Linearly-polarized Laser Pulse Research Articles

Highly Collimated Monochromatic X-rays Generated by Collision of High-Energy Electrons with Tightly Focused Linearly Polarized Laser Pulse

This article delves into the generation and modulation process of X-rays as high-energy photon sources. Using the principles of classical electrodynamics, this study enables nonrelativistic short pulse lasers to collide with high-energy electrons while the collision center is away from the focal point. This scattering method may produce X-rays with good collimation and monochromaticity, and it progressively approaches inverse Thomson scattering. We studied and analyzed the effects of different electron characteristics and laser parameter settings on the high-energy angular distribution and spectrum of X-rays, especially the setting of the collision center and initial electron velocity, as well as the setting of laser intensity and pulse width. Linear polarized laser pulses with relativistic intensity can generate discrete supercontinuum X-rays with spectral distortion. In addition, the relationships between electronic and laser properties and radiation energy were also studied. Our research can provide valuable insights for manipulating collimated or distorted, monochromatic, or tunable X-rays, as well as understanding their properties.

Single-frequency linearly-polarized gain-switched DFB pulsed fiber laser

We report a single-frequency linearly-polarized gain-switched distributed feedback (DFB) pulsed fiber laser. A 976 nm laser diode (LD) which can generate both pulsed and continuous wave (CW) laser is used as the pump source to initiate gain-switched progress. Pulsed fiber laser with single-frequency and linearly-polarization is obtained by applying pressure on a 50 mm long phase shifted fiber Bragg grating (PS-FBG). Through parameter optimization, the laser generates single-frequency linearly-polarized laser pulses with a center wavelength of 1030.78 nm, a narrowest linewidth of 36.1 MHz, a maximum peak power of 3.78 W, and a polarization extinction ratio of better than 19 dB. And the repetition rate of output pulses can be flexibly changed from 1 kHz to 400 kHz with a pulse width of 20–200 ns.

Non-classical high harmonic generation in graphene driven by linearly-polarized laser pulses.

Recent studies in high-order harmonic generation (HHG) in solid targets reveal new scenarios of extraordinary rich electronic dynamics, in comparison to the atomic and molecular cases. For the later, the main aspects of the process can be described semiclassically in terms of electrons that recombine when the trajectories revisit the parent ion. HHG in solids has been described by an analogous mechanism, in this case involving electron-hole pair recombinations. However, it has been recently reported that a substantial part of the HHG emission corresponds to situations where the electron and hole trajectories do not overlap in space. According to the present knowledge, HHG from this imperfect recollisions reflects the quantum nature of the process, arising in systems with large Berry curvatures or for elliptically polarized driving fields. In this work, we demonstrate that imperfect recollisions are also relevant in the more general case. We show the signature of such recollisions in the HHG spectrum from monolayer graphene -a system with null Berry curvature- irradiated by linearly polarized driving fields. Our calculations also reveal that imperfect multiple-order recollisions contribute to the harmonic emission when electron-hole excursion times exceed one cycle of the driving field. We believe that our work adds a substantial contribution to the full understanding of the sub-femtosecond dynamics of HHG in solid systems.

Effects of parameters on the proton focusing driven by Coulomb explosion

The effects of laser and plasma parameters on the proton focusing driven by Coulomb explosion in the interaction of a laser pulse with a thin arched carbon-hydrogen target are investigated by two-dimensional particle-in-cell simulations. The influence of the laser polarization, the laser spot size and the geometrical dimension of the arched target has been examined. It is shown that a linearly-polarized laser pulse contributes to the occurrence of the Coulomb explosion in a shorter time, and a larger focal spot is advantageous to produce a homogeneous Coulomb field. The geometrical property of the arched target and the buffer action of heavy ions are appropriate for the forward proton focusing.

Ultrafast structural dynamics of photo-reactions observed by time-resolved x-ray cross-correlation analysis.

We applied angular X-ray Cross-Correlation analysis (XCCA) to scattering images from a femtosecond resolution X-ray free-electron laser pump-probe experiment with solvated PtPOP {[Pt2(P2O5H2)4]4–} metal complex molecules. The molecules were pumped with linear polarized laser pulses creating an excited state population with a preferred orientational (alignment) direction. Two time scales of 1.9 ± 1.5 ps and 46 ± 10 ps were revealed by angular XCCA associated with structural changes and rotational dephasing of the solvent molecules, respectively. These results illustrate the potential of XCCA to reveal hidden structural information in the analysis of time-resolved x-ray scattering data from molecules in solution.

Maxwell-consistent, symmetry- and energy-preserving solutions for ultrashort-laser-pulse propagation beyond the paraxial approximation

We analytically and numerically investigate the propagation of ultrashort tightly focused laser pulses in vacuum, with particular emphasis on Hermite-Gaussian and Laguerre-Gaussian modes. We revisit the Lax series approach for forward-propagating linearly-polarized laser pulses, in order to obtain Maxwell-consistent and symmetry-preserving analytical solutions for the propagation of all field components beyond the paraxial approximation in four-dimensional geometry (space and time). We demonstrate that our solution conserves the energy, which is set by the paraxial-level term of the series. The full solution of the wave equation towards which our series converges is calculated in the Fourier space. Three-dimensional numerical simulations of ultrashort tightly-focused pulses validate our analytical development.

