Intense Laser Pulses Research Articles

On the proton radiography of magnetic fields in targets irradiated by intense picosecond laser pulses

Proton radiography is a common diagnostic technique in laser-driven magnetic field generation studies. It is based on measuring proton beam deflection in electromagnetic fields induced around the target with the help of radiochromic film stacks. Unraveling information recorded in experimental radiographs and extracting the field profiles is not always a straightforward task. In this paper, some aspects of data analysis by reproducing experimental radiographs in numerical simulations are described. The approach allows determining the field strength and structure in the target area for various target geometries.

Laser-pulse and electron-bunch plasma wakefield accelerator

Propagation distances of intense laser pulses and high-charge electron beams through the plasma are, respectively, limited by diffraction and self-deceleration. This imposes severe constraints on the performance of the two major advanced accelerator concepts: laser and plasma wakefield accelerators. Using numerical simulations, we demonstrate that when the two beams co-propagate in the plasma, they can interact synergistically and extend each other's travel distances. The key interactions responsible for the synergy are found to be laser channeling by the electron bunch, and direct laser acceleration of the bunch electrons by the laser pulse. Remarkably, the amount of energy transferred from the laser pulse to the plasma can be increased by several times by the guiding electron bunch despite its small energy content. Implications of such synergistic interactions for the high-gradient acceleration of externally injected witness charges are discussed, and a new concept of a Laser-pulse and Electron-bunch Plasma Accelerator (LEPA) is formulated.

Exploring photoelectron angular distributions emitted from molecular dimers by two delayed intense laser pulses

We describe the results of experiments and simulations performed with the aim of extending photoelectron spectroscopy with intense laser pulses to the case of molecular compounds. Dimer frame photoelectron angular distributions generated by double ionization of N$_2$-N$_2$ and N$_2$-O$_2$ van der Waals dimers with ultrashort, intense laser pulses are measured using four-body coincidence imaging with a reaction microscope. To study the influence of the first-generated molecular ion on the ionization behavior of the remaining neutral molecule we employ a two-pulse sequence comprising of a linearly polarized and a delayed elliptically polarized laser pulse that allows distinguishing the two ionization steps. By analysis of the obtained electron momentum distributions we show that scattering of the photoelectron on the neighbouring molecular potential leads to a deformation and rotation of the photoelectron angular distribution as compared to that measured for an isolated molecule. Based on this result we demonstrate that the electron momentum space in the dimer case can be separated, allowing to extract information about the ionization pathway from the photoelectron angular distributions. Our work, when implemented with variable pulse delay, opens up the possibility of investigating light-induced electronic dynamics in molecular dimers using angularly resolved photoelectron spectroscopy with intense laser pulses.

(Invited) Rapid Photo-Charge Mapping of Stainless-Steel Under the Hydrogen Permeation

Photocurrent techniques have been applied to analyze the properties of passive films on metals and their corrosion phenomena. Photocurrent mapping is also used to evaluate the inhomogeneity of, for example, non-uniform corrosion behavior by using a lock-in amplifier to detect a weak photocurrent signal combined with a laser light scanning. This method is, however, takes a long time to acquire a scanning image due to a rather slow time-constant of the lock-in amplifier measurement. In this work, a photo-charge mapping method is developed in which an intense UV laser pulse light (405 nm, 0.5 W in the present study) is focused on the specimen surface and a photo-charge evolved by a single laser pulse is evaluated by integrating the photocurrent using an electronic integrator, as shown in Fig. 1. An intense laser pulse provides a large photocurrent in some condition sufficiently for a single photocurrent pulse analysis. Fast scanning of the laser light is also achieved by using a galvano-scanner. Currently, a time required for one image acquisition is a few minutes which is limited due to the software execution time. In this work, the spatial distribution of hydrogen permeation in a stainless-steel sheet (SUS304, 0.1 mm in thickness) placed in a Devanathan cell was traced under the tensile stress test. Hydrogen was introduced from the backside of the sheet by using the galvanostatic cathodic polarization at –1mA cm–2, and penetrated hydrogen to the front side was oxidized under the anodic polarization at +0.8 V vs. Ag-AgCl R.E. The passive film was pre-formed before the hydrogen penetration. Absorption of hydrogen into stainless steel caused a drastic increase of photocurrent on the n-type semiconductor passive film due to the photo-assisted oxidation of permeated hydrogen atom via a stainless-steel sheet, i.e., hydrogen atoms can be effectively oxidized by the holes generated by UV irradiation in the passive film. In the photo-charge image shown in Fig. 1, left half of the observed area (4mm x 4 mm, 40 x 40 pixel) was penetrated by hydrogen from the backside where the increase of photo-charge was observed. The Devanathan cell was combined with a tensile tester, and a photo-charge map was traced during the hydrogen loading under the loading condition until the notched sample was cleaved. The photo-charge intensity increased around the notch before cleavage, probably due to an increase of hydrogen concentration penetrated or thickening of the passive film. Figure 1

