Femtosecond Intense Laser Pulse Research Articles

Controlling electron motion with attosecond precision by a shaped femtosecond intense laser pulse

Controlling electron motion with attosecond precision by a shaped femtosecond intense laser pulse

Investigation of terahertz radiation generation from laser-wakefield acceleration

We investigate the generation of terahertz (THz) radiation from laser-wakefield acceleration (LWFA) in a helium gas jet. We consider a three-dimensional setup incorporating a realistic gas density distribution and use particle-in-cell simulations to study the interaction of a femtosecond intense laser pulse with the gas medium. Our results show that LWFA can efficiently produce THz radiation. In the simulations, we use multiple probes to record the electric and magnetic fields arising from the interaction. In addition, we compare the results of fixed and moving window simulation boxes used to capture electromagnetic fields in the THz range. We demonstrate that a moving window with a 600 μm width can be significantly useful for THz studies. We further analyze the spectrum of spatially and temporally resolved electromagnetic radiation and its emission angle. Our results are consistent with experimental data. Our findings provide valuable insights into the potential of LWFA as a strong source of THz radiation.

Ionization and dissociation dynamics of H2O in ultrashort intense near-IR laser fields by the time-dependent adiabatic state method and the time-dependent configuration interaction method

In order to investigate dissociation dynamics of water molecules in a near-IR femtosecond intense laser pulse, we performed ab initio molecular dynamics calculations combined with the time-dependent adiabatic state approach. We showed that H+ is produced from H2O2+ not only by the two-body dissociation but also by the three-body dissociation and that the kinetic energy distributions of H+ for the two-body dissociation are in good agreement with the corresponding experimental data. We also performed time-dependent configuration interaction calculations to estimate the ionization probability of H2O2+ and confirmed that H2O2+ is ionized to H2O3+ by the charge-resonance enhanced ionization mechanism.

Acceleration and Deceleration of Unidirectional Molecular Rotation by a Femtosecond Laser Pulse

We report velocity controls of a directional molecular motion by a femtosecond intense laser pulse. A laser-induced, direction-controlled rotational wave packet in N2 molecules was subjected to a l...

Nitrogen fluorescence induced by the femtosecond intense laser pulses in air

Our experiments show that external focusing and initial laser energy strongly influences filament generated by the femtosecond Ti–sapphire laser in air. The experimental measurements show the filament length can be extended both by increasing the laser energy and focal length of focusing lens. On the other hand, the plasma fluorescence emission can be enhanced by increasing the laser energy with fixed focal length or decreasing the focal length. In addition, the collapse distance measured experimentally are larger than the calculated ones owing to the group-velocity-dispersion effect. In addition, we find that the line widths of the spectral lines from $\text{N}_{2}$ is independent of filament positions, laser energies and external focusing.

Multi variable control of filamentation of femtosecond laser pulses propagating in air

A comprehensive approach for control of filamentation and generation of a high density conductive channel during femtosecond intense laser pulse propagation in air is being reviewed. Imposing astigmatism on the beam with a tilted lens allows obtaining a single stable filament out of a high power pulse (orders of magnitude higher than the critical power), which would otherwise generate random multiple filamentation pattern. The collapse distance of filaments is controlled with a double lens setup. Once the filament is stabilized, a substantially extended lifetime of the high density plasma channel generated in its wake is experimentally demonstrated using combination of femtosecond and nanosecond laser pulses. Free electron density above 1015 cm−3 in the formed plasma filament is measured to sustain for over 30 ns. This high density plasma lifetime prolongation of more than one order of magnitude is achieved by properly timed irradiation of the filament with a relatively low intensity nanosecond laser pulse, in comparison to a filament without such irradiation. The experimental results are in good agreement with our theoretical model that follows the evolution of the temperature and density of various molecules, atoms and ion species. The results point to the possibility of generating substantially long time duration, stable high density plasma filaments in air.

Lateral propagation of fast electrons at the laser-irradiated target surfaces

Lateral propagation of fast electrons at the target surfaces irradiated by femtosecond intense laser pulses is measured by kα x-ray imaging technique when a preplasma is presented. An annular halo surrounding a bright spot is observed in the x-ray images when the scale length of the electron density is large. For an incidence angle of 70° the x-ray images show a non-symmetrical distribution peaked to the laser propagation direction. The x-ray photons in the halo are mainly excited by the fast electrons that flow in the preplasma when their paths intersect the high density regions near the target surface.