Ultra-short-pulse laser ablation and modification of fully sprayed single walled carbon nanotube networks

In this study, we report on femtosecond (470fs) laser pulse ablation and modification of solution processed single-walled carbon nanotube (SWCNT) networks sprayed on oxidized silicon and polyimide substrates. Taking advantage of the small heat affected zone of the femtosecond pulse regime we demonstrate precise and selective laser processes, that advance laser patterning beyond simple galvanic isolation. A thorough investigation regarding the laser pulse-SWCNT film interaction is performed, identifying critical interaction regimes in the single- and multi-pulse operation mode. For the first time we achieve a pronounced debundling of the network, by operating the laser below the ablation threshold, a process that was previously limited to pre-deposition. Furthermore, we investigate the effect of polarized laser pulses on the ablation properties. Making use of linear polarized laser pulses we are able to selectively ablate SWCNTs, which are oriented parallel to the incident laser polarization, resulting in aligned networks with anisotropic conductivity. This feature was previously tied to direct growth of SWCNTs, strongly limiting its implementation. The scaleability of the presented femtosecond laser processes is demonstrated, allowing for the use of low-cost solution processed SWCNT thin-films, while maintaining the advantages of on-chip SWCNT growth such as SWCNT separation and alignment.

Electron acceleration in vacuum by a linearly-polarized ultra-short tightly-focused THz pulse

The analytic expressions for the electric and magnetic fields of an ultra-short, tightly-focused, linearly-polarized laser pulse propagating in vacuum, derived elsewhere (Salamin, 2015) [13] to lowest-order of a truncated power-series expansion from vector and scalar potentials, are employed here for single electron acceleration calculations by THz radiation. It is shown that, while currently available THz peak powers cannot accelerate electrons appreciably, yet they result in substantial energy gradients. The field equations are used to show that an electron can be accelerated, in vacuum, from rest to 4.83 MeV by interaction with a single THz pulse of 1 TW power. Similarly, a 1 GW power pulse focused to sub-wavelength waist radius at focus is shown to accelerate the electron from rest to 5.76 keV.

Producing High Intense Attosecond Pulse Train by Interaction of Three-Color Pulse and Overdense Plasma

Amplifying the attosecond pulse by the chirp pulse amplification method is impossible. Furthermore, the intensity of attosecond pulse is low in the interaction of laser pulse and underdense plasma. This motivates us to propose using a multi-color pulse to produce the high intense attosecond pulse. In the present study, the relativistic interaction of a three-color linearly-polarized laser-pulse with highly overdense plasma is studied. We show that the combination of , and frequencies decreases the instance full width at half maximum reflected attosecond pulse train from the overdense plasma surface. Moreover, we show that the three-color pulse increases the intensity of generated harmonics, which is explained by the relativistic oscillating mirror model. The obtained results demonstrate that if the three-color laser pulse interacts with overdense plasma, it will enhance two orders of magnitude of intensity of ultra short attosecond pulses in comparison with monochromatic pulse.

Polarization effect of a Gaussian laser pulse on magnetic field influenced electron acceleration in vacuum

Electron acceleration by a laser pulse in the presence of azimuthal magnetic field in vacuum has been analyzed. The azimuthal magnetic field influences the trajectory of an accelerated electron during the laser electron interaction in vacuum. The electron trajectory in the absence and presence of azimuthal magnetic field with a linearly polarized (LP) and circularly polarized (CP) laser pulses is analyzed. Due to the presence of azimuthal magnetic field, a confined trajectory of accelerated electron is observed in the direction of propagation of laser pulse. Resonance between the electron and the laser field occurs at optimum values of magnetic field, electron gains high energy from the laser and gets accelerated in the direction of propagation of laser pulse. The azimuthal magnetic field keeps the electron motion close to the axis parallel to the direction of propagation due to which the electron gains and retains high energy for longer distances. The electron energy gain is relatively higher with a CP laser pulse than that with LP laser pulse. The high energy gain of about 2 GeV is observed with a CP laser pulse of peak intensity 2.74×1020 W/cm2 in the presence of azimuthal magnetic field of 534 kG.

Nonlinear reflection of a nanosecond laser pulse from thin aluminum film in the temperature range 2–14 kK

This letter aims to experimentally demonstrate the possibility of measuring the temporal dependencies of the surface temperature of an aluminum film confined by a transparent dielectric in the range below and above the critical temperature of aluminum (from 2 kK to 14 kK). Such temperatures are achieved under the action of a powerful linearly-polarized laser pulse of one nanosecond in duration onto the film’s surface. To find the temporal dependencies of the temperature of the aluminum film the nonlinear reflection coefficient of its irradiated surface is measured to determine the radiation of a Q-switched Nd:YAG laser at the fundamental wavelength.

