High-intensity Laser Pulses Research Articles

Characterization of pure and mixed Ar, Kr and Xe gas jets generated by different nozzles and a study of X-ray radiation yields after interaction with a sub-ps laser pulse

Gas jets accelerated through a linear supersonic and a conical nozzle, comprising a monomer/cluster mix, were characterized at UNR using a Mach-Zehnder type interferometer and Rayleigh scattering. A comparison of the two nozzle types is presented, showing that the linear nozzle produces gas jets of an order of magnitude denser than the conical nozzle. The linear gas jets of Ar, Kr, and Xe as well as triple mixtures with different percentages of each of the aforementioned gases were characterized. The densest gas jets used Ar as the target gas, while the least dense jets came from Kr. Cluster radii of the pure gases were measured, and Xe gas jets were found to produce the largest gas clusters. A study of X-ray generation by gas jet-laser plasma was performed at the UNR Leopard laser (1.057 μm, 350 fs, ∼1019 W/cm2) on the linear nozzle. The gas jets were irradiated with a high-intensity sub-ps laser pulse. An absolute X-ray output of the laser-gas jet interactions measured by the calibrated PCDs is presented and show that triple mixtures of Xe, Kr, and Ar each exhibited a higher X-ray yield compared to the pure gases. A strong anisotropy of X-ray radiation with respect to laser beam polarization direction is observed in all the gas jets. In fact, this anisotropy is different in three spectral regions (>1.4, 3.5 and 9 keV).

Controlling the Temporal Pulse Shape of the Passively Q-Switched Laser by Nonlinear Feedback Control

Q-switched lasers are high intensity laser pulses with extremely short durations of a few nanoseconds. The application of such laser pulses is ubiquitous [1,2]. In many dermatological procedures, intense energy pulses of Q-switched lasers are used to evaporate pigmented cells with high precision while the ultra-short pulse duration prevents damaging the surrounding healthy tissues. These lasers are also used in many industrial applications that require precision cutting, ranging from laser ablation and micromachining to the high-speed drilling of holes. However, the necessary precision of desired pulse characteristics has become increasingly demanding. Ever shrinking dimensions of micromachining call for reliable pulses with predictable properties. Currently, the difficulty of producing identical pulses of prescribed intensity, shape, and duration limits the use of Q-switched lasers. While the authors have previously worked on actively Q-switched lasers [3], this work focuses on passive Q-switching, where saturable absorber materials function as the internal switch in the laser cavity. In this paper theoretical tools from the field of nonlinear dynamical control systems [4] are applied to design a feedback controller circuit, which can produce a Q-switched pulse with customized characteristics by varying the pump rate, which can be influenced by the pump power. The pump power is considered as the input to the system and the photon number is the output of the system. The population inversion, the saturable absorber concentration and the photon number are considered as the states of the system. First it is demonstrated that by applying nonlinear state feedback, the Q-switched laser system can be transformed into a pair of two cascaded integrators. Therefore, the desired output can be customized by applying its second derivative as the input. Cr:YSO Q-switched Cr:LiSAF laser is used as an example to demonstrate our method. Both theoretical analysis of the method and the numerical simulation results are presented.

Ion acceleration in the transparent regime and the critical influence of the plasma density scale length

The influence of a plasma density gradient on ions accelerated along the specular (back reflection) direction in the transparent Target Normal Sheath Acceleration regime is investigated. Enhanced acceleration of ions is experimentally observed in this regime using high-intensity and ultra-high contrast laser pulses and extremely thin foils of few nanometer thicknesses. The experimental trend for the maximum proton energy appeared quite different from the already published numerical results in this regime where an infinitely steep plasma gradient was assumed. We showed that for a realistic modelling, a finite density gradient has to be taken into account. By means of particle-in-cell (PIC) simulations, we studied for the first time the influence of the plasma density scale length on ion acceleration from these nanofoil targets. Through a qualitative agreement between our numerical particle-in-cell simulations and our experiments, the main conclusion with regard to the experimental requirements is that, in the transparent regime evidenced with nanofoils as compared to the opaque regime, the plasma expansion has to be taken into account and both the pulse contrast and the damage threshold of the material are essential parameters.

Stability of a modified Peaceman–Rachford method for the paraxial Helmholtz equation on adaptive grids

This study concerns the asymptotic stability of an eikonal, or ray, transformation based Peaceman–Rachford splitting method for solving the paraxial Helmholtz equation with high wave numbers. Arbitrary nonuniform grids are considered in transverse and beam propagation directions. The differential equation targeted has been used for modeling propagations of high intensity laser pulses over a long distance without diffractions. Self-focusing of high intensity beams may be balanced with the de-focusing effect of created ionized plasma channel in the situation, and applications of grid adaptations are frequently essential. It is shown rigorously that the fully discretized oscillation-free decomposition method on arbitrary adaptive grids is asymptotically stable with a stability index one. Simulation experiments are carried out to illustrate our concern and conclusions.

