Evidence for rescattering in intense, femtosecond laser interactions with a negative ion

It is now well established that energetic electron emission, nonsequential ionization, and high harmonic generation, produced during the interaction of intense, femtosecond laser pulses with atoms (and atomic positive ions), can be explained by invoking rescattering of the active electron in the laser field, the so-called rescattering mechanism. In contrast for negative ions, the role of rescattering has not been established experimentally. By irradiating F- ions with ultrashort laser pulses, F+ ion yields as a function of intensity for both linearly and circularly polarized light have been measured. We find that, at intensities well below saturation for F+ production by sequential ionization, there is a small but significant enhancement in the yield for the case of linearly polarized light, providing the first clear experimental evidence for the existence of the rescattering mechanism in negative ions.

High harmonic generation by halogen anions and noble gas atoms in a laser field

A comparative study of high harmonic generation (HHG) by atoms and ions with active p-electrons is carried out in the theoretical framework of the rescattering mechanism. The substate with m(l) = 0, i.e. zero orbital momentum projection along the electric vector of a linearly polarized laser wave, is found to give the major contribution to the HHG rate. Our calculations for HHG by an H atom in an excited 2p-state demonstrate that the rate for recombination into a final state with a different value of m(l) (= +/- 1), is higher for lower harmonic orders N, while for higher N (beyond the plateau domain) the difference vanishes. For species with closed electron shells, the m(l)-changing transitions are forbidden by the Pauli exclusion principle. We report absolute HHG rates for halogen ions and noble gas atoms at various intensities. These results demonstrate that the Coulomb binding potential of the atoms considerably enhances both the ionization and recombination steps in the rescattering process. However, the weak binding energy of the anions allows lower orders of HHG to be efficiently produced at relatively low intensities, from which we conclude that observation of HHG by an anion is experimentally feasible.

Efficient grid treatment of the ionization dynamics of laser-driven H-2(+)

We implement a parallel, time-dependent hybrid finite-difference Lagrange mesh code to model the electron dynamics of the fixed-nuclei hydrogen molecular ion subjected to intense ultrashort laser Pulses, Ionization rates are calculated and compared with results from a previous finite-difference approach and also with published Floquet results. The sensitivity of the results to the gauge describing the electron-field interaction is studied. Visualizations of the evolving wave packets are also presented in which the formation of a stable bound-state resonance is observed.

Reconstruction of atomic ionization probabilities in intense laser fields

Application of a parallel-projection inversion technique to z-scan spectra of multiply charged xenon and krypton ions, obtained by non-resonant field ionization of neutral targets, has for the first time permitted the direct observation of intensity-dependent ionization probabilities. These ionization efficiency curves have highlighted the presence of structure in the tunnelling regime, previously unobserved under full-volume techniques.

Multiple ionization of ions and atoms by intense ultrafast laser pulses

Non-sequential processes in the multiple ionization of Xe and Xe+ targets subject to intense femtosecond laser pulses have been investigated. A precise ratio has been determined for the direct comparison of ionization using circular and linear polarized fields. Suppression of non-sequential effects where an ionic target is compared to a neutral atom target has been confirmed.

Carbon ion acceleration from thin foil targets irradiated by ultrahigh-contrast, ultraintense laser pulses

In this study, ion acceleration from thin planar target foils irradiated by ultrahigh-contrast (10(10)), ultrashort (50 fs) laser pulses focused to intensities of 7 x 10(20) W cm(-2) is investigated experimentally. Target normal sheath acceleration (TNSA) is found to be the dominant ion acceleration mechanism when the target thickness is >= 50 nm and laser pulses are linearly polarized. Under these conditions, irradiation at normal incidence is found to produce higher energy ions than oblique incidence at 35 degrees with respect to the target normal. Simulations using one-dimensional (1D) boosted and 2D particle-in-cell codes support the result, showing increased energy coupling efficiency to fast electrons for normal incidence. The effects of target composition and thickness on the acceleration of carbon ions are reported and compared to calculations using analytical models of ion acceleration.