Favorable target positions for intense laser acceleration of electrons in hydrogen-like, highly-charged ions

Classical relativistic Monte Carlo simulations of petawatt laser acceleration of electrons bound initially in hydrogen-like, highly-charged ions show that both the angles and energies of the laser-accelerated electrons depend on the initial ion positions with respect to the laser focus. Electrons bound in ions located after the laser focus generally acquire higher (≈GeV) energies and are ejected at smaller angles with respect to the laser beam. Our simulations assume a tightly-focused linearly-polarized laser pulse with intensity approaching 1022 W/cm2. Up to fifth order corrections to the paraxial approximation of the laser field in the focal region are taken into account. In addition to the laser intensity, the Rayleigh length in the focal region is shown to play a significant role in maximizing the final energy of the accelerated electrons. Results are presented for both Ne9+ and Ar17+ target ions.

Spin effects in nonlinear Compton scattering in ultrashort linearly-polarized laser pulses

AbstractThe nonlinear Compton scattering by a linearly polarized laser pulse of finite duration is analyzed, with a focus on the spin effects of target electrons. We show that, although the Compton scattering accompanied by the electron no-spin flip is dominant, for some energy regions of Compton photons their emission is dominated by the process leading to the electron spin flip. This feature is observed for different pulse durations, and can be treated as a signature of quantum behavior. Similar conclusions are reached when analyzing the scattered electron energy spectra. This time, the sensitivity of spin effects to the carrier-envelope phase of the driving pulse is demonstrated. The possibility of electron acceleration by means of Compton scattering is also discussed.

Water molecules in ultrashort intense laser fields

Ionization and excitation of water molecules in intense laser pulses are studied theoretically by solving the three-dimensional time-dependent electronic Schrödinger equation within the single-active-electron approximation. The possibility to image orbital densities by measurement of the orientation-dependent ionization of H2O in few-cycle, 800nm linear-polarized laser pulses is investigated. While the highest-occupied molecular orbital 1b1 is found to dominate the overall ionization behavior, contributions from the energetically lower lying 3a1 orbital dominate the ionization yield in the nodal plane of the 1b1 orbital. The ratio of the ionization yields of the two orbitals depends on the intensity. Furthermore, even for laser pulses as long as 8 cycles the orientation-dependent ion yield depends on the carrier-envelope phase. In the interpretation of the orientation-dependent ionization as an imaging tool these effects have to be considered.

Alignment-dependent ionization of molecular hydrogen in intense laser fields

The alignment dependence of the ionization behavior of H$_2$ exposed to intense ultrashort laser pulses is investigated on the basis of solutions of the full time-dependent Schr\"odinger equation within the fixed-nuclei and dipole approximation. The total ionization yields as well as the energy-resolved electron spectra have been calculated for a parallel and a perpendicular orientation of the molecular axis with respect to the polarization axis of linear polarized laser pulses. For most, but not all considered laser peak intensities the parallel aligned molecules are easier to ionize. Furthermore, it is shown that the velocity formulation of the strong-field approximation predicts a simple interference pattern for the ratio of the energy-resolved electron spectra obtained for the two orientations, but this is not confirmed by the full ab initio results.

Molecular-alignment-assisted high-energy supercontinuum pulse generation in air

We demonstrate controllable generation of a high-energy supercontinuum pulse of 1.8 mJ through femtosecond filamentation in prealigned diatomic molecules in air. For high-energy femtosecond laser pulses of linear polarization, the focusing condition is loosened by the cross-defocusing effect from the perpendicularly orientated molecules, which promotes the high-energy supercontinuum generation with suppressed multifilamentation and decreased multiphoton ionization loss due to the reduced ionization probability.

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.

Angle-resolved covariance mapping of spatially-aligned in an intense picosecond laser field

Angle-resolved covariance mapping has been applied to measure the angular distributions of - and - ion pairs resulting from double ionization of using picosecond-duration, linearly-polarized laser pulses of intensity is spatially aligned along the laser polarization vector much more efficiently than . The field-induced dipole moment of the dication is calculated to be 21% more than that of the cation. The following sequence of events is inferred upon immersion of in the intense field: ionization and double ionization spatial alignment dissociation.

Raman Lidar Observations: Simultaneous Measurements of Water Vapor, Temperature and Aerosol Vertical Profiles, Part I.

A Raman lidar was developed to observe vertical profiles of water vapor, temperature and aerosols. The lidar system can detect signal from 5 detectors simultaneously. Two vibrational Raman backscattering echoes from H 2 O and N 2 (or O 2 ) excited by Nd:YAG 3rd harmonics wavelength (355 nm) are used to observe the profiles of water vapor mixing ratio and atmospheric temperature (or density). Nd:YAG fundamental and 2nd harmonics wavelengths (1064 and 532 nm) are used to observe backscattering coefficient profiles of aerosols and clouds at these wavelengths. The scattering at 532 nm is observed in parallel and perpendicular components corresponding to the linear polarization plane of the transmitted linearly-polarized laser pulse to observe depolarization ratio profiles. Observed water vapor and temperature distributions coincide with the simultaneously observed radiosonde sounding data excellently. The Raman lidar makes it possible to observe the vertical profiles of water vapor, temperature, aerosols, and clouds simultaneously.