Strong-field Breit-Wheeler pair production in short laser pulses: Relevance of spin effects

Production of electron-positron pairs in the collision of a high-energy photon with a high-intensity few-cycle laser pulse is studied. By utilizing the frameworks of laser-dressed spinor and scalar quantum electrodynamics, a comparison between the production of pairs of Dirac and Klein-Gordon particles is drawn. Positron energy spectra and angular distributions are presented for various laser parameters. We identify conditions under which predictions from Klein-Gordon theory either closely resemble or largely differ from those of the proper Dirac theory. In particular, we address the question to which extent the relevance of spin effects is influenced by the short duration of the laser pulse.

Infrared Laser Activation of Soluble and Membrane Protein Assemblies in the Gas Phase.

Collision-induced dissociation (CID) is the dominant method for probing intact macromolecular complexes in the gas phase by means of mass spectrometry (MS). The energy obtained from collisional activation is dependent on the charge state of the ion and the pressures and potentials within the instrument: these factors limit CID capability. Activation by infrared (IR) laser radiation offers an attractive alternative as the radiation energy absorbed by the ions is charge-state-independent and the intensity and time scale of activation is controlled by a laser source external to the mass spectrometer. Here we implement and apply IR activation, in different irradiation regimes, to study both soluble and membrane protein assemblies. We show that IR activation using high-intensity pulsed lasers is faster than collisional and radiative cooling and requires much lower energy than continuous IR irradiation. We demonstrate that IR activation is an effective means for studying membrane protein assemblies, and liberate an intact V-type ATPase complex from detergent micelles, a result that cannot be achieved by means of CID using standard collision energies. Notably, we find that IR activation can be sufficiently soft to retain specific lipids bound to the complex. We further demonstrate that, by applying a combination of collisional activation, mass selection, and IR activation of the liberated complex, we can elucidate subunit stoichiometry and the masses of specifically bound lipids in a single MS experiment.

Temporal laser-pulse-shape effects in nonlinear Thomson scattering

The influence of the laser pulse temporal shape on the Nonlinear Thomson Scattering on-axis photon spectrum is analyzed in detail. Using the classical description, analytical expressions for the temporal and spectral structure of the scattered radiation are obtained for the case of symmetric laser pulse shapes. The possibility of reconstructing the incident laser pulse from the scattered spectrum averaged over interference fringes in the case of high peak intensity and symmetric laser pulse shape is discussed.

Transient Plasma Photonic Crystals for High-Power Lasers.

A new type of transient photonic crystals for high-power lasers is presented. The crystal is produced by counterpropagating laser beams in plasma. Trapped electrons and electrically forced ions generate a strong density grating. The lifetime of the transient photonic crystal is determined by the ballistic motion of ions. The robustness of the photonic crystal allows one to manipulate high-intensity laser pulses. The scheme of the crystal is analyzed here by 1D Vlasov simulations. Reflection or transmission of high-power laser pulses are predicted by particle-in-cell simulations. It is shown that a transient plasma photonic crystal may act as a tunable mirror for intense laser pulses. Generalizations to 2D and 3D configurations are possible.

Demonstration of a positron beam-driven hollow channel plasma wakefield accelerator.

Plasma wakefield accelerators have been used to accelerate electron and positron particle beams with gradients that are orders of magnitude larger than those achieved in conventional accelerators. In addition to being accelerated by the plasma wakefield, the beam particles also experience strong transverse forces that may disrupt the beam quality. Hollow plasma channels have been proposed as a technique for generating accelerating fields without transverse forces. Here we demonstrate a method for creating an extended hollow plasma channel and measure the wakefields created by an ultrarelativistic positron beam as it propagates through the channel. The plasma channel is created by directing a high-intensity laser pulse with a spatially modulated profile into lithium vapour, which results in an annular region of ionization. A peak decelerating field of 230 MeV m−1 is inferred from changes in the beam energy spectrum, in good agreement with theory and particle-in-cell simulations.