Electron detachment from negative ions in a short laser pulse

We present an efficient and accurate method to study electron detachment from negative ions by a few-cycle linearly polarized laser pulse. The adiabatic saddle-point method of Gribakin and Kuchiev [Phys. Rev. A 55, 3760 (1997)] is adapted to calculate the transition amplitude for a short laser pulse. Its application to a pulse with N optical cycles produces 2(N + 1) saddle points in complex time, which form a characteristic "smile." Numerical calculations are performed for H(-) in a 5-cycle pulse with frequency 0.0043 a.u. and intensities of 10(10), 5 x 10(10), and 10(11) W/cm(2), and for various carrier-envelope phases. We determine the spectrum of the photoelectrons as a function of both energy and emission angle, as well as the angle-integrated energy spectra and total detachment probabilities. Our calculations show that the dominant contribution to the transition amplitude is given by 5-6 central saddle points, which correspond to the strongest part of the pulse. We examine the dependence of the photoelectron angular distributions on the carrier-envelope phase and show that measuring such distributions can provide a way of determining this phase.

The RMT method for many-electron atomic systems in intense short-pulse laser light

We describe a new ab initio method for solving the time-dependent Schrödinger equation for multi-electron atomic systems exposed to intense short-pulse laser light. We call the method the R-matrix with time-dependence (RMT) method. Our starting point is a finite-difference numerical integrator (HELIUM), which has proved successful at describing few-electron atoms and atomic ions in strong laser fields with high accuracy. By exploiting the R-matrix division-of-space concept, we bring together a numerical method most appropriate to the multi-electron finite inner region (R-matrix basis set) and a different numerical method most appropriate to the one-electron outer region (finite difference). In order to exploit massively parallel supercomputers efficiently, we time-propagate the wavefunction in both regions by employing Arnoldi methods, originally developed for HELIUM.

Stable ion radiation pressure acceleration with intense laser pulses

We have demonstrated the promising radiation pressure acceleration (RPA) mechanism of laser-driven ion acceleration at currently achievable laser and target parameters through a large number of two-dimensional particle-in-cell simulations and experiments. High-density monoenergetic ion beams with unprecedented qualities such as narrow-peaked spectrum, lower-divergence and faster energy-scaling are obtained, compared with the conventional target normal sheath acceleration. The key condition for stable RPA from thin foils by intense circularly polarized lasers has been identified, under which the stable RPA regime can be extended from ultrahigh intensities > 10(22) W cm(-2) to a currently accessible range 10(20)-10(21) W cm(-2). The dependences of the RPA mechanism on laser polarization, intensity and on the target composition and areal density have been studied.

Development of XUV lasers at the RAL central laser facility

Recent progress in the development of XUV lasers by research teams using high-power and ultrashort-pulse Nd:glass and KrF laser facilities at the Rutherford Appleton Laboratory is reviewed. Injector-amplifier operation and prepulse enhanced output of the Ge XXIII collisional laser driven by a kilojoule glass laser, enhanced gain in CVI recombination with picosecond CPA drive pulses from a glass laser, and optical field ionization and XUV harmonic generation with a KrF CPA laser are described.

Dominance of Radiation Pressure in Ion Acceleration with Linearly Polarized Pulses at Intensities of 10(21) W cm(-2)

A novel regime is proposed where, by employing linearly polarized laser pulses at intensities 10(21) W cm(-2) (2 orders of magnitude lower than discussed in previous work [T. Esirkepov et al., Phys. Rev. Lett. 92, 175003 (2004)]), ions are dominantly accelerated from ultrathin foils by the radiation pressure and have monoenergetic spectra. In this regime, ions accelerated from the hole-boring process quickly catch up with the ions accelerated by target normal sheath acceleration, and they then join in a single bunch, undergoing a hybrid light-sail-target normal sheath acceleration. Under an appropriate coupling condition between foil thickness, laser intensity, and pulse duration, laser radiation pressure can be dominant in this hybrid acceleration. Two-dimensional particle-in-cell simulations show that 1.26 GeV quasimonoenergetic C6+ beams are obtained by linearly polarized laser pulses at intensities of 10(21) W cm(-2).

