High-intensity Femtosecond Laser Pulses Research Articles

Attosecond and nano-Coulomb electron bunches via the Zero Vector Potential mechanism

The commissioning of multi-petawatt class laser facilities around the world is gathering pace. One of the primary motivations for these investments is the acceleration of high-quality, low-emittance electron bunches. Here we explore the interaction of a high-intensity femtosecond laser pulse with a mass-limited dense target to produce MeV attosecond electron bunches in transmission and confirm with three-dimensional simulation that such bunches have low emittance and nano-Coulomb charge. We then perform a large parameter scan from non-relativistic laser intensities to the laser-QED regime and from the critical plasma density to beyond solid density to demonstrate that the electron bunch energies and the laser pulse energy absorption into the plasma can be quantitatively described via the Zero Vector Potential mechanism. These results have wide-ranging implications for future particle accelerator science and associated technologies.

Quantum mechanical modeling of high-intensity laser pulse interaction with hydrogen atom with considering the magnetic field and polarization

In the study of the non-relativistic interaction between high-intensity femtosecond laser pulses and atoms, the influence of the magnetic field is commonly overlooked. This work investigates the effects of the magnetic field in the high-intensity few-cycle laser pulses with non-relativistic intensity of 3.5×1014W/cm2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$3.5 \ imes 10^{14} { }\\;\\;{\ ext{W}}/{\ ext{cm}}^{2}$$\\end{document} at the center wavelength of 800 nm on the high-order harmonic generation (HHG), attosecond pulse train (APT), isolated attosecond pulse (IAP), and the electron trajectory in the hydrogen atom, employing the numerical solution of the time-dependent Schrödinger equation in three dimensions (3D-TDSE). Two polarizations, linear and circular, are considered. A comparison with the scenario where the magnetic field is not considered shows that the magnetic field can apply significant corrections to the results. Particularly, considering the magnetic field for circular polarization can make the cutoff frequency of HHG coincide with the semi-classical relationship of ħωc=Ip+3.17Up\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbar {\\upomega }_{{\ ext{c}}} = {\ ext{I}}_{{\ ext{p}}} + 3.17{\ ext{U}}_{{\ ext{p}}}$$\\end{document}, a case that for circular polarization does not exist without considering the magnetic field. Moreover, accounting for the magnetic field leads to a reduction in the attosecond pulse duration for circular polarization for APT (360as\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$360{\ ext{ as}}$$\\end{document} versus 241as\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$241{\ ext{ as}}$$\\end{document}) and for IAP (834as\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$834{\ ext{ as}}$$\\end{document} versus 602as\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$602{\ ext{ as}}$$\\end{document}). Additionally, the difference in production efficiency of HHG and APT between linear and circular polarization is reduced by two orders of magnitude, when magnetic field is considered. Although considering the magnetic field complicates the electron trajectory, especially for circular polarization, however, our quantum model provides enhanced insight into how the interaction works, especially when and where the electron collides with the parent nucleus. In this case, the quantum mechanical modeling largely covers the huge difference of not considering the magnetic field in the results predicted by other works.

A solid-state high harmonic generation spectrometer with cryogenic cooling.

Solid-state high harmonic generation (sHHG) spectroscopy is a promising technique for studying electronic structure, symmetry, and dynamics in condensed matter systems. Here, we report on the implementation of an advanced sHHG spectrometer based on a vacuum chamber and closed-cycle helium cryostat. Using an in situ temperature probe, it is demonstrated that the sample interaction region retains cryogenic temperature during the application of high-intensity femtosecond laser pulses that generate high harmonics. The presented implementation opens the door for temperature-dependent sHHG measurements down to a few Kelvin, which makes sHHG spectroscopy a new tool for studying phases of matter that emerge at low temperatures, which is particularly interesting for highly correlated materials.

The modeling non-sequential double ionization of helium atom under high-intensity femtosecond laser pulses with shielding charge approximation

In this study, we successfully extracted the ‘knee structure’ for non-sequential double ionization (NSDI) in the helium atom. To achieve this, for the first time, we solved the time-dependent Schrödinger equation in three dimensions for the helium atom, utilizing the shielding charge approximation. Our findings corroborate prior observations by Wang et al [Wang and Eberly, Phys. Rev. Lett. 105, 083001 (2010)], demonstrating that NSDI occurs within a narrower time window in circular polarization compared to linear polarization. As a result, the yield of linear polarization was higher than that of circular polarization, aligning with the previously reported results. Notably, in the case of circular polarization, the time window further narrows with increasing intensity, attributed to a decrease in the time-of-flight.