Supercomputations and big-data analysis in strong-field ultrafast optical physics: filamentation of high-peak-power ultrashort laser pulses

High-intensity ultrashort laser pulses propagating in gas media or in condensed matter undergo complex nonlinear spatiotemporal evolution where temporal transformations of optical field waveforms are strongly coupled to an intricate beam dynamics and ultrafast field-induced ionization processes. At the level of laser peak powers orders of magnitude above the critical power of self-focusing, the beam exhibits modulation instabilities, producing random field hot spots and breaking up into multiple noise-seeded filaments. This problem is described by a (3 + 1)-dimensional nonlinear field evolution equation, which needs to be solved jointly with the equation for ultrafast ionization of a medium. Analysis of this problem, which is equivalent to solving a billion-dimensional evolution problem, is only possible by means of supercomputer simulations augmented with coordinated big-data processing of large volumes of information acquired through theory-guiding experiments and supercomputations. Here, we review the main challenges of supercomputations and big-data processing encountered in strong-field ultrafast optical physics and discuss strategies to confront these challenges.

Comparison of Vanadium Oxide Thin Films Prepared Using Femtosecond and Nanosecond Pulsed Laser Deposition

ABSTRACTPulsed laser deposition (PLD) is a technique which utilizes a high energy pulsed laser ablation of targets to deposit thin films on substrates in a vacuum chamber. The high-intensity laser pulses create a plasma plume from the target material which is projected towards the substrate whereupon it condenses to deposit a thin film. Here we investigate the properties of vanadium oxide thin films prepared utilizing two variations of the pulsed laser deposition (PLD) technique: femtosecond PLD and nanosecond PLD. Femtosecond PLD (f-PLD) has a significantly higher peak intensity and shorter duration laser pulse compared to that of the excimer-based nanosecond PLD (n-PLD). Experiments have been conducted on the growth of thin films prepared from V2O5 targets on glass substrates using f-PLD and n-PLD. Characterization using SEM, XRD and Raman spectroscopy shows that the f-PLD films have significantly rougher texture prior to annealing and exhibit with an amorphous nano-crystalline character whereas the thin films grown using n-PLD are much smoother and highly predominantly amorphous. The surface morphology, structural, vibrational, and chemical- and electronic-state elemental properties of the vanadium oxide thin films, both prior to and after annealing to 450 °C, will be discussed.

High-power LD side-pump Nd: YAG regenerative amplifier at 1 kHz repetition rate with volume Bragg gratings (VBG) for broadening and compressor

Pulse width of 8.7 ps was broadened to 102.2, 198 ps with single and double pass the VBG respectively. When the 102.2 ps pulse was injected into 1 kHz repetition rate of LD side-pump Nd: YAG regenerative amplifier (RA), pulse width of 87.5 ps at 1 kHz was obtained with the pulse energy of 9.4 mJ, the beam quality of M^2 factor was 1.2. The pulse width was compressed to 32.7 ps with a single pass VBG and the pulse energy reduced to 8.8 mJ, and the power density was up to 15.2 GW/cm2, the stability for pulse to pulse rms is about 0.6 %, beam pointing was about 35 μrad. In addition, when 198 ps pulse was injected into RA, pulse width of 156 ps was obtained which energy was 9.6 mJ, the pulse width was compressed to 38 ps by double passing the VBG, the pulse energy decreased to 8.5 mJ. Chirped VBG is a new way to obtain high-intensity picosecond pulse laser system simple and smaller.

Energy distribution of fast electrons accelerated by high intensity laser pulse depending on laser pulse duration

The dependence of high-energy electron generation on the pulse duration of a high intensity LFEX laser was experimentally investigated. The LFEX laser (λ = 1.054 and intensity = 2.5 – 3 x 1018 W/cm2) pulses were focused on a 1 mm3 gold cubic block after reducing the intensities of the foot pulse and pedestal by using a plasma mirror. The full width at half maximum (FWHM) duration of the intense laser pulse could be set to either 1.2 ps or 4 ps by temporally stacking four beams of the LFEX laser, for which the slope temperature of the high-energy electron distribution was 0.7 MeV and 1.4 MeV, respectively. The slope temperature increment cannot be explained without considering pulse duration effects on fast electron generation.

Vlasov modelling of laser-driven collisionless shock acceleration of protons

Ion acceleration due to the interaction between a short high-intensity laser pulse and a moderately overdense plasma target is studied using Eulerian Vlasov–Maxwell simulations. The effects of variations in the plasma density profile and laser pulse parameters are investigated, and the interplay of collisionless shock and target normal sheath acceleration is analyzed. It is shown that the use of a layered-target with a combination of light and heavy ions, on the front and rear side, respectively, yields a strong quasi-static sheath-field on the rear side of the heavy-ion part of the target. This sheath-field increases the energy of the shock-accelerated ions while preserving their mono-energeticity.

