Laser Propagation Direction Research Articles

Coulomb rescattering in nondipole interaction of atoms with intense laser fields

We investigate the ionization dynamics of atoms irradiated by an intense laser field using a semiclassical model that includes magnetic Lorentz force in the rescattering process. We find that, the electrons tunneled with different initial transverse momenta (i.e., perpendicular to the instantaneous electric field direction) distributed on a specific circle in the momentum plane can finally converge to the same transverse momentum after experiencing Coulomb forward scattering. These electron trajectories lead to a bright spot structure in the 2D transverse momentum distribution, and particularly in the long-wavelength limit, a nonzero momentum peak in the direction antiparallel to the laser propagation (or radiation pressure) direction. Making analysis of the subcycle dynamics of rescattering trajectories, we unveil the underlying mechanism of the anti-intuitive peak. Beyond the strong field approximation and the dipole approximation, we quantitatively predict the spot center and the peak position. Our results are compared with a recent experiment and some theoretical predictions are given.

Amplification of Relativistic Electron Bunches by Acceleration in Laser Fields.

Direct acceleration of electrons in a coherent, intense light field is revealed by a remarkable increase of the electron number in the MeV energy range. Laser irradiation of thin polymer foils with a peak intensity of ∼1×10^{20} W/cm^{2} releases electron bunches along the laser propagation direction that are postaccelerated in the partly transmitted laser field. They are decoupled from the laser field at high kinetic energies, when a second foil target at an appropriate distance prevents their subsequent deceleration in the declining laser field. The scheme is established with laser pulses of high temporal contrast (10^{10} peak to background ratio) and two ultrathin polymer foils at a distance of 500 μm. 2D particle in cell simulations and an analytical model confirm a significant change of the electron spectral distribution due to the double foil setup, which leads to an amplification of about 3 times of the electron number around a peak at 1MeV electron energy. The result verifies a theoretical concept of direct electron bunch acceleration in a laser field that is scalable to extreme acceleration potential gradients. This method can be used to enhance the density and energy spread of electron bunches injected into postaccelerator stages of laser driven radiation sources.

Collective dipole-dipole interactions in an atomic array

The coherent dipole-dipole interactions of atoms in an atomic array are studied. It is found that the excitation probability of an atom in an array parallel to the direction of laser propagation ($\boldsymbol{\hat{k}}$) will either grow or decay logarithmically along $\boldsymbol{\hat{k}}$, depending on the detuning of the laser. The symmetry of the system for atomic separations of $\delta r = j\lambda/2$, where $j$ is an integer, causes the excitation distribution and scattered radiation to abruptly become symmetric about the center of the array. For atomic separations of $\delta r < \lambda/2$, the appearance of a collection of extremely subradiant states ($\Gamma\sim 0$), disrupts the described trend. In order to interpret the results from a finite array of atoms, a band structure calculation in the $N\rightarrow \infty$ limit is conducted where the decay rates and the Collective Lamb Shifts of the eigenmodes along the Brillouin zone are shown. Finally, the band structure of an array strongly affects its scattered radiation, allowing one to manipulate the Collective Lamb Shift as well as the decay rate (from superradiant to subradiant) by changing the angle of the driving laser.

Suppression of spin-exchange relaxation in tilted magnetic fields within the geophysical range

We present a detailed experimental and theoretical study on the relaxation of spin coherence due to the spin-exchange mechanism arising in the electronic ground states of alkali-metal vapor atoms. As opposed to the well-explored formation of a stretched state in a longitudinal geometry (magnetic field parallel to the laser propagation direction) we employ adapted hyperfine-selective optical pumping in order to suppress spin-exchange relaxation. By comparing measurements of the intrinsic relaxation rate of the spin coherence in the ground state of cesium atoms with detailed density-matrix simulations we show that the relaxation due to spin-exchange collisions can be reduced substantially even in a tilted magnetic field of geomagnetic strength, the major application case of scalar magnetic surveying. This explains the observed striking improvement in sensitivity and further deepens the understanding of the light-narrowed ${M}_{x}$ magnetometer, which was presented recently. Additionally, new avenues for investigating the dynamics in alkali-metal atoms governed by the spin-exchange interaction and interacting with arbitrary external fields open up.

Laser filamentation induced bubbles and their motion in water.

