D. B. Milošević

Imaging of carrier-envelope phase effects in above-threshold ionization with intense few-cycle laser fields

Sub-femtosecond control of the electron emission in above-threshold ionization of the rare gases Ar, Xe and Kr in intense few-cycle laser fields is reported with full angular resolution. Experimental data that were obtained with the velocity-map imaging technique are compared to simulations using the strong-field approximation (SFA) and full time-dependent Schrodinger equation (TDSE) calculations. We find a pronounced asymmetry in both the energy and angular distributions of the electron emission that critically depends on the carrier-envelope phase (CEP) of the laser field. The potential use of imaging techniques as a tool for single-shot detection of the CEP is discussed.

Interference structure of above-threshold ionization versus above-threshold detachment

Laser-induced electron detachment or ionization of atoms and negative ions is considered. In the context of the saddle-point evaluation of the strong-field approximation (SFA), the velocity maps of the direct electrons (those that do not undergo rescattering) exhibit a characteristic structure due to the constructive and destructive interference of electrons liberated from their parent atoms/ions within certain windows of time. This structure is defined by the above-threshold ionization rings at fixed electron energy and by two sets of curves in momentum space on which destructive interference occurs. The spectra obtained with the SFA are compared with those obtained by numerical solution of the time-dependent Schr?dinger equation. For detachment, the agreement is excellent. For ionization, the effect of the Coulomb field is most pronounced for electrons emitted in a direction close to laser polarization, while for near-perpendicular emission the qualitative appearance of the spectrum is unaffected.

Atomic processes in bicircular fields

Various laser-induced and laser-assisted processes in atoms exposed to bicircular fields are investigated. The strong-field ionization process is exemplified by the direct and by the high-order above-threshold detachment of negative fluorine ions both for corotating and counterrotating bicircular fields. The symmetries of the electron distributions in the momentum plane are analyzed in detail. The results for laser-assisted electron-ion recombination and for high-order harmonic generation are presented for the bicircular field with counterrotating components.

Carrier-envelope-phase control of plasmonic-field enhanced high-order harmonic generation

Abstract Plasmonic field-enhanced high-order harmonic generation (HHG) is investigated theoretically using the solutions of the three-dimensional time-dependent Schrödinger equation and a (semi)classical three-step model. Plasmonic field is modeled by a spatially inhomogeneous field with a time-dependent cosine squared pulse envelope. The dependence of the HHG yield on the carrier-envelope phase (CEP) is investigated. It is shown that the position of the cutoff of the HHG spectra is very sensitive to the value of CEP and that for the inhomogeneous field the dependence on the CEP is modulo. This enables both the CEP control of the plasmonic field-enhanced HHG and the determination of the CEP modulo by the measurement of the HHG cutoff position. Contrary to the homogeneous field case, for inhomogeneous field the difference between the maximum and minimum HHG cutoff (for the CEP in the interval from 0 to) remains substantial even for pulses longer than 10 optical cycles.

Interference in strong-field ionization of a two-centre atomic system

Strong-field photoionization of argon dimers by a few-cycle laser pulse is investigated using electron–ion coincidence momentum spectroscopy. The momentum distribution of the photoelectrons exhibits interference due to the emission from the two atomic argon centres, in analogy with a Youngs double-slit experiment. However, a simulation of the dimer photoelectron momentum spectrum based on the atomic spectrum supplemented with a theoretically derived interference term leads to distinct deviations from the experimental result. The deviations may have their origin in a complex electron dynamics during strong-field ionization of the Ar2 dimer.

High-order above-threshold ionisation of atoms and negative ions: channel-closing effects and the low-frequency approximation

The so-called low-frequency approximation (LFA) is an improved version of the strong-field approximation. The LFA describes the rescattering step of high-order above-threshold ionisation by utilising the exact field-free scattering amplitude, which has to be calculated separately. We apply the LFA to high-order above-threshold detachment of the fluorine negative ion. By comparing the so-obtained angle- and energy-resolved spectra with the exact results obtained as solutions of the time-dependent Schrödinger equation we show that the LFA is a more adequate approximation than the so-called improved strong-field approximation used previously. The LFA is also superior to the recently introduced quantitative rescattering (QRS) model. We show this by applying the LFA to the analysis of the intensity-dependent enhancements in above-threshold ionisation spectra of argon atoms. These enhancements can be explained as channel-closing-induced effects. In particular, we have observed and explained the behaviour of these channel-closing enhancements for higher values of the electron emission angle.

High-order harmonic generation in non-planar molecules driven by a bicircular field

ABSTRACT We investigate high-order harmonic generation of non-planar ammonia molecules by a bicircular field, which consists of two coplanar counter-rotating circularly polarised fields of frequencies ω and 2ω. This field possesses dynamical symmetries, which can be used together with the symmetry of the molecular Hamiltonian to obtain the selection rules for harmonic emission. We show analytically and confirm by numerical calculations that circularly polarised harmonics of order n = 3q ± 1, with q integer, as well as linearly polarised harmonics of order n = 3q are emitted. The presence of well-separated both linearly and circularly polarised harmonics in the same spectrum is unique to non-planar molecules.

Phase space path-integral formulation of the above-threshold ionization

Atoms and molecules submitted to a strong laser field can emit electrons of high energies in the above-threshold ionization (ATI) process. This process finds a highly intuitive and also quantitative explanation in terms of Feynmans path integral and the concept of quantum orbits [P. Salieres et al., Science 292, 902 (2001)]10.1126/science.108836. However, the connection with the Feynman path-integral formalism is explained only by intuition and analogy and within the so-called strong-field approximation (SFA). Using the phase space path-integral formalism we have obtained an exact result for the momentum-space matrix element of the total time-evolution operator. Applying this result to the ATI we show that the SFA and the so-called improved SFA are, respectively, the zeroth- and the first-order terms of the expansion in powers of the laser-free effective interaction of the electron with the rest of the atom (molecule). We have also presented the second-order term of this expansion which is responsible fo...

A gauge-covariant derivation of the strong-field approximation

A brief discussion of gauge invariance in nonrelativistic quantum mechanics is presented. The strong-field approximation for ionization of an atom by a laser field is rederived in such a way that the resulting ionization amplitude comes out identical in any gauge. This result agrees with the usual length-gauge form.

Attosecond coincidence spectroscopy of diatomic molecules

Sub-cycle ionization of Ar2 by few-cycle laser fields is investigated with COLTRIMS. Low energy photoelectrons show clear deviations from double slit interference. We suggest that breakdown of the single-active electron approximation could be responsible for such effect.