High-order harmonic generation, attosecond pulse, and non-sequential double ionization in the helium atom under high-intensity femtosecond laser pulses

High-order harmonic generation (HHG), attosecond pulse train (APT), isolated attosecond pulse (IAP), and non-sequential double ionization (NSDI) in the Helium atom under intense femtosecond laser pulses are computed using the time-dependent Schrodinger equation (TDSE) in one dimension (1D). By taking into account the electron–electron and electron-nucleus interactions, as well as determining the Helium atom’s ground state wave function through the imaginary time propagation (ITP) method, we are able to observe the widely recognized ‘knee structure’ in the ionization probability of the Helium atom as a function of intensity within an ionization boundary condition model. The results are in good agreement with the experimental data reported by (Walker et al (1994) Phys. Rev. Lett. 73, 1227).

Self-channeling of spatially modulated femtosecond laser beams in the post-filamentation region

The propagation of high-intensity femtosecond laser pulses in air under conditions of superposed spatial phase modulation is considered theoretically. The numerical simulations are carried out on the basis of the reduced form of a nonlinear Schrödinger equation for a time-averaged electric field envelope. Initial spatial modulations are applied to pulse wavefront profiling by a staggered ( T E M 22 ) phase plate, which is simulated numerically. The dynamics of laser pulse self-focusing, filamentation, and post-filamentation self-channeling after the phase plates with variable phase jumps is studied. We show that, with specific phase modulations, the pulse filamentation region in air can be markedly shifted further and elongated compared with a nonmodulated pulse. Moreover, during the post-filamentation propagation of spatially structured radiation, the highly localized light channels are formed, possessing enhanced intensity and reduced angular divergence, which enables post-filamentation pulse self-channeling on the distance multiple exceeding the Rayleigh range.

Wavelength dependence of the multiphoton ionization of CH3I by intense femtosecond laser pulses through Freeman resonances.

Multiphoton ionization (MPI) of methyl iodide, CH3I, has been investigated with the photoelectron imaging (PEI) technique, using high intensity femtosecond laser pulses at different central wavelengths. The use of high laser field strengths alters the way in which matter-radiation interaction takes place. This generates highly nonlinear phenomena, among which we can highlight the Stark shift effect. It can distort the potential energy surfaces of atoms and molecules, varying both the energy of electronic and rovibrational states of these systems and their ionization potentials. In this way, the Stark shift can generate resonances between intermediate states and an integer number of laser photons of a given wavelength, which would be absent in the low intensity regime. The main purpose of this work is the generation, detection and characterization of resonances produced by the Stark shift, commonly known as Freeman resonances, induced by multiphoton ionization of gas-phase CH3I at different laser wavelengths. The results obtained reveal that a multitude of resonances are induced in the ionization of CH3I in the range of intensities employed, involving several Rydberg states. Ionization pathways associated with different degrees of vibrational excitation in both the intermediate states and the molecular cation generated in each of the experiments are proposed.

Ultra-supercontinuum generation with high intense femtosecond pulses

Extreme spectral broadening has been theoretically simulated from the index of refraction changes from the envelope response to the fifth- and third-order susceptibilities from self-phase modulation (SPM) under the influence of an extremely high-intensity femtosecond laser pulse to potentially produce spectral broadening changes extending from extreme X-rays, UV, visible, THz down to DC. The theoretical results show that an extremely high-intensity pulse as high as on the order of 1014~1016 W/m2 can influence the fourth-order refractive index arising from fifth-order susceptibility large enough that the nonlinear n4I02 term to overtakes the n2I0 term from the third-order susceptibility to produce the ultra-supercontinuum broadening in the liquids such as CS2 and rare gas liquids and solids such as calcite, ZnO, Si, Argon, and Krypton. There has been experimental verification at lower intensities that the SC is symmetrical in frequency about laser frequency for say a Gaussian pulse extends from, UV visible, NIR, to MIR by many researchers using various states of matter. This paper provides an opportunity to extend the SPM model from X-rays to DC to form Ultra Supercontinuum Generation (USCG) using extreme intensity fs pulses in the four states of matter: gases, liquids, solids, and plasmas. At the highest intensity, a continuum with weaker intensity goes towards the X-ray region on the anti-Stokes side.

Electric-field-induced second-harmonic generation using high-intensity femtosecond laser pulses over the observable optical breakdown threshold.

