High-intensity Ultrashort Laser Pulses Research Articles

Squeezed Thermal Phonons Precurse Nonthermal Melting of Silicon as a Function of Fluence

What happens when an ultrashort high-intensity laser pulse heats up a silicon crystal? Accurate quantum-mechanical simulations of the motion of silicon atoms in the crystal reveal synchronous atomic oscillations between two distinct types of squeezed thermal-phonon modes.

Single-shot 35 fs temporal resolution electron shadowgraphy

We obtain single-shot time-resolved shadowgraph images of the electromagnetic fields resulting from the interaction of a high intensity ultrashort laser pulse with a metal surface. Using a high brightness relativistic electron beam and a high streaking speed radiofrequency deflector, we report <35 fs temporal resolution enabling a direct visualization of the retarded-time dominated field evolution which follows the laser-induced charge emission. A model including the finite signal propagation speed well reproduces the data and yields measurements of fundamental parameters in short pulse laser-matter interaction such as the amount of emitted charge and the emission time scale.

Evolution of high intensity ultrashort laser pulse spot size propagating through magnetized plasma

Evolution of the spot size of ultrashort laser pulse propagating in underdense magnetized cold plasma has been studied taking into account the effect of nonlinearity up to third order. The plasma is embedded in an external magnetic field with constant strength and variable direction on a plane perpendicular to electric field vector of the laser pulse. The time derivative of the third order nonlinear current density of plasma electron is used in the paraxial wave equation in plasma and the source dependence expansion method is employed to solve the equation. Results show that, influence of the direction of the external magnetic field on spot size is significant. So that, increasing angle between laser pulse magnetic field vector and external magnetic field decreases the effect of external magnetic field and leads to decreasing the focal length of plasma lens. Also, the effects of laser pulse intensity and external magnetic field strength on the self-focusing property of laser pulse for different direction of the external magnetic field have been studied.

Dynamics of laser-induced damage of spherical nanoparticles by high-intensity ultrashort laser pulses

Damage of a metal spherical nanoparticle by femtosecond laser pulses is analyzed by splitting the overall process into two steps. The fast step includes electron photoemission from a nanoparticle. It takes place during direct action of a laser pulse and its rate is evaluated as a function of laser and particle parameters by two approaches. Obtained results sug- gest the formation of significant positive charge of the nanoparticles due to the photoemission. The next step includes ion emission that removes the excessive positive charge and modifies particle structure. It is delayed with respect to the photo-emission and is analyzed by a simple analytical model and modified molecular dynamics. Obtained energy distribution suggests generation of fast ions capable of penetrating into surrounding material and generating defects next to the nanoparticle. The modeling is extended to the case of a nanoparticle on a solid surface to understand the basic mechanism of surface laser damage initiated by nano-contamina- tion. Simulations predict embedding the emitted ions into substrate within a spot with size significantly exceeding the original particle size. We discuss the relation of those effects to the problem of bulk and surface laser-induced damage of optical materials by single and multiple ultrashort laser pulses. © 2012 Society of Photo-Optical Instrumentation Engineers (SPIE). (DOI: 10 .1117/1.OE.51.12.121816)

Polarisation response of a gas medium in the field of a high-intensity ultrashort laser pulse: high order Kerr nonlinearities or plasma electron component?

The polarisation response of quantum systems modelling silver and xenon atoms in the field of a high-intensity femtosecond Ti : sapphire laser (photon energy ħω ≈ 1.5 eV), has been investigated by direct numerical integration of the Schrödinger equation. The applicability ranges of the perturbation theory and polarisation expansion in powers of field are determined. The contributions of excited atoms and electrons in the continuous-spectrum states to the polarisation response at the fundamental frequency, which arise as a result of excitation and photoionisation, are analysed. It is shown that specifically ionisation changes the sign of dielectric susceptibility with an increase in radiation intensity for the systems under consideration.