We demonstrate femtosecond filamentation induced convection in water by using a microscope directly observing the dynamic processes of the generated bubbles on a macroscopic time scale. The bubbles are driven by the filament in water and do directional movements. The angles between the bubbles' moving directions and the laser propagation direction varied at different positions along the filament, exhibiting a fusiform distribution. It indicates a fluid dynamic phenomenon depending on the local filament intensity, and reveals the convection processes induced by filamentation in water indirectly.

Generalized velocity-gauge form of the light-matter interaction Hamiltonian beyond the dipole approximation

The exact velocity-gauge minimal-coupling Hamiltonian describing the laser-matter interaction is transformed into another form by means of a series of gauge transformations. The Hamiltonian corresponding to this point of view is valid for an arbitrary time- and space-dependent laser field, also known as a nondipole field. In effect, the Hamiltonian represents a generalization of the original velocity-gauge minimal-coupling Hamiltonian in the sense that the particle's (classical) velocity in the laser propagation direction is also explicitly accounted for by a new operator term. Imposing the so-called long-wavelength approximation (LWA) on the field, i.e., assuming the laser wavelength being much larger than the extent of the atomic system, the spatial dependence of the field can be neglected and the interaction Hamiltonian reduces to a simpler form. Nevertheless, the resulting LWA Hamiltonian includes the effect of the magnetic-field component of the laser, which is in clear contrast with the usual dipole approximation Hamiltonian derived by imposing the LWA directly on the initial velocity-gauge minimal-coupling Hamiltonian. As such, the weak-field condition necessary to justify neglecting the magnetic field, and the LWA condition, can be considered independently in this formalism, making it an attractive alternative for a broad range of applications in strong-field physics. We demonstrate that, from a numerical perspective, this form of the light-matter interaction is advantageous compared to its standard velocity-gauge counterpart as it gives rise to faster convergence properties when describing ionization dynamics in superintense fields beyond the dipole approximation.

Non-Maxwellian electron distributions resulting from direct laser acceleration in near-critical plasmas

The irradiation of few-nm-thick targets by a finite-contrast high-intensity short-pulse laser results in a strong pre-expansion of these targets at the arrival time of the main pulse. The targets decompress to near and lower than critical densities with plasmas extending over few micrometers, i.e. multiple wavelengths. The interaction of the main pulse with such a highly localized but inhomogeneous target leads to the generation of a short channel and further self-focusing of the laser beam. Experiments at the Glass Hybrid OPCPA Scaled Test-bed (GHOST) laser system at University of Texas, Austin using such targets measured non-Maxwellian, peaked electron distribution with large bunch charge and high electron density in the laser propagation direction. These results are reproduced in 2D PIC simulations using the EPOCH code, identifying direct laser acceleration (DLA) [1] as the responsible mechanism. This is the first time that DLA has been observed to produce peaked spectra as opposed to broad, Maxwellian spectra observed in earlier experiments [2]. This high-density electrons have potential applications as injector beams for a further wakefield acceleration stage as well as for pump-probe applications.

High-order harmonic generation of O2 molecules with different nuclear axis orientations

High-order harmonic generation (HHG) is one of the hottest topics in strong field atomic and molecular physics. In this paper, frequency domain theory which is based on formal scattering theory is extended to study the HHG of O2 molecules under a linearly polarized single mode laser field. The dependence of HHG on the angle 0 between the laser polarization direction and nuclear axis is investigated. In our calculation, we only consider the contribution of highest occupied molecular orbital (HOMO) and use the single electron approximation. The HOMO is obtained from quantum chemical software Molpro. The intensity of the laser is 5.181014 W/cm2 and the wavelength is 800 nm. On the one hand, in the case that the nuclear axis lies in the plane perpendicular to the laser propagation direction, we find that the yields of all order harmonics increase with 0 increasing until the yields reach the maximum values when 0 is equal to about 45. Then the yields decrease with 0 increasing and have the minimum values when 0 is equal to about 90. The analysis shows that the yield of HHG is dominated by the density of electrons in HOMO along the laser polarizing direction in momentum space. On the other hand, in the case that the nuclear axis lies in the plane parallel to laser propagation direction, the dependence of HHG on 0 is the same as that when the nuclear axis is in the plane perpendicular to laser propagation direction. The reasons for the same results for the two cases lie in the following fact. The HOMO of O2 molecule has g symmetry which is not rotationally symmetric around nuclear axis. So HHG yield relies on the g extension orientation. Since the g extension orientation cannot be fixed, the HHG of O2 should be averaged over the contributions to HHG at all possible g extension orientations. This average is equivalent to that the electron density is rotationally symmetric around the nuclear axis and hence leads to the fact that the HHG yield of O2 depends on 0 rather than the plane that the nuclear axis lies in.