Jitter-free 40-fs 375-keV electron pulses directly accelerated by an intense laser beam and their application to direct observation of laser pulse propagation in a vacuum

We report the generation of ultrashort bright electron pulses directly driven by irradiating a solid target with intense femtosecond laser pulses. The duration of electron pulses after compression by a phase rotator composed of permanent magnets was measured as 89 fs via the ponderomotive scattering of electron and laser pulses, which were almost at the compression limit due to the dispersion of the electron optics. The electron pulse compression system consisting of permanent magnets enabled extremely high timing stability between the laser pulse and electron pulse. The long-term RMS arrival time drift was below 14 fs in 4 h, which was limited by the resolution of the current setup. Because there was no time-varying field to generate jitter, the timing jitter was essentially reduced to zero. To demonstrate the capability of the ultrafast electron pulses, we used them to directly visualize laser pulse propagation in a vacuum and perform 2D mapping of the electric fields generated by low-density plasma in real time.

Self-focusing and self-compression of intense pulses via ionization-induced spatiotemporal reshaping.

Ionization is a fundamental process in intense laser-matter interactions and is known to cause plasma defocusing and intensity clamping. Here, we investigate theoretically the propagation dynamics of an intense laser pulse in a helium gas jet in the ionization saturation regime, and we find that the pulse undergoes self-focusing and self-compression through ionization-induced reshaping, resulting in a manyfold increase in laser intensity. This unconventional behavior is associated with the spatiotemporal frequency variation mediated by ionization and spatiotempral coupling. Our results illustrate a new regime of pulse propagation and open up an optics-less approach for raising laser intensity.

Non-Spiking Laser Controlled by a Delayed Feedback

In 1965, Statz et al. (J. Appl. Phys. 30, 1510 (1965)) investigated theoretically and experimentally the conditions under which spiking in the laser output can be completely suppressed by using a delayed optical feedback. In order to explore its effects, they formulate a delay differential equation model within the framework of laser rate equations. From their numerical simulations, they concluded that the feedback is effective in controlling the intensity laser pulses provided the delay is short enough. Ten years later, Krivoshchekov et al. (Sov. J. Quant. Electron. 5394 (1975)) reconsidered the Statz et al. delay differential equation and analyzed the limit of small delays. The stability conditions for arbitrary delays, however, were not determined. In this paper, we revisit Statz et al.’s delay differential equation model by using modern mathematical tools. We determine an asymptotic approximation of both the domains of stable steady states as well as a sub-domain of purely exponential transients.

Controlling the Ultrafast Dynamics of HD+ by the Carrier-Envelope Phases of an Ultrashort Laser Pulse: A Quasi-Classical Dynamics Study.

A theoretical study on the coupled electron-nuclear dynamics of HD+ molecular ions under ultrashort, intense laser pulses is performed by employing a well-established quasi-classical model. The influence of the laser carrier-envelope phase on various channel (H + D+, D + H+, and H+ + D+) probabilities is investigated at different laser field intensities. The carrier-envelope phase is found to govern the dissociation (H + D+ and D + H+) and Coulomb explosion (H+ + D+) channel probabilities. The kinetic energy release distributions of the fragments are also found to be sensitive to the carrier-envelope phase of the laser pulse. Our results are in agreement with the previously reported quantum dynamics studies and experiments.

THz generation by optical rectification of intense near-infrared pulses in organic crystal BNA.

Generation of terahertz radiation by optical rectification of intense near-infrared laser pulses in N-benzyl-2-methyl-4-nitroaniline (BNA) is investigated in detail by carrying out a complete characterization of the terahertz radiation. We studied the scaling of THz yield with pump pulse repetition rate and fluence which enabled us to predict the optimal operating conditions for BNA crystals at room temperature for 800 nm pump wavelength. Furthermore, recording the transmitted laser spectrum allowed us to calculate the nonlinear refractive index of BNA at 800 nm.

Momentum-resolved above-threshold ionization of deuterated water

We present momentum-resolved coincidence measurements of strong-field single and double ionization of deuterated water $({\mathrm{D}}_{2}\mathrm{O})$ using intense few-cycle laser pulses. We measure the vector momentum of electrons and ions resulting from the laser-molecule interaction. This enables us to measure the photoelectron spectrum for electrons in coincidence with fragment ions having different momenta, which can be related to different final states of the cation and dication. The photoelectron spectra for different fragment ion momenta show striking differences, which can be interpreted in terms of the removal of electrons from different molecular orbitals, including the highest occupied molecular orbital (HOMO), as well as more deeply bound ones (e.g., HOMO-2). We discuss our measurements in light of calculations which model the strong-field light matter interaction.

