Intense Laser Pulses Research Articles

Proton beams generated via thermonuclear deuterium–deuterium fusion by means of modified cavity pressure acceleration-type targets as a candidate for proton–boron fusion driver

In this Letter, we report the possibility of generating intense, highly energetic proton beams using terawatt, sub-nanosecond class laser system by irradiating modified cavity pressure acceleration-type targets. In this approach, the main source of few-mega electron volt protons is thermonuclear deuterium–deuterium reaction; therefore, the energy spectrum of accelerated particles and their number is not as strongly related to the laser intensity (laser pulse energy and pulse duration in particular) as in the case of the most common ion acceleration mechanism, namely, target normal sheath acceleration. Performed Monte Carlo simulations suggest that using mentioned mechanism to generate proton beam might be beneficial and efficient driver for laser induced proton–boron fusion when moderate-to-low laser pulse intensities ( ⩽ 1016W/cm2) and thin, lower than 100 μm boron foils are used as catchers.

Interference mechanism of plasma self-organization in transparent dielectrics under the intense femtosecond laser pulse exposure

A new mechanism of plasma self-organization in transparent dielectrics with wide bandgap exposed to the intense tightly focused laser radiation was revealed, which causes the generation of 3D periodic ring structures with subwavelength period both along the laser pulse propagation and in the radial direction. The mechanism involves formation of dense plasma burst in the pre-focal region that provides efficient scattering of the incident wave. The interference of a plane incident laser wave in the focal region and a divergent reflected one will form the standing wave pattern with local minima and maxima of laser field both in the direction of the incident wave propagation and perpendicular to it producing the ring patterns of effective ionization regions in the dielectric volume. Analytical and numerical simulations of the process of laser wave scattering on a near-spherical plasma object with dimensions both smaller and larger than the laser radiation wavelength are performed to verify the proposed model.

Transient absorption of warm dense matter created by an X-ray free-electron laser

Warm dense matter is at the boundary between a plasma and a condensed phase and plays a role in astrophysics, planetary science and inertial confinement fusion research. However, its electronic structure and ionic structure upon irradiation with strong laser pulses remain poorly understood. Here, we use an intense and ultrafast X-ray free-electron laser pulse to simultaneously create and characterize warm dense copper using L-edge X-ray absorption spectroscopy over a large irradiation intensity range. Below a pulse intensity of 1015 W cm−2, an absorption peak below the L edge appears, originating from transient depletion of the 3d band. This peak shifts to lower energy with increasing intensity, indicating the movement of the 3d band upon strong X-ray excitation. At higher intensities, substantial ionization and collisions lead to the transition from reverse saturable absorption to saturable absorption of the X-ray free-electron laser pulse, two nonlinear effects that hold promise for X-ray pulse-shaping. We employ theoretical calculations that combine a model based on kinetic Boltzmann equations with finite-temperature real-space density-functional theory to interpret these observations. The results can be used to benchmark non-equilibrium models of electronic structure in warm dense matter.

Specificity of DNA damage formation induced by femtosecond near-infrared laser filamentation in water

We investigated the deoxyribonucleic acid (DNA) damage induced by laser filamentation, which was generated by focusing femtosecond near-infrared Ti:Sapphire laser light in water at several repetition rates ranging from 1000 Hz to 10 Hz. Using plasmid DNA (pUC19), the single-strand break, double-strand break, nucleobase lesions, and the fragmented DNA were analyzed and quantified by agarose gel electrophoresis. Additionally, the H2O2 concentration after irradiation was determined. We observed that (1) the DNA damage per laser shot and (2) the enzyme-sensitive base lesions per total DNA damage decreased as the laser repetition rate increased. Furthermore, (3) extraordinarily short DNA fragments were likely to be produced, compared with those produced using X-rays, and (4) most OH radicals could be eliminated by recombination to generate H2O2, preventing them from damaging the DNA. The Monte-Carlo simulation of the strand break formation implies that the observed dependency of strand break efficiency on the laser repetition rate is mainly due to diffusion of DNA molecules. These findings quantitatively and qualitatively revealed that an intense laser pulse induces a specific DNA damage profile that is not induced by X-rays, a sparsely ionizing radiation source.