Relativistic effects in the interaction of high intensity ultra-short laser pulse with collisional underdense plasma

In this paper, the effect of weakly relativistic ponderomotive force in the interaction of intense laser pulse with nonisothermal, underdense, collisional plasma is studied. Ponderomotive force modifies the electron density and temperature distribution. By considering the weakly relativistic effect and ohmic heating of plasma electrons, the nonlinear dielectric permittivity of plasma medium is obtained and the equation of electromagnetic wave propagation in plasma is solved. It is shown that with considering the ohmic heating of electrons and collisions, the effect of ponderomotive force in weakly relativistic regime leads to steepening the electron density profile and increases the temperature of plasma electrons noticeably. Bunches of electrons in plasma become narrower. By increasing the laser pulse strength, the wavelength of density oscillations decreases. In this regime of laser-plasma interaction, electron temperature increases sharply by increasing the intensity of laser pulse. The amplitude of electric and magnetic fields increases by increasing the laser pulse energy while their wavelength decreases and they lost their sinusoidal form.

Schwinger vacuum pair production in chirped laser pulses

The recent developments of high intensity ultra-short laser pulses have raised the hopes of observing Schwinger vacuum pair production which is one of the important non-perturbative phenomena in Quantum electrodynamics (QED). The quantitative analysis of realistic high intensity laser pulses is vital for understanding the effect of the field parameters on the momentum spectrum of the produced particles. In this study, we analyze chirped laser pulses with a sub-cycle structure, and investigate the effects of the chirp parameter on the momentum spectrum of the produced particles. The combined effect of the chirp and carrier phase of the laser pulse is also analyzed. These effects are qualitatively explained by investigating the turning point structure of the potential within the framework of the complex WKB scattering approach to pair production.

Long-lived laser-induced microwave plasma guides in the atmosphere: Self-consistent plasma-dynamic analysis and numerical simulations

A detailed model of plasma dynamics, which self-consistently integrates plasma-kinetic, Navier–Stokes, electron heat conduction, and electron-vibration energy transfer equations, is used to quantify the limitations on the lifetime of microwave plasma waveguides induced in the atmosphere through filamentation with high-intensity ultrashort laser pulses further sustained by long laser pulses. We demonstrate that a near-infrared or midinfrared laser pulse can tailor plasma decay in the wake of a filament, efficiently suppressing, through electron temperature increase, the attachment of electrons to neutral species and dissociative recombination, thus substantially increasing the plasma-guide lifetime and facilitating long-distance transmission of microwaves.

Efficient laser-ion acceleration from closely stacked ultrathin foils

A new scheme to efficiently accelerate protons by a single linear polarized high-intensity ultrashort laser pulse using multiple ultrathin foils is proposed. The foils are stacked at a spacing comparable to their thickness and subsequently irradiated by the same laser pulse. The foil thicknesses are chosen such that the laser light pressure can displace all electrons out of the foil. The authors present a simple, yet precise dynamical model of the acceleration process from which both optimum foil thickness and spacing can be derived. Extensive two-dimensional (2D) particle-in-cell simulations verify the model predictions and suggest an enhancement of the maximum proton kinetic energy by 30% for the two-foil case compared to a single foil.

Neutron production from 7Li(d,xn) nuclear fusion reactions driven by high-intensity laser–target interactions

Numerical simulations of neutron production from deuterium–lithium nuclear fusion reactions have been performed. A set of differential cross sections for the 7Li(d,xn) reaction for incident deuteron energies of up to 50 MeV is assembled. The angular distribution of neutrons from a thick lithium target is simulated and benchmarked against experimental data. Two-stage neutron production from laser–target experiments has been studied as a function of laser intensity and energy. During the first stage a well collimated deuteron beam is generated using a high-intensity ultrashort pulse laser. During the second stage it is transported through a lithium target using a 3D Monte-Carlo ion beam–target deposition model. The neutron yield is estimated to be ∼108 neutrons J−1 laser energy. Some 1010 neutrons can be expected from a ∼100 J petawatt-class laser. For incident deuteron energies above 1 MeV the proposed scheme for neutron production from d–Li reactions is superior to that from d–d reactions, producing a collimated beam of neutrons with higher neutron yield.