Micron-scale fast electron filaments and recirculation determined from rear-side optical emission in high-intensity laser-solid interactions

The transport of relativistic electrons generated in the interaction of petawatt class lasers with solid targets has been studied through measurements of the second harmonic optical emission from their rear surface. The high degree of polarization of the emission indicates that it is predominantly optical transition radiation (TR). A halo that surrounds the main region of emission is also polarized and is attributed to the effect of electron recirculation. The variation of the polarization state and intensity of radiation with the angle of observation indicates that the emission of TR is highly directional and provides evidence for the presence of mu m-size filaments. A brief discussion on the possible causes of such a fine electron beam structure is given.

Measurements of fast electron scaling generated by petawatt laser systems

Fast electron energy spectra have been measured for a range of intensities between 10(18) and 10(21) W cm(-2) and for different target materials using electron spectrometers. Several experimental campaigns were conducted on petawatt laser facilities at the Rutherford Appleton Laboratory and Osaka University, where the pulse duration was varied from 0.5 to 5 ps relevant to upcoming fast ignition integral experiments. The incident angle was also changed from normal incidence to 40 degrees in p-polarized. The results confirm a reduction from the ponderomotive potential energy on fast electrons at the higher intensities under the wide range of different irradiation conditions.

Strong-field photodetachment of H- by few-cycle laser pulses

The recent adiabatic saddle-point method of Shearer et al. [ Phys. Rev. A 84 033409 (2011)] is applied to study strong-field photodetachment of H- by few-cycle linearly polarized laser pulses of frequencies near the two-photon detachment threshold. The behavior of the saddle points in the complex-time plane for a range of laser parameters is explored. A detailed analysis of the influence of laser intensities [(2×1011)–(6.5 × 1011) W/cm2], midinfrared laser wavelengths (1800–2700 nm), and various values of the carrier envelope phase (CEP) on (i) three-dimensional probability detachment distributions, (ii) photoangular distributions (PADs), (iii) energy spectra, and (iv) momentum distributions are presented. Examination of the probability distributions and PADs reveal main lobes and jetlike structures. Bifurcation phenomena in the probability distributions and PADs are also observed as the wavelength and intensity increase. Our simulations show that the (i) probability distributions, (ii) PADs, and (iii) energy spectra are extremely sensitive to the CEP and thus measuring such distributions provides a useful tool for determining this phase. The symmetrical properties of the electron momentum distributions are also found to be strongly correlated with the CEP and this provides an additional robust method for measuring the CEP of a laser pulse. Our calculations further show that for a three-cycle pulse inclusion of all eight saddle points is required in the evaluation of the transition amplitude to yield an accurate description of the photodetachment process. This is in contrast to recent results for a five-cycle pulse.

Radiochromic film spectroscopy of laser-accelerated proton beams using the FLUKA code and dosimetry traceable to primary standards

A new approach to spectroscopy of laser induced proton beams using radiochromic film (RCF) is presented. This approach allows primary standards of absorbed dose-to-water as used in radiotherapy to be transferred to the calibration of GafChromic HD-810 and EBT in a 29 MeV proton beam from the Birmingham cyclotron. These films were then irradiated in a common stack configuration using the TARANIS Nd:Glass multi-terawatt laser at Queens University Belfast, which can accelerate protons to 10-12 MeV, and a depth-dose curve was measured from a collimated beam. Previous work characterizing the relative effectiveness (RE) of GafChromic film as a function of energy was implemented into Monte Carlo depth-dose curves using FLUKA. A Bragg peak (BP) "library" for proton energies 0-15 MeV was generated, both with and without the RE function. These depth-response curves were iteratively summed in a FORTRAN routine to solve for the measured RCF depth-dose using a simple direct search algorithm. By comparing resultant spectra with both BP libraries, it was found that the effect of including the RE function accounted for an increase in the total number of protons by about 50%. To account for the energy loss due to a 20 mu m aluminum filter in front of the film stack, FLUKA was used to create a matrix containing the energy loss transformations for each individual energy bin. Multiplication by the pseudo-inverse of this matrix resulted in "up-shifting" protons to higher energies. Applying this correction to two laser shots gave further increases in the total number of protons, N of 31% and 56%. Failure to consider the relative response of RCF to lower proton energies and neglecting energy losses in a stack filter foil can potentially lead to significant underestimates of the total number of protons in RCF spectroscopy of the low energy protons produced by laser ablation of thin targets.