Pulse repetition-rate effect on the critical power for self-focusing of femtosecond laser in air

The femtosecond laser filamentation is of significant interest due to its remarkable characteristics, and determining the critical power of self-focusing is essential for the process of filamentation. In this work, the critical power for self-focusing of intense femtosecond laser pulses at different repetition rates is experimentally measured according to the focus-shift method. A bimodal fitting method is proposed to more accurately determine the self-focusing critical power. It is found that the self-focusing critical power decreases as the laser repetition rate increases. A numerical simulation of the filamentation process based on the modified nonlinear Schrödinger equation effectively explains the experimental results obtained. This work provides valuable insights for the generation and application of high repetition rate femtosecond laser filamentation.

Coherent radiation of an electron bunch colliding with an intense laser pulse

We study the conditions for coherent radiation of an electron bunch driven by a counterpropagating strong pulsed electromagnetic plane wave. We derive the spectrum of the coherent radiation and show that it is emitted backward with respect to the laser propagation direction and has a very narrow angular spread. We demonstrate that for a solid density plasma coherent radiation extends to frequencies up to hundreds of keV, thereby enhancing the low-frequency part of the spectrum by many orders of magnitude. Our analytical findings are tested with three-dimensional particle-in-cell simulations of an electron bunch passing through a laser pulse, clearly demonstrating how the coherence can essentially modify the observed radiation spectrum. Published by the American Physical Society 2024

Analysis on the minimum structure of harmonic spectra from MgO crystals.

Recent theoretical and experimental findings have demonstrated the minimum characteristic in the harmonic spectrum of bulk MgO crystals subjected to intense laser pulses. However, the dominant mechanism behind this minimum structure is still under debate. This study simulates the harmonic spectrum from a MgO crystal in a linearly polarized laser pulse by solving multi-band semiconductor Bloch equations. The results show that the minimum feature at 20 eV in the MgO harmonic spectra from 1700 and 800 nm laser pulses is due to band dispersion and interference between interband harmonics. Notably, the disappearance of the minimum structure at 14 eV in the harmonic spectrum from the 800 nm laser is attributed to the intensity suppression of higher energy harmonics, caused by decreased electron population at the boundary of the first Brillouin zone in the multi-band case. These findings offer insights into the spectral structure of solid-state harmonics, contributing to the all-optical reconstruction of the crystal band based on its harmonic spectrum.

Laser Proton Acceleration from a Near-Critical Imploding Gas Target.

The interaction between relativistic intense laser pulses and near-critical-density targets has been sought after in order to increase the efficiency of laser-plasma energy coupling, particularly for laser-driven proton acceleration. To achieve the density regime for high-repetition-rate applications, one elusive approach is to use gas targets, provided that stringent target density profile requirements are met. These include reaching the critical plasma density while maintaining micron-scale density gradients. In this Letter, we present a novel scheme for achieving the necessary requirements using optical laser pulses to transversely shape the target and create a colliding shock wave in both planar and cylindrical geometries. Utilizing this approach, we experimentally demonstrated stable proton acceleration and achieved up to 5MeV in a monoenergetic distribution and particle numbers above 10^{8} Sr^{-1} MeV^{-1} using a 1.5J energy on-target laser pulse. The Letter also reports for the first time an extend series of 200 consecutive shots that demonstrates the robustness of the approach and its maturity for applications. These results open the door for future work in controlling gas targets and optimizing the acceleration process for more energetic multipetawatt laser systems.