Molecular dynamics driven by ultrashort laser pulses: Interference effects due to rescattering

A time-dependent Schrodinger equation is integrated numerically to investigate the dynamics of a model molecular system driven by a high-intensity ultrashort laser pulse. Two-dimensional photoelectron momentum distributions are analyzed. Highly nonmonotonic electron angular distributions are obtained that cannot be explained by diffraction in the double-well potential of a molecular ion. The nonmonotonicity is also demonstrated for atomic ionization and is attributed to the interference that occurs between components of an electron wave packet after its rescattering from the parent ion. An analytical model explaining the observed effects is developed.

Laser Acceleration of Electron Beams to the GeV-class Energies in Gas Jets

In a laser-plasma wakefield accelerator, the ponderomotive force of an ultrashort high intensity laser pulse excites a longitudinal wave or plasma bubble in a way similar to the excitation of a wake wave behind a boat as it propagates on the water surface. Electric fields inside the plasma bubble can be several orders of magnitude higher than those available in conventional RF-based particle accelerator facilities which are limited by material breakdown. Therefore, if an electron bunch is properly phase-locked with the bubble's acceleration field, it can gain relativistic energies within an extremely short distance. Here, in the bubble regime we show the generation of stable and reproducible sub GeV, and GeV-class electron beams. Supported by three-dimensional particle-in-cell simulations, our experimental results show the highest acceleration gradients produced so far. Simulations suggested that the plasma bubble elongation should be minimized in order to achieve higher electron beam energies.

Photogeneration of membrane potential hyperpolarization and depolarization in non-excitable cells

We monitored femtosecond laser induced membrane potential changes in non-excitable cells using patchclamp analysis. Membrane potential hyperpolarization of HeLa cells was evoked by 780 nm, 80 fs laser pulses focused in the cellular cytoplasm at average powers of 30-60 mW. Simultaneous detection of intracellular Ca2+ concentration and membrane potential revealed coincident photogeneration of Ca2+ waves and membrane potential hyperpolarization. By using non-excitable cells, the cell dynamics are slow enough that we can calculate the membrane potential using the steady-state approximation for ion gradients and permeabilities, as formulated in the GHK equations. The calculations predict hyperpolarization that matches the experimental measurements and indicates that the cellular response to laser irradiation is biological, and occurs via laser triggered Ca2+ which acts on Ca2+ activated K+ channels, causing hyperpolarization. Furthermore, by irradiating the cellular plasma membrane, we observed membrane potential depolarization in combination with a drop in membrane resistance that was consistent with a transient laser-induced membrane perforation. These results entail the first quantitative analysis of location-dependent laser-induced membrane potential modification and will help to clarify cellular biological responses under exposure to high intensity ultrashort laser pulses.

Determination of a dose-like curve for active colour centres produced in LiF single crystals by ultrashort high intensity laser pulses and a preliminary investigation of their spectral and spatial properties by confocal and atomic microscopies

In this paper, the spatial and spectral properties of colour centres produced in LiF single crystals by ultrashort high intensity laser pulses are reported. The emission spectra of the centres were measured conventionally and also by confocal microscopy. A large amount of F centres that gave rise to aggregates such as F2, F4, F2+ and F3+ could be identified, showing a microscopic homogeneous distribution. The coloured surfaces were also spatially characterized by atomic force microscopy, and the images show the presence of defects on the modified surface. The formation of micrometric and sub-micrometric structures was observed and related to filamentation processes.

Neutron production from interactions of high-intensity ultrashort pulse laser with a planar deuterated polyethylene target

The deuteron acceleration from a planar uniform deuterated polyethylene (CD2) target was studied with a two-dimensional electromagnetic particle-in-cell method in the ultrarelativistic regime for peak laser intensities between 1023 and 1025W∕m2. Under identical conditions the deuteron energy and angular distributions are very different when compared to that produced from a double-layer target. The neutron production from D(d,n)–He3 nuclear fusion reactions was also investigated with a three-dimensional Monte Carlo ion beam-target deposition model. The neutron yield and its angular distribution were studied as a function of peak laser intensity and laser pulse duration. A neutron yield of 104–106neutronsperJoule laser energy is inferred from simulations of a 4 μm thick planar uniform CD2 target. The angular scattering of neutrons is found to be nonisotropic and has a significant component in the forward (laser propagation) direction.

