency is analyzed by numerical results of the particle-in-cell simulation in the RPA regime. It is shown that, for long-laser pulses (τp > 50 fs), the presence of pre-plasma makes a destructive effect on ion acceleration while it may have a contributing effect for short-laser pulses (τp < 50 fs). Therefore, the 35 fs (20 fs) laser pulse can accelerate ions up to 40 MeV (55 eV), which is almost two (three) times larger in energy rather than use of a 100 fs pulse with the same pre-plasma scale length.

Strong electromagnetic pulses generated in high-intensity short-pulse laser interactions with thin foil targets

AbstractMeasurements are reported of the target neutralization current, the target charge, and the tangential component of the magnetic field generated as a result of laser–target interaction by pulses with the energy in the range of 45–92 mJ on target and the pulse duration from 39 to 1000 fs. The experiment was performed at the Eclipse facility in CELIA, Bordeaux. The aim of the experiment was to extend investigations performed for the thick (mm scale) targets to the case of thin (μm thickness) targets in a way that would allow for a straightforward comparison of the results. We found that thin foil targets tend to generate 20–50% higher neutralization current and the target charge than the thick targets. The measurement of the tangential component of the magnetic field had shown that the initial spike is dominated by the 1 ns pulse consistent with the 1 ns pulse of the neutralization current, but there are some differences between targets of different types on sub-ns scale, which is an effect going beyond a simple picture of the target acting as an antenna. The sub-ns structure appears to be reproducible to surprising degree. We found that there is in general a linear correlation between the maximum value of the magnetic field and the maximum neutralization current, which supports the target-antenna picture, except for pulses 100s of fs long.

Terahertz generation in crystal driven by two-color laser pulses‡

SynopsisWe study the generation of terahertz (THz) radiation in 1D crystal driven by two-color laser pulses by solving the time-dependent Schrödinger equations. Our results indicate that stronger THz radiation can be obtained from the interaction of short laser pulse with crystals.

Generation of Subsurface Voids, Incubation Effect, and Formation of Nanoparticles in Short Pulse Laser Interactions with Bulk Metal Targets in Liquid: Molecular Dynamics Study.

The ability of short pulse laser ablation in liquids to produce clean colloidal nanoparticles and unusual surface morphology has been employed in a broad range of practical applications. In this paper, we report the results of large-scale molecular dynamics simulations aimed at revealing the key processes that control the surface morphology and nanoparticle size distributions by pulsed laser ablation in liquids. The simulations of bulk Ag targets irradiated in water are performed with an advanced computational model combining a coarse-grained representation of liquid environment and an atomistic description of laser interaction with metal targets. For the irradiation conditions that correspond to the spallation regime in vacuum, the simulations predict that the water environment can prevent the complete separation of the spalled layer from the target, leading to the formation of large subsurface voids stabilized by rapid cooling and solidification. The subsequent irradiation of the laser-modified surface is found to result in a more efficient ablation and nanoparticle generation, thus suggesting the possibility of the incubation effect in multipulse laser ablation in liquids. The simulations performed at higher laser fluences that correspond to the phase explosion regime in vacuum reveal the accumulation of the ablation plume at the interface with the water environment and the formation of a hot metal layer. The water in contact with the metal layer is brought to the supercritical state and provides an environment suitable for nucleation and growth of small metal nanoparticles from metal atoms emitted from the hot metal layer. The metal layer itself has limited stability and can readily disintegrate into large (tens of nanometers) nanoparticles. The layer disintegration is facilitated by the Rayleigh–Taylor instability of the interface between the higher density metal layer decelerated by the pressure from the lighter supercritical water. The nanoparticles emerging from the layer disintegration are rapidly cooled and solidified due to the interaction with water environment, with a cooling rate of ∼2 × 1012 K/s observed in the simulations. The computational prediction of two distinct mechanisms of nanoparticle formation yielding nanoparticles with different characteristic sizes provides a plausible explanation for the experimental observations of bimodal nanoparticle size distributions in laser ablation in liquids. The ultrahigh cooling and solidification rates suggest the possibility for generation of nanoparticles featuring metastable phases and highly nonequilibrium structures.