We investigated the performance of electric-field-induced second-harmonic generation (E-FISHG) by spectroscopic measurement using high-intensity femtosecond laser pulses. The second-harmonic intensity increased quadratically versus the applied electric field, as expected from the theory, up to 15 kV/cm with the laser energy up to 2.5 mJ, which is ∼5 times higher than the observable optical breakdown threshold. In addition, when the laser energy was 2.8 mJ, ∼80 times signal intensity at 0.23 mJ was obtained. These results suggest that the electric-field measurement by E-FISHG with high-intensity second harmonics is expected by using high-intensity laser pulses above the observable optical breakdown threshold. Spectroscopic measurement shows no E-FISHG of white light generated by self-phase modulation in laser-induced filament.

Complementary diagnostics of high-intensity femtosecond laser pulses via vacuum acceleration of protons and electrons

Taking advantage of complementary measurements of the characteristics of both protons and electrons accelerated by a laser in a vacuum, we propose an advanced diagnostics of the parameters of the tightly focused high-intensity femtosecond laser pulse. The laser field description is based on Stratton–Chu integrals, which allow simulating laser pulses with different spatial–temporal profiles focused by an off-axis parabolic mirror down to the diffraction limit. The free particles, whose dynamics are calculated by the test particle method, are accelerated from a rarefied gas (almost a vacuum). We analyze the dependence of the particle spectra on the laser parameters: the laser peak intensity, focal spot size, and pulse duration. The results show that diagnostics obtained by simultaneously using protons and electrons allow increasing the estimation accuracy of measuring the focal spot size and the peak intensity and thus allow evaluating the laser pulse duration in the femtosecond range. Our proposal is a response to the urgent need to measure pulse durations in the focal spot for new PW-level class lasers of super-short duration up to ∼10 to 20 fs, which clearly demonstrate a current trend in laser technology.

Observation of ultrafast impact ionization in diamond driven by mid-infrared femtosecond pulses

We report on the observation of ultrafast impact ionization in monocrystalline diamond driven by high-intensity mid-infrared femtosecond laser pulses. The measurements are based on monitoring the excited carrier population during and after the interaction of the pre-excited sample with a strong infrared pulse by transient transmission spectroscopy and photoluminescence measurements. A twofold increase in the initial carrier population due to impact ionization is observed with the peak infrared intensity of 2.5 TW/cm2. The experimental results are supported by numerical simulations of electron dynamics using time-dependent density functional theory, which show that the electrons in the conduction band reach the energy threshold for impact ionization during the interaction with the infrared pulse.

Polarization-dependent micro-structure fabrication with direct femtosecond laser writing on plastic polarizer films.

Iodine-doped polyvinyl alcohol (IDPVA) film has been widely used as a plastic polarizer due to its great linear dichroism. We found that the anisotropic character of the plastic polarizer can be permanently damaged upon exposure of high intensity femtosecond laser pulses. This process is a two-photon-induced chemical reaction and denominated as two-photon-induced isotropy (TPII). The TPII effect can form a high polarization contrast on the base of the original IDPVA films. With this property, polarization-sensitive diffractive optical elements are fabricated in IDPVA films. The low cost of the IDPVA film and the high polarization contrast of TPII make it a promising new candidate for femtosecond laser fabrication of polarization-selective optical elements.

Generation of a strong reverse shock wave in the interaction of a high-contrast high-intensity femtosecond laser pulse with a silicon target

We present ultrafast pump-probe reflectivity and Doppler spectrometry of a silicon target at relativistic laser intensity. We observe an unexpected rise in reflectivity to a peak approximately ∼9 ps after the main pulse interaction with the target. This occurs after the reflectivity has fallen off from the initially high “plasma-mirror” phase. Simultaneously measured time-dependent Doppler shift data show an increase in the blue shift at the same time. Numerical simulations show that the aforementioned trends in the experimental measurements correspond to a strong shock wave propagating back toward the laser. The relativistic laser-plasma interaction indirectly heats the cool-dense (ne≥1023 cm−3 and Te∼10 eV) target material adjacent to the corona, by hot electron induced return current heating, raising its temperature to around 150 eV and causing it to explode violently. The increase in reflectivity is caused by the transient steepening of the plasma density gradient at the probe critical surface due to this explosive behavior.