Interaction of intense ultrashort pulse lasers with clusters

The dynamics of clusters composed of different material irradiated by a high-intensity ultrashort pulse laser was studied using a fully relativistic three-dimensional molecular dynamics model. Key parameters of the cluster evolution such as particle positions, energy absorption, and cluster explosion were simulated. By a direct comparison of these parameters for clusters of equal initial radius but made of different material (deuterium, neon, argon, and xenon), the main stages and attributes of cluster evolution were elucidated. The simulations showed that clusters made of different material act alike, especially those of heavy elements. Clusters made of heavy elements (neon, argon, and xenon) differentiate from clusters made of light elements (deuterium) by the magnitude of the absorbed energy per cluster and the final mean energy of exploding ions. What most distinguishes clusters composed of different material is the amount of emitted radiation and its spectral range.

Displacement of the spectrum of a light pulse composed of a small number of oscillations toward shorter wavelengths in a dielectric with plasma nonlinearity

The principal regularities are determined of the self-action of high-intensity ultrashort laser pulses, caused by ionization of a dielectric medium as the pulses propagate in it. It is shown that the plasma nonlinearity generated in dielectrics in the strong fields of pulses composed of a small number of oscillations causes profiling of the emission spectrum and shifts its maximum toward shorter wavelengths.

Ablation of metallic targets by high-intensity ultrashort laser pulses

Fundamental characteristics and underlying mechanisms of ultrafast ablation of aluminum and copper materials by short, from $50\phantom{\rule{0.3em}{0ex}}\mathrm{fs}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}24\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$, infrared laser pulses with relatively high-energy fluences are investigated in theory and experiment. Two different ablation regimes, nonthermal and thermal, are revealed to exist depending on the laser parameters. Theoretical analyses provide a detailed description of the underlying mechanisms, laser-induced stress and phase explosion, playing a dominant role in these ablation processes. Their induced ablation rates as a function of pulse duration are also simulated at several laser fluences. The interplay of the two competing mechanisms is found to be responsible for mutual transitions between thermal and nonthermal ablations. With the help of a modified two-temperature mode through considering e-e-collision dominated electron diffusions, a thermal-nonthermal ablation transition law is obtained successfully. The critical pulse length, acting as a division of the two ablation regimes, is demonstrated to reduce with increasing laser fluence. Both hole-drilling and line-scribing experiments of Al targets by using ultrashort laser pulses are performed for the confirmation of our theory. Morphological changes of the ablated areas are characterized through scanning electron microscopy. Transitions between the two different ablation regimes are achieved by proper selection of the laser pulse time duration for a given laser fluence. Experimental results are consistent with theoretical predictions.

Backscattering of a high-intensity ultrashort laser pulse from a solid target at oblique incidence

The results of experimental measurement and analytical estimation of the coefficient of backward reflection from targets made of various materials upon their irradiation by a picosecond laser pulse having the intensity of (0.8–4) × 1018 W/cm2 are presented. It is shown that the induced surface instability leads to the formation of a profile backscattering the incident radiation. The coefficient of backward reflection achieves a few percent and has an intensity optimum depending on the target material.

Controlled rotational Raman echo recurrences and modulation of high-intensity ultrashort laser pulses by molecular rotations in the gas phase

We show that sequences of femtosecond laser pulses can control the rotational Raman response of a gas mixture, giving rise to a tunable manifold of echo recurrences in the retarded nonlinear-optical response of the gas. Tailored phase masks for high-intensity ultrashort laser pulses are experimentally demonstrated using molecular rotations in the gas phase driven by optimally time-ordered pulse sequences.