Unsteady oscillation distribution model of liquid jet in supersonic crossflows

Unsteady distribution of spray is experimentally studied when a round liquid jet is injected into a supersonic crossflow vertically. An oscillation distribution model for the liquid column and spray is established. Tyndall scattering caused by the sol medium is put forward to eliminate the interference effect of monochromatic laser passing through the supersonic gas flow field. The scattering causes the disordering of laser propagation direction and phase, thus makes the planar light source uniform and eliminate the interference effect of laser at the same time. Then a uniform light source is formed and can be set as the uniform background with a pulse width of 7 ns. The camera, with dimension of CCD pixel space of 40002672 pixel, is located directly in front of planar light source, and the shooting area is between both. The frozen liquid jet/spray images with high spatiotemporal resolution are captured using the pulsed laser background imaging (PLBI) method in supersonic crossflows. And the drag phenomenon caused by the too-long exposure time in the ordinary and traditional high-speed imaging process is avoided. Based on the maximizing inter-class variance method (Otsu) and Canny method, the out boundary of liquid jet/spray are extracted from an instantaneous image. A dimensionless parameter named intermittency factor (the logogram is r) is defined and used to quantitatively analyze the oscillation distribution characteristics of jet/spray. The intermittency factor of the whole spray field could be calculated by sample probability statistic method. An empirical jet/spray oscillation distribution model, in supersonic crossflows, is summarized based on parameter studies. Various conditions are studied, including stagnation pressure range of gas (642 kPa to 1010 kPa), practical pressure range (0.36 MPa to 4.61 MPa), nozzle diameters (0.48 mm/1.0 mm/1.25 mm/1.52 mm), distances down from nozzle (10 mm to 125 mm), and jet-gas momentum flux ratio range (0.11 to 7.49). The empirical model is used to predict the oscillation distribution of water jet penetrated in a Ma2.1 supersonic crossflow. It is indicated that the predictive result matches well with the experimental result. It could be concluded that the PLBI method presented in this paper reasonably utilizes the high energy and short pulse characteristics of the laser to successfully complete the frozen image of liquid jet/spray under the condition of supersonic crossflow. The dimensionless parameter r defined in the study can be used to quantitatively analyze the oscillation distribution characteristics of jet/spray well. This study has important significance for understanding the diffusion characteristics of liquid jet in supersonic crossflows.

Responsivity optimization in magneto-optic sensors based on ferromagnetic materials.

In an effort to optimize magnetic field detection sensitivities, the Faraday responsivity vector, which determines the relationship between the Faraday rotation angle and an externally applied magnetic field, was investigated in magneto-optic sensors based on bismuth-doped iron-garnet films. Under externally applied fields, Faraday rotation is produced principally by domain rotation and domain wall motion, whose relative contributions depend on the domain geometry and the direction of laser propagation. When optically probed along a principal magnetization axis, Faraday rotation is driven by a single magnetization mechanism, and the responsivity is linearized (reduced to an effective Verdet constant). When the films are probed along an oblique angle to the principal axes, the relationship between the Faraday rotation and the external field becomes tensorial and much more complex. Although this may lead to more complicated phenomena, the interplay of domain rotation and domain wall bowing can be exploited to improve responsivity or bandwidth. A generalized model for the magnitude and direction of the responsivity vector is formulated, which gives predictions that are consistent with the experimental data. Applications to arrayed sensors and three-axis field measurements are discussed.

Weibull-type speckle distributions as a result of saturation in stimulated scattering processes

AbstractDuring the propagation of an optically smoothed laser beam through a warm plasma the speckle field pattern and the corresponding speckle intensity distribution are modified in time and along the laser propagation direction. It is shown here that the laser–plasma interaction can change the character of speckle statistics from an initially exponential-type limit law to a Weibull-type law. The Weibull distribution is characterized by a power-law-type behavior in a limited interval of the random variable, which is, in the present case, the speckle intensity. The properties of the speckle distributions are studied using methods of extremal and order statistics. The scattering instability process (here stimulated Brillouin forward scattering) causing the change in speckle statistics has an onset behavior associated with a “critical gain” value, as pointed out in work by Rose and DuBois (1993b). The saturation of the instability process as a function of intensity explains the limited interval of the Weibull-type speckle distribution. The differences in the type of the speckle statistics are analyzed by using “excess over threshold” methods relying on the generalized Pareto distribution, which clearly brings to evidence the transition from an exponential type distribution to the Weibull-type distribution as a function of the instability gain value, that is, from the regime below critical gain to values above the critical gain.