Spontaneous formation of coherent structures by an intense laser pulse interacting with overdense plasma

The formation and the dynamics of coherent magnetic field structures in the context of laser plasma interaction has attracted considerable attention. In the literature the formation of these structures has, however, mostly been reported in the wake of a laser pulse propagating in an underdense plasma medium (Bulanov et al., Phys. Rev. Lett., vol. 76, 1996, pp. 3562–3565; Nakamura & Mima Phys. Rev. Lett., vol. 100, 2008, 205006; Bulanov et al., Plasma Phys. Rep., vol. 31, no. 5, 2005, pp. 369–381; Naumova et al., Phys. Plasmas, vol. 8, no. 9, 2001, pp. 4149–4155; Nakamura et al., Phys. Rev. Lett., vol. 105, no. 13, 2010, 135002). The study here focuses on the formation of coherent structures by an intense laser pulse when it interacts with an overdense plasma medium. The laser in this case gets reflected and partially dumps its energy to the lighter electrons species. Particle-in-cell simulation studies have been carried out in two dimensions to show that the energetic electrons (generated at the critical layer and having relativistic energies), together with the background plasma electrons often self-organize to form distinct electron current vortices. These electron vortices have associated magnetic fields with monopolar or dipolar symmetries depending on the rotation profile of the electron current. The formation, stability and dynamics of these structures in the context of overdense plasma is of special importance as they provide a possibility of energy transport into those regions of plasma which are inaccessible to lasers. For such applications, higher energy content (involvement of relativistic electrons in their formation) of these structures is desirable. It is shown that their salient propagation characteristics even at relativistic energies follow the rules of electron magnetohydrodynamics (EMHD) (Isichenko & Marnachev, Sov. Phys. JETP, vol. 66, 1987, p. 702; Biskamp et al., Phys. Rev. Lett., vol. 76, 1996, p. 1264) (Generalized - EMHD Yadav et al., Phys. Plasmas, vol. 15, no. 6, 2008, 062308; Yadav et al., Phys. Plasmas, vol. 16, no. 4, 2009, 040701) for homogeneous (inhomogeneous) plasma medium, respectively.

Electron-ion dynamics of hydroperoxyl radical under intense femtosecond laser pulses

The electron-ion dynamics of hydroperoxyl radical in intense femtosecond laser pulses is studied by using time-dependent density functional theory combined with molecular dynamics approach. We calculate the optimized structure, the ionization energy, and the optical absorption strength. The results are in good agreement with experiments. The irradiation dynamics of HO2 including the ionization, the dipole moment, the bond lengths, the kinetic energies, and the level depletion is explored by varying the laser frequency. Computational results indicate that the excitation behaviors are distinct due to different frequencies. Furthermore, the angular dependence of the total ionization and the orbital ionization yields of HO2 are explored. The calculated result predicts a maximum around [Formula: see text] and [Formula: see text] for the total ionization and the angular dependence of the total ionization reflects the symmetry of the HOMO.

Dynamics of solvated electrons during femtosecond laser-induced plasma generation in water.

We studied the dynamics of solvated electrons in the early stage of plasma generation in water induced with an intense femtosecond laser pulse. According to the decay kinetics of solvated electrons, a fast recombination process of solvated electrons (geminate recombination) occurred with a more prolonged lifetime (500 ps to 1 ns) than that observed in previous pulse photolysis studies (10-100 ps). This unusually longer lifetime is attributed to additional production of solvated electrons due to abundant free electrons generated with the laser-induced plasma, implying significant influence of free electrons on the dynamics of solvated electrons.

Optical-Stark Induced Distortions in Vortex Momentum Distributions of p-Orbital Electrons of Neon Atoms

We present results of numerical simulations on the photoelectron momentum distributions of the p-orbital electrons of neon atoms ionized by a pair of time-delayed and oppositely circularly polarized intense laser pulses. Deploying the strong-field theory, we can readily produce vortex-shaped momentum distributions. Similarity and disparity in the vortex patterns between the neon and hydrogen atoms are observed. The optical Stark effect is found to induce distortions in the vortex momentum distributions, which are quantitatively described by introducing several physical quantities. Among these quantities, the autocorrelation parameter turns out to be the most sensitive probe for extremely weak Stark effect. The nonlinear phase induced by the optical Stark effect deciphers the distortions of the vortex momentum distributions.