Effect of laser spot size on fusion neutron yield in laser–deuterium cluster interactions

The effect of the laser spot size on the neutron yield of table-top nuclear fusion from explosions of a femtosecond intense laser pulse heated deuterium clusters is investigated by using a simplified model, in which the cluster size distribution and the energy attenuation of the laser as it propagates through the cluster jet are taken into account. It has been found that there exists a proper laser spot size for the maximum fusion neutron yield for a given laser pulse and a specific deuterium gas cluster jet. The proper spot size, which is dependent on the laser parameters and the cluster jet parameters, has been calculated and compared with the available experimental data. A reasonable agreement between the calculated results and the published experimental results is found.

Two-dimensional electron momentum spectra of argon ionized by short intense lasers: Comparison of theory with experiment

We studied the two-dimensional electron momentum spectra of Ar by femtosecond intense laser pulses with mean wavelength from $400\phantom{\rule{0.3em}{0ex}}\mathrm{nm}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}800\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, to compare with experimental results of Maharjan et al. [J. Phys. B 39, 1955 (2006)]. At the higher intensities we found that the effects of ground-state depletion and laser-focus volume are very important such that the peak laser field strength is not reached in experiment. The ubiquitous fanlike stripes in the low-energy electron momentum spectra and the evidence of Freeman resonances in the experimental data are well reproduced in the theoretical calculations. We emphasize that depletion of the initial state should be carefully evaluated for ionization of atoms in the tunneling region.

Controlling ultrashort intense laser pulses by plasma Bragg gratings with ultrahigh damage threshold

Propagation of ultrashort intense laser pulses in a plasma Bragg grating induced by two counterpropagating laser pulses has been investigated. Such a plasma grating exhibits an ultrawide photonic band gap, near which strong dispersion appears. It is found that the grating dispersion dominates the dispersion of background plasma by several orders of magnitude. Particle-in-cell (PIC) simulations show light speed reduction, pulse stretching, and chirped pulse compression in the plasma grating. The nonlinear coupled-mode theory agrees well with the PIC results. Because the plasma grating has a much higher damage threshold than the ordinary optical elements made of metal or dielectric, it can be a novel tool for controlling femtosecond intense laser pulses.

Electron Energy Spectrometer for Laser-Driven Energetic Electron Generation

An electron energy spectrometer for studying the energy spectrum of electrons emitted from solid foils irradiated by a femtosecond intense laser pulse and its calibration with a β source is described. The intensity distribution of the magnetic field induced by a dipole magnet and the time decay of the photostimulated luminescence of an imaging plate were determined. Both the energy scale and the electron intensity conversion ratio of the electron energy spectrometer were calibrated with a 90Sr–90Y β source. The energy spectrum of hot electrons emitted from a 3-µm-thick Tantalum foil target irradiated by laser pulses with a pulse duration of 50 fs and a peak intensity of 2×1018 W/cm2 was determined with the calibrated electron energy spectrometer.

Ionization Induced Scattering of Femtosecond Intense Laser Pulses in Cluster Plasmas

The 45° scattering of a femtosecond (60 fs) intense laser pulse with a 20 nm FWHM (the full width at half maximum) spectrum centered at 790 nm has been studied experimentally while focused in argon clusters at intensity-1016 W/cm2. Scattering spectra under different backing pressures and laser-plasma interaction lengths were obtained, which showed spectral blueshifting, beam refraction and complex modulation. These ionization-induced effects reveal the modulation of laser pulses propagating in plasmas and the existing obstacle in laser cluster interaction at high laser intensity and high electron density.

HOW TO READ A MOLECULAR CLOCK WITH SUB-FEMTOSECOND ACCURACY

We analyze the elementary processes leading to the double ionization of D2 molecules by a single femtosecond intense laser pulse. From the total kinetic energy release of the two D+ ions which exhibits distinct peaks depending on the laser intensity, pulse length and mean wavelength, we show that the double ionization of D2 by a short femtosecond laser pulse can serve as a molecular clock. We discuss how to read such a clock correctly and how to choose laser parameters so the clock can be read more accurately.

Temporal and spatial characterization of harmonics structures of relativistic nonlinear Thomson scattering

The harmonics of the scattering of a femtosecond intense laser pulse by an electron has been numerically investigated. The harmonic spectrum shows interesting red shifts and parasitic lines in the blue sides of harmonic lines. The red shift of the lines is found to be caused by the dilation of laser oscillation experienced by an electron due to its relativistic drift motion along the direction of a driving laser propagation and the parasitic lines come from the variation of the laser intensity. The angular distribution of each higher harmonic line shows double peak patterns in the forward direction. The backward scattering has its own distinct pattern: line-shaped nodes perpendicular to the laser electric field, the number of which is the harmonic order number minus one. As the harmonic order increases, the primary peaks of higher harmonics move from the backward to the forward direction of the laser propagation. In the time domain, each radiation pulse in the case of a linearly-polarized laser pulse has a double peak structure due to the disappearance of the acceleration during the half cycle of an electron's oscillation.