Observation of the inhomogeneous spatial distribution of MeV ions accelerated by the hydrodynamic ambipolar expansion of clusters

An inhomogeneous spatial distribution of laser accelerated carbon/oxygen ions produced via the hydrodynamic ambipolar expansion of CO2 clusters has been measured by using CR-39 detectors. An inhomogeneous etch pits spatial distribution has appeared on the etched CR-39 detector installed on the laser propagation direction, while homogeneous ones are appeared on those installed at 45° and 90° from the laser propagation direction. From the range of ions in CR-39 obtained by using the multi-step etching technique, the averaged energies of carbon/oxygen ions for all directions are determined as 0.78 ± 0.09 MeV/n. The number of ions in the laser propagation direction is about 1.5 times larger than those in other directions. The inhomogeneous etch pits spatial distribution in the laser propagation direction could originate from an ion beam collimation and modulation by the effect of electromagnetic structures created in the laser plasma.

Multi-view second-harmonic generation imaging of mouse tail tendon via reflective micro-prisms.

Here we experimentally show that second-harmonic generation (SHG) imaging is not sensitive to collagen fibers oriented parallel to the direction of laser propagation and, as a consequence, can potentially miss important structural information. As an alternative approach, we demonstrate the use of reflective micro-prisms to enable multi-view SHG imaging of mouse tail tendon by redirecting the focused excitation and collection of subsequent emission. Our approach data corroborates the theoretical treatment on vanishing and nonvanishing orientations, where fibers along the laser direction are largely transparent by SHG. In strong contrast, the two-photon excited fluorescence of dye-labeled collagen fibers is isotropic and is not subject to this constraint. We utilized Pearson correlation to quantify differences in fluorescent and backward detected SHG images of the tendon fiber structure, where the SHG and TPEF were highly statistically correlated (0.6-0.8) for perpendicular excitation but were uncorrelated for excitation parallel to the fiber axis. The results suggest that improved imaging of 3D collagen structure is possible with multi-view SHG microscopy.

Effects of magnetically induced optical incoherence in arrayed Faraday rotator crystals

Bismuth doped iron garnet films were studied in arrayed structures to improve sensitivity in polarimetric magnetic field sensors. The sensitivity gained from the arrayed configuration was found to depend on the ferromagnetic domain geometry of the films. Under small, externally applied fields, films with planar anisotropy yielded an optical modulation depth which increased linearly with the number of crystals in the array. In films with perpendicular anisotropy, linearity and sensitivity were obstructed by a mechanism of optical polarization incoherence which was found to be generated within the magnetic domains. Our results indicate that optimal sensitivity in arrayed configurations requires planar anisotropies, or a direction of laser propagation that is perpendicular to the local domain magnetization.

Terahertz emission from laser-induced microplasma in ambient air

The integration of ambient air plasmas as source and sensor in terahertz time-domain techniques allows spectral measurements covering the elusive terahertz gap (0.1–10 THz), further increasing the impact of those scientific tools in the study of the four states of matter. In this article we describe the experimental study of the terahertz emission from a laser-induced plasma of submillimeter size. The main direction of emission is almost orthogonal to the laser propagation direction, unlike that of elongated plasmas. We show that laser pulse energies lower than 1 μJ are sufficient to generate measurable terahertz pulses from ambient air. This significant decrease in the required laser energy will make plasma-based terahertz techniques more accessible to the scientific community.

Backward-propagating MeV electrons from 1018 W/cm2 laser interactions with water

We present an experimental study of the generation of ∼MeV electrons opposite to the direction of laser propagation following the relativistic interaction at normal incidence of a ∼3 mJ, 1018 W/cm2 short pulse laser with a flowing 30 μm diameter water column target. Faraday cup measurements record hundreds of pC charge accelerated to energies exceeding 120 keV, and energy-resolved measurements of secondary x-ray emissions reveal an x-ray spectrum peaking above 800 keV, which is significantly higher energy than previous studies with similar experimental conditions and more than five times the ∼110 keV ponderomotive energy scale for the laser. We show that the energetic x-rays generated in the experiment result from backward-going, high-energy electrons interacting with the focusing optic, and vacuum chamber walls with only a small component of x-ray emission emerging from the target itself. We also demonstrate that the high energy radiation can be suppressed through the attenuation of the nanosecond-scale pre-pulse. These results are supported by 2D particle-in-cell simulations of the laser-plasma interaction, which exhibit beam-like backward-propagating MeV electrons.