Characterization of multilayer coatings of aluminium, tungsten and molybdenum on steel substrate using laser-induced breakdown spectroscopy

Laser induced breakdown spectroscopy (LIBS) has been applied for the depth profiling of multilayer metallic coating structure which is simulated to the ITER-like wall materials in fusion devices such as EAST tokamak. The coatings were produced by magnetron sputtering and characterized by LIBS using a Nd:YAG laser at 1064 nm with pulse duration of 5 ns. The light from the resulting plasma was analyzed as a function of both number of laser pulse (N) and elemental spectral line intensity. The depth profile of the multilayer coating was determined by three different spectral signal processing methods; intensity background subtraction, intensity normalization and linear Pearson correlation. The intensity normalization method provides the best outcomes/solutions for thickness measurement and analysis of interfaces of multilayer coatings among other methods. The evolution of spectral intensity representing different materials was properly examined for 90 consecutive laser shots at the same spot. The ablation behaviour of the coating materials was analyzed in terms of ablation rate under the laser pulse of various fluences. In a much deep crater, some factors originating from laser ablation, such as material re-deposition and plasma shielding effect were analyzed. As a comparison, the depth, shape and diameter of craters were also measured using SEM and a profilometer. The influence of laser power density on the ablation rate and crater profile was investigated. The elemental composition, inside the crater, was verified by the EDX technique. The results indicate that LIBS has a potential power for in-situ depth profiling of multilayer coatings on the wall of the EAST.

Partial-wave representation of the strong-field approximation

The strong-field approximation (SFA) has been widely applied to model ionization processes in short and intense laser pulses. Several approaches have been suggested in order to overcome certain limitations of the original SFA formulation with regard to the representation of the initial bound and final continuum states of the emitted electron as well as a suitable description of the driving laser pulse. We here present a reformulation of the SFA in terms of partial waves and spherical tensor operators that supports a quite simple implementation and the comparison of different treatments of the active (photo)electron and the laser pulses. In particular, this reformulation helps to adapt the SFA to experimental setups, and it paves the way to extend the strong-field theory toward the study of nondipole contributions in light-atom interactions as well as of many-particle correlations in strong-field ionization processes. A series of detailed computations have been carried out in order to confirm the validity of the reformulation and to show how the representation of the bound and continuum states affects the predicted above-threshold ionization spectra and related observables.

A paradoxical phenomenon: Intense pulsed light‐induced hypertrichosis, a brief review

AbstractThe advancement of laser and light therapies have allowed patients of all races and ethnicities access to different methods for removing unwanted hairs that can cause emotional distress and impact one's quality of life. Despite their effectiveness, these technologies are not without their risks, such as the well‐known side effects of blister formation, erythema, crusting, and postinflammatory hypo‐ and hyperpigmentation. However, one adverse effect that seems to remain a clinical mystery is paradoxical hypertrichosis. Paradoxical hypertrichosis is the growth of fine, dark hair in treated areas or untreated areas close to the area treated on the face and neck, more frequently affecting those with darker skin (Fitzpatrick III‐VI) and darker hair color, and most associated with those having undergone the intense pulsed laser and long‐pulse alexandrite laser, although, this adverse reaction is likely common to all current and laser and light hair removal devices, despite the unknown prevalence and varying incidence rates. This phenomenon is not well understood, although explanations include the activation of dormant hair follicles by the intense pulse light flashlamp's suboptimal fluences and the synchronization of hair growth cycles caused by direct light stimulation.

Self-waveguiding of relativistic laser pulses in neutral gas channels

The authors show that an ultrashort, relativistic intensity laser pulse can propagate for hundreds of Rayleigh ranges in a prepared neutral hydrogen channel by generating its own plasma waveguide as it propagates.

Relativistic flying forcibly oscillating reflective diffraction grating.

Relativistic flying forcibly oscillating reflective diffraction gratings are formed by an intense laser pulse (driver) in plasma. The mirror surface is an electron density singularity near the joining area of the wake wave cavity and the bow wave; it moves together with the driver laser pulse and undergoes forced oscillations induced by the field. A counterpropagating weak laser pulse (source) is incident at grazing angles, being efficiently reflected and enriched by harmonics. The reflected spectrum consists of the source pulse base frequency and its harmonics, multiplied by a large factor due to the double Doppler effect.

Coupling of Sub-Terawatt Laser Into Hollow Core Waveguide for High-Harmonic Generation Above 200 eV

Hollow core waveguides are widely used in extreme field research to guide laser pulses. The laser peak intensity is usually several orders higher than the damage threshold of waveguide cladding material in these applications. The nonlinear drift of the spatial focus of such high power laser with the power or intensity fluctuation can easily destroy the waveguide in an avalanche manner. In this letter, we experimentally investigate the nonlinear drift of focal position and size of a sub-terawatt laser beam with respect to pulse energy and pulse duration. A theoretical model is derived and qualitatively explains the experimental observations. A reliable procedure to couple 3 mJ laser pulses at 27 fs into a hollow core waveguide with 150 $\mu \text{m}$ bore diameter is developed. When the waveguide is filled with Helium, the high harmonics up to 212 eV are generated which confirms that the intense laser pulses with peak intensity of $1.8\times 10 ^{\mathbf {15}}$ W/cm $^{\mathbf {2}}$ are successfully coupled into the waveguide.