Analytical formula for signal optimization in stimulated photon-photon scattering setup with three laser pulses

We consider a setup to detect stimulated photon-photon scattering using high-power lasers. Signal photons are emitted from an overlap of the incoming intense laser pulses focused in vacuum from three sides. We derive and justify a general approximate analytical formula for the angular distribution and total yield of such signal photons in terms of the parameters of the incoming pulses, including their intensity, carrier frequencies, durations, focusing, polarizations, mutual orientation and overlap. Using the obtained formula a parametric study of the signal is carried out and optimization is performed. Published by the American Physical Society 2024

Interference of harmonics emitted by different tunneling momentum channels in laser fields

Abstract By numerically solving the semiconductor Bloch equation (SBEs), we theoretically study the high-harmonic generation of ZnO crystals driven by one-color and two-color intense laser pulses. The results show the enhancement of harmonics and the cut-off remains the same in the two-color field, which can be explained by the recollision trajectories and electron excitation from multi-channels. Based on the quantum path analysis, we investigate contribution of different ranges of the crystal momentum k of ZnO to the harmonic yield, and find that in two-color laser fields, the intensity of the harmonic yield of different ranges from the crystal momentum makes a big difference and the harmonic intensity is depressed from all k channels, which is related to the interferences between harmonics from symmetric k channels.

Role of blue-shift length in macroscopic properties of high-harmonic generation

The production of brighter coherent XUV radiation by intense laser pulses through the process of high-harmonic generation (HHG) is one of the central challenges in contemporary nonlinear optics. We study the generation and spatial propagation of high harmonics analytically and via ab initio simulations. We focus on the length scales defining the growth of the harmonic signal with propagation distance and show that the well-known coherence length limits HHG only for relatively low driving intensities. For higher intensities, the photoionisation of the medium, naturally accompanying HHG, leads to essentially transient phase matching and laser frequency blue shift. By systematically taking both of these factors into account, we demonstrate that the behaviour of the harmonic signal at higher intensities is defined by another length scale—the blue-shift length. In this generation regime the XUV intensity at a given frequency first grows quadratically and then saturates passing the blue-shift length, but the total harmonic efficiency continues growing linearly due to the linear increase of the harmonic line bandwidth. The changeover to this generation regime takes place for all harmonic orders roughly simultaneously. The rate of the efficiency growth is maximal if the atomic dispersion is compensated by photoelectrons near the centre of the laser pulse. Our theory offers a robust way to choose the generation conditions that optimise the growth of the harmonic signal with propagation.

Optimized laser production of thermonuclear neutrons from plasma of submicron-sized clusters

The concept of maximizing the D-D fusion neutron yield from the laser-heated large volume of cluster medium by matching the focal spot size and cluster plasma structural scales to the laser pulse intensity was confirmed. For this purpose, the three-dimensional particle-in-cell GEANT4 simulations have been performed by zoning of the large interaction domain. While considering a small domain of the entire interaction volume, which is partitioned into successive zones along laser propagation direction, a special algorithm was proposed allowing to reconstruct the integral spectrum of deuterons and D-D neutron yield. We demonstrate that it makes possible to specify high-performance laser–cluster neutron source following this concept. For example, for the submicron heavy water droplets heated by femtosecond laser pulse of the intensity 3×1019 W/cm2 a D-D neutron yield may reach 107 neutrons per 1 J of deposited laser energy if the intensity contrast ratio prevents premature cluster destruction. Such yield is considerably higher than achieved to date for microstructured targets.

High energy neutral atom emission from rear side of foils

Intense laser pulses focused onto a solid micron foil target are best known for generation of large electrostatic sheath fields at the target rear and acceleration of protons to MeV energies. The fact that such acceleration schemes also generate fast neutral atoms has received comparatively less attention. A quantitative assessment of the neutralization fraction reveals novel and often unexpected aspects of electron-ion interaction in regions far from dense plasmas. At the rear side of laser irradiated foil, a few hundred keV accelerated neutral Hydrogen atoms are observed. Neutralisation fraction is much larger than that anticipated from the collisional charge transfer and is about 300% larger for ions of ⩽ 100 keV energy. Corroborated by particle-in-cell simulations, the larger neutralisation fraction is attributed to electron-recombination as the protons co-propagate with the low energy electrons.