High-order harmonic generation: A quantum-mechanical description and the effects of propagation

A theoretical investigation of the generation of high-order harmonics in an ensemble of atoms interacting with femtosecond laser pulses of high intensity is conducted. It is shown that the cut-off frequency of the photoemission spectra is saturated in a laser field of subatomic strength. A model of the effect dispersion has on the radiation of high-order harmonics in extended environments is proposed.

Analytical description of attosecond pulse generation on a plasma surface irradiated by high-intense laser pulses

We study theoretically the process of turning a laser pulse into a train of attosecond or even zeptosecond pulses due to high harmonic generation (HHG) upon backreflection of intense laser radiation from a plasma surface. It is shown that under appropriate conditions these attosecond pulses may have an amplitude that is several orders of magnitude larger than that of the laser pulse. We study this process in detail, especially the nanobunching of the plasma electron density. We derive the analytical expression that describes the electron density profile and obtain a good agreement with particle-in-cell simulations. We investigate the most efficient case of HHG at a moderate laser intensity (normalised vector potential ) on the overdense plasma slab with an exponential pre-plasma profile. Subsequently we calculate the spectra of single attosecond pulses from back radiation using our expression for density shape in combination with the equation for spectrum of nanobunch radiation.

Effect of nanosecond pulse laser ablation on the surface morphology of Zr-based metallic glass

In this study, we investigated the ripple patterns formation on the surface of Zr41.2Ti13.8Cu12.5Ni10Be22.5 (vit1) bulk metallic glass using a nanosecond pulse laser ablation in air with a wavelength of 1064nm. The strong thermal ablation phenomenon could be observed on vit1 BMG surface at laser energy of 200mJ as a result of the adhibition of confining overlay. Many periodic ripples had formed on the edge of the ablated area at laser energy of 400mJ because of the high intensity pulsed laser beam. The underlying mechanism of the periodic ripples formation could be explained by the K-H hydrodynamic instability theory. It had been shown that laser ablation with 600mJ and 200 pulses results in the formation of many micro-cracks on the ablated area. Further analysis showed that the spatial occupation of the laser ablated area and the spacing between two adjacent ripples increased as the laser energy and the number of incident laser pulses increasing. The surface ripples feature on the edge of ablated area became more obvious with increasing laser pulses, but it was not correlated closely with the laser energies variation.

Controlling the spectral shape of nonlinear Thomson scattering with proper laser chirping

Effects of nonlinearity in Thomson scattering of a high intensity laser pulse from electrons are analyzed. Analytic expressions for laser pulse shaping in amplitude and frequency are obtained which control spectrum broadening for arbitrarily high laser pulse intensities. These analytic solutions allow prediction of the spectral form and required laser parameters to avoid broadening. The predictions are validated by numerical calculations. This control over the scattered radiation bandwidth allows of narrow bandwidth sources to be produced using high scattering intensities, which in turn greatly improves scattering yield for future x- and gamma-ray sources.

Bubble structure in laser wake-field acceleration

AbstractHighly nonlinear ellipsoid bubble regime of the laser wake-field acceleration with high-intensity laser pulse is considered with analytical and numerical calculations. The important property of this regime is the production of the mono-energetic high-quality electron beam. We introduce a new twofold ellipsoid structure of the bubble (egg shape) by referring to some published two-dimensional (2D) and 3D simulations. In this paper, a new analytical formalism is introduced, in which dimensions of the front part of the ellipsoid bubble are related to the laser pulse and plasma parameters. These relationships are in agreement with 2D particle-in-cell code results in recent work (Benedetti et al., 2013).

Cooling of relativistic electron beams in intense laser pulses: Chirps and radiation

Next-generation high-power laser facilities (such as the Extreme Light Infrastructure) will provide unprecedented field intensities, and will allow us to probe qualitatively new physical regimes for the first time. One of the important fundamental questions which will be addressed is particle dynamics when radiation reaction and quantum effects play a significant role. Classical theories of radiation reaction predict beam cooling in the interaction of a relativistic electron bunch and a high-intensity laser pulse, with final-state properties only dependent on the laser fluence. The observed quantum suppression of this cooling instead exhibits a dependence on the laser intensity directly. This offers the potential for final-state properties to be modified or even controlled by tailoring the intensity profile of the laser pulse. In addition to beam properties, quantum effects will be manifest in the emitted radiation spectra, which could be manipulated for use as radiation sources. We compare predictions made by classical, quasi-classical and stochastic theories of radiation reaction, and investigate the influence of chirped laser pulses on the observed radiation spectra.