Quantitative determination of the spatial orientation of graphene by polarized Raman spectroscopy

Polarized Raman spectroscopy has been employed to characterize transverse sections of graphene monolayers upon both a copper substrate and a polyester film. Well-defined Raman spectra can be obtained from the one atom thick transverse sections of graphene because of the strong resonance Raman scattering. The intensity of Raman 2D band (I2D) is independent of the axis of laser polarization when the laser beam is perpendicular to the surface of the graphene monolayer but I2D is found to vary as approximately the 4th power of the cosine of the angle between the axis of laser polarization and the plane of graphene when the direction of laser propagation is parallel to the graphene sheet. It is demonstrated that a generalized spherical expanded harmonics orientation distribution function (ODF) can be used to quantify the spatial orientation of the graphene. The roughness of the graphene, evaluated using atomic force microscopy, shows a good correlation with the ODF determined using polarized Raman spectroscopy, showing how the Raman technique may be employed to quantify the spatial orientation of graphene. It is also demonstrated how the technique can be used to quantify the orientation of graphene in high-ordered pyrolytic graphite and graphene paper.

Hollow screw-like drill in plasma using an intense Laguerre-Gaussian laser.

With the development of ultra-intense laser technology, MeV ions can be obtained from laser–foil interactions in the laboratory. These energetic ion beams can be applied in fast ignition for inertial confinement fusion, medical therapy, and proton imaging. However, these ions are mainly accelerated in the laser propagation direction. Ion acceleration in an azimuthal orientation was scarcely studied. In this research, a doughnut Laguerre–Gaussian (LG) laser is used for the first time to examine laser–plasma interaction in the relativistic intensity regime in three-dimensional particle-in-cell simulations. Studies have shown that a novel rotation of the plasma is produced from the hollow screw-like drill of an mode laser. The angular momentum of particles in the longitudinal direction produced by the LG laser is enhanced compared with that produced by the usual laser pulses, such as linearly and circularly polarized Gaussian pulses. Moreover, the particles (including electrons and ions) can be trapped and uniformly compressed in the dark central minimum of the doughnut LG pulse. The hollow-structured LG laser has potential applications in the generation of x-rays with orbital angular momentum, plasma accelerators, fast ignition for inertial confinement fusion, and pulsars in the astrophysical environment.

Ultrahigh-energy electron beam generated by ultra-intense circularly polarized laser pulses

The earlier research showed that circularly polarized laser pulses with peak intensities in a range of 1022-1025 W/cm2 can directly accelerate and generate GeV-TeV monoenergetic electron beams with a linear energy scaling with the laser intensity. To obtain higher energy electron beams, a scheme is proposed to use an electron beam with an initial energy E0 along the laser propagation direction. This scheme can overcome the linear energy scaling with E0=0 obtained previously and enhance the beam energy by E0 folds. This is because an electron beam with an initial energy can move with the laser pulse together and therefore obtain a longer acceleration distance. Two-dimensional particle-in-cell simulation shows that this scheme is effective only for the electron beams initially with low energy on the order of MeV. With overhigh energy, electrons will miss the optimum acceleration field because the electron acceleration distance is much longer than the Rayleigh distance and the laser intensity is significantly attenuated.

Spin-asymmetric laser-driven relativistic tunneling frompstates

The tunneling ionization of an electron from a p-state in a highly charged ion in the relativistic regime is investigated in a linearly polarized strong laser field. In contrast to the case of an s-state, the tunneling ionization from the p-state is spin asymmetric. We have singled out two reasons for the spin asymmetry: first, the difference of the electron energy Zeeman splitting in the bound state and during tunneling, and second, the relativistic momentum shift along the laser propagation direction during the under-the barrier motion. Due to the latter, those states are predominantly ionized where the electron rotation is opposite to the electron relativistic shift during the under-the-barrier motion. We have investigated the dependence of the ionization rate on the laser intensity for different projections of the total angular momentum and identified the intensity parameter which governs this behaviour. The significant change of the ionization rate is originated from the different precession dynamics of the total angular momentum in the bound state at high and low intensities.