Relativistic Interaction of Long-Wavelength Ultrashort Laser Pulses with Nanowires

We report on experimental results in a new regime of a relativistic light-matter interaction employing mid-infrared (3.9-micrometer wavelength) high-intensity femtosecond laser pulses. In the laser generated plasma, the electrons reach relativistic energies already at rather low intensities due to the fortunate lambda^2-scaling of the kinetic energy with the laser wavelength. The lower intensity suppresses optical field ionization and creation of the pre-plasma at the rising edge of the laser pulse efficiently, enabling an enhanced efficient vacuum heating of the plasma. The lower critical plasma density for long-wavelength radiation can be surmounted by using nanowires instead of flat targets. In our experiments about 80% of the incident laser energy has been absorbed resulting in a long living, keV-temperature, high-charge state plasma with a density of more than three orders of magnitude above the critical value. Our results pave the way to laser-driven experiments on laboratory astrophysics and nuclear physics at a high repetition rate.

Ultrafast spin dynamics and inverse spin Hall effect in nanostructures with giant spin-orbit coupling

The features of ultrafast spin dynamics excitation using high-intensity femtosecond laser pulses in magnetic materials are reviewed. Key mechanisms of a pump pulse action on the spin system of a magnetic material are discussed, including the inverse Faraday effect in yttrium-iron garnet; induced magnetic anisotropy in the thulium orthoferrite, and the thermal driving of a magnetic system out of equilibrium in the case of the metallic ferrimagnet GdFeCo, which is a promising material for magnetic memory and terahertz spintronics. It was shown that, apart from using conventional magneto-optical methods for probing the magnetization dynamics in magnetic heterostructures, the inverse spin Hall effect can also be used, an approach that potentially enables the development of memory elements in which the ultrafast optical control of magnetization is combined with electrical detection.

A setup for micro-structured silicon targets by femtosecond laser irradiation

We present a process to fabricate micro-structured thin silicon foils with highly light absorbing properties over a broad wavelength region based on short-pulse laser treatment. We employ this fascinating technique to the fabrication of targets for high-intensity laser-plasma experiments. A polished silicon wafer is processed with high-intensity femtosecond laser pulses resulting in conical spikes within the irradiated region. Height, distance, and shape of these spikes depend primarily on the number, energy, central wavelength and duration of the incident laser pulses, as well as the ambient medium. This method is of great value for the future development of target fabrication. The broad parameter base offers a huge selection regarding shape and dimensions of the resulting structure. It can be included in existing laser operations within the manufacturing chain while offering a high degree of customisability. Within the reach of higher repetition rates of high-power laser systems and an increased number of requested targets, this manufacturing method can be scaled while being cost efficient and easily adaptable.

800 nm高能量飞秒激光脉冲刻写长周期光纤光栅机理

800 nm高能量飞秒激光脉冲刻写长周期光纤光栅机理

Spontaneous emission of atoms in a strong laser field

The spontaneous emission of an atomic system in the field of a high-intensity femtosecond laser pulse is considered within the framework of a consistent quantum-mechanical approach based on an examination of the interaction of a quantum system with a set of quantized field modes in a vacuum state. Both even and odd harmonics of the driving field are shown to appear in the atomic emission spectrum, and the mechanisms of their generation are elucidated. A comparison with the semiclassical theory of laser harmonic generation is made. The semiclassical approach in describing the spontaneous emission in strong laser fields, especially under conditions of significant depletion of the ground (initial) state in a laser field, is shown to be limited.

Characteristics of femtosecond laser pulses propagating in multiply ionized rare gases

A three-dimensional non-adiabatic model in combination with a rate equation model is used to characterize high-intensity (>1016W/cm2) femtosecond laser pulse propagation in atomic gas medium. In these conditions the atoms in the target gas will become multiply charged ions, while the laser pulse will propagate in a medium with high electron concentration created by itself. We obtain pulse characteristics in time, frequency and space domain for two representative cases: 800nm and 267nm (third harmonic) multi-cycle pulses which are both of practical importance. We show that the spatial-temporal variation of the refractive index in the macroscopic medium is the primary reason for pulse temporal/spectral and spatial shaping during propagation.

Range extension in laser-induced breakdown spectroscopy using femtosecond–nanosecond dual-beam laser system

We extend the detection range of laser-induced breakdown spectroscopy by combining high-intensity femtosecond laser pulses with high-energy nanosecond CO2 laser pulses. The femtosecond laser pulses ionize the molecules and generate filament in air. The free electrons generated in the self-confined plasma channel by the femtosecond laser serve as the seed electrons which cause efficient avalanche ionization in the nanosecond CO2 laser field. We show that the detection distance has been extended by three times with the assistance of femtosecond laser filamentation.