Interaction of intense ultrashort laser pulses with solid targets: A systematic analysis using first-principles calculations

Interaction of intense ultrashort laser pulses with solid targets: A systematic analysis using first-principles calculations

Effect of trapezoidal plasma density region in bubble wakefield acceleration

In the process of bubble wakefield acceleration highly nonlinear region is developed inside plasma, which intuitively suggests that nonuniform plasma density having gradients may be more suited to achieve large nonlinearity in the system. Moreover, when an intense laser pulse propagates in a plasma, it is subjected to various instabilities and these instabilities can be controlled by plasma density profiles which effectively control the energy and flux of the accelerated particles. Considering all these points we investigate in the present work the scaling effect of up-ramp and down-ramp regions in plasma density profile on the bubble wakefield. These regions are separated by a plateau region (maximum density) enabling the density to have trapezoidal profile. With this density profile, the bubble wakefield acceleration is examined considering four different lengths of up-ramp and plateau regions keeping a constant down-ramp length. Increasing steepness of up-ramp length (larger density gradient), i.e., lowering the length of up-ramp and increasing the plateau length creates a bubble having higher wakefield strength, resulting into higher accumulation of plasma electrons at its tail and higher energy spectrum with higher kinetic energy gradient and Poynting flux of accelerated electrons.

Novel signatures of radiation reaction in electron–laser sidescattering

In this article we investigate novel signatures of radiation reaction via the angular deflection of an electron beam colliding at 90 degrees with an intense laser pulse. Due to the radiation reaction effect, the electrons can be deflected towards the beam axis for plane wave backgrounds, which is not possible in the absence of radiation reaction effects. The magnitude and size of the deflection angle can be controlled by tailoring the laser pulse shapes. The effect is first derived analytically using the Landau–Lifshitz equation, which allows to determine the important scaling behavior with laser intensity and particle energy. We then move on to full scale 3D Monte Carlo simulations to verify the effect is observable with present day laser technology. We investigate the opportunities for an indirect observation of laser depletion in such side scattering scenarios.

Biological Action of Intense Laser Pulses at the Molecular Level

Biological Action of Intense Laser Pulses at the Molecular Level

Investigating Xenon Aggregates Irradiated by Intense Attosecond Laser Pulses

Investigating Xenon Aggregates Irradiated by Intense Attosecond Laser Pulses

Femtosecond Laser Carburization of WS2 Flakes to W2C for Enhanced Photothermal Conversion Efficiency

Photothermal therapy is a new, promising approach for treating cancer in a noninvasive way. Herein, a photothermally efficient nanomaterial is synthesized by exposing bulk WS2 powder, dispersed in carbon‐rich acetonitrile, to high‐intensity femtosecond laser pulses. The photothermal measurements show a significantly higher rise in temperature and enhance photothermal efficiency for the laser‐treated material. The attribution is on the enhancement to the formation of tungsten semi‐carbide (W2C), which is verified by morphological and structural characterization. The tungsten semi‐carbide is formed by laser‐induced carburization; intense laser pulses knock S atoms out of the WS2 lattice and dissociate the acetonitrile molecules, resulting in the formation of W2C. Interestingly, the hexagonal W2C has a two‐dimensional (2D) layered morphology like that of the WS2, suggesting its morphology is not random and might be determined by the morphology of the parent material. The study provides a simple route to transform a 2D transition‐metal dichalcogenide into a 2D transition‐metal carbide, which is useful for applications including photothermal therapy.

Scaling relations in laser-induced vaporization of thin free-flying liquid metal sheets

We experimentally study the vaporization of free-flying liquid tin sheets when exposed to a 100-ns laser pulse with an intensity between 0.2×107 and 4×107 W/cm2, a case inspired by current developments around “advanced target shaping” in industrial laser-produced plasma sources for extreme ultraviolet (EUV) nanolithography machines. Our findings reveal a gradual vaporization and a linear relationship between the average vaporization rate and laser pulse intensity (with a prefactor of 1.0(3)×10−7ms−1/Wcm−2), for various targets ranging from 20nmto200nm in thickness. We introduce a numerical one-dimensional heating and vaporization model based on Hertz-Knudsen evaporation and find excellent agreement between simulations and experimental data. We furthermore demonstrate that the amount of vaporization of liquid tin targets in the investigated laser intensity range is governed solely by the deposited fluence, and collapse all data onto a single nondimensional curve, enabling the accurate prediction of vaporization dynamics in applications in future development of EUV sources. Published by the American Physical Society 2024