Study of correlation effects during reemission of ultrashort electromagnetic pulses by the helium atom

The calculation of the energy of the ground state of atoms is the criterion for the correctness of compact analytical atomic wave functions. However, such a test is in fact static. We studied the degree of taking into account electron correlations in various simple analytical wave functions in the dynamic process of reemission by the helium atom of ultrashort pulses of an electromagnetic field. Direct guidelines are given on the use of particular analytical wave functions, which can be helpful in simple calculations and estimations of dynamic processes.     

Inelastic processes and interference effects during the interaction of positronium with ultrashort electromagnetic pulses

The excitation, breakup, and reradiation during the interaction of a positronium atom with ultrashort electromagnetic pulses are considered. The probabilities of inelastic processes and reradiation spectra have been obtained. The interference between the amplitudes of the photon emission by the electron and positron is shown to contribute noticeably to the reradiation spectra. The developed approach is applicable for describing the interaction of positronium with ultrashort pulses of attosecond or shorter duration.     

Self-action dynamics of ultrashort electromagnetic pulses

The self-action dynamics of three-dimensional wave packets whose width is on the order of the carrier frequency is studied under fairly general assumptions concerning the dispersion properties of the medium. The condition for the wave field collapse is determined. Self-action regimes in a dispersion-free medium and in media with predominance of anomalous or normal group velocity dispersions are numerically investigated. It is shown that, for extremely short pulses, nonlinearity leads not only to the self-compression of the wave field but also to a “turn-over” of the longitudinal profile. In a dispersionless medium, the formation of a shock front within the pulse leads to the nonlinear dissipation of linearly polarized radiation and to self-focusing stabilization. For circularly polarized radiation, the wave collapse is accompanied by the formation of an envelope shock wave.     

Self-action dynamics of ultrashort electromagnetic pulses

The equation generalizing the nonlinear Schrödinger equation to the case of pulses with a duration of few field oscillation periods is analyzed. A change in the effective parameters (centroid, duration, and width) of the wave field on the pulse propagation path are determined by the moments method. The collapse of spatial structure is shown to occur, and its formation associated with the steepening of the pulse leading edge are numerically studied.     

Molecular dynamics driven by ultrashort laser pulses: Interference effects due to rescattering

A time-dependent Schrödinger equation is integrated numerically to investigate the dynamics of a model molecular system driven by a high-intensity ultrashort laser pulse. Two-dimensional photoelectron momentum distributions are analyzed. Highly nonmonotonic electron angular distributions are obtained that cannot be explained by diffraction in the double-well potential of a molecular ion. The nonmonotonicity is also demonstrated for atomic ionization and is attributed to the interference that occurs between components of an electron wave packet after its rescattering from the parent ion. An analytical model explaining the observed effects is developed.     

Spatiotemporal structure of isodiffracting ultrashort electromagnetic pulses

We present a model of isodiffracting single-cycle and few-cycle ultrashort electromagnetic pulses. The model is based on exact solutions of the time-dependent paraxial wave equation with space-time coupling effects included. The spatiotemporal structure of these pulses is characterized by a scaling parameter which relates off-axis pulse shapes to the axial temporal waveforms. Depending on the spectrum a pulse may transform itself from a single-cycle pulse to a multicycle pulse along the radial coordinate. This model is also used to describe recirculating pulses in a curved mirror cavity resonator. The Gouy phase shift contributes an absolute phase that results in a pulse-to-pulse temporal instability.     

Calculation of the probability of photoprocesses induced by ultrashort electromagnetic pulses

An expression for the total probability of photoprocesses induced by ultrashort electromagnetic pulses in terms of the excited transition cross-section has been derived by using perturbation theory. The expression obtained is valid for any radiation spectral width and for various types of transition: bound-bound and bound-free. By way of illustration the excitation of a two-level system by an ultrashort pulse with a controlled carrier–envelope phase and the photoionization of a hydrogen atom under the action of a sequence of two-cycle electromagnetic pulses are considered.     

Spectra induced by interactions of atoms with ultrashort electromagnetic pulses

An ultrashort pulse of an electromagnetic field incident on an atom shakes the atom and gives rise to various electron transitions in it. These processes are accompanied by the reemission of the incident ultrashort pulse. This paper studies the relation between the spectra of reemitted photons of an ultrashort pulse and the transitions of atomic electrons into particular states. The obtained partial reemission spectra can allow one to relate direction patterns to the probability of the excitation of an atom into different states.     

Symmetric fragmentation of polyatomic molecular ions in ultrashort intense laser pulses

The intense field-ionization behavior of linear homonuclear polyatomic molecular ions has been numerically investigated for different molecular configurations. The enhanced ionization effect of molecular ions is found as a general feature in a critical bond length range, but the enhancement degree of ionization is bond-selective. The more symmetric the molecular ion stretches, the more enhanced the ionization of the molecular ion. It implies that the symmetric fragmentation of linear molecular ions is most probable at moderate laser intensity. These results are explained in terms of the field-induced over-barrier ionization mechanism.     

Scattering of ultrashort electromagnetic pulses on metal clusters

We have calculated and analyzed the probability of ultrashort electromagnetic pulse (USP) scattering on small metal clusters in the frequency range of plasmon resonances during the field action. The main attention is devoted to dependence of the probability of scattering on the pulse duration for various detunings of the USP carrier frequency from the plasmon resonance frequency. Peculiarities of the USP scattering from plasmon resonances with various figures of merit are revealed.     

Structural features of the self-action dynamics of ultrashort electromagnetic pulses

Self-focusing dynamics of electromagnetic pulses of arbitrary duration is analyzed numerically and analytically. The wave-field evolution is considered by the wave equation in the reflectionless approximation under quite general assumptions about the dispersion of the medium. Methods for qualitative investigation of the self-focusing dynamics of quasimonochromatic radiation are generalized to the case of wave packets with the length of a few oscillation periods. In particular, sufficient conditions for collapse and many other integral relations are obtained by the momentum method. A self-similar-type transformation is used to show that new structural features are primarily associated with the nonlinear dispersion of the medium (with the dependence of the group velocity of a wave packet on its amplitude). Numerical analysis confirms that the self-focusing of radiation is preceded by an increase in the steepness of the longitudinal profile.     

Reemission spectra and interference effects upon interaction of ultrashort electromagnetic pulses with nanosystems

The processes of reemission of ultrashort electromagnetic pulses by arbitrary nanosystems consisted of isolated complex atoms have been considered. The angular distributions of reemission spectra have been obtained for a series of regular nanosystems. It has been shown that the interference of the photon-emission amplitudes leads to the appearance of characteristic “diffraction” maxima. One-, two-, and three-dimensional nanostructures, as well as planar and cylindrical constructions as models of planar nanosystems and nanotubes, were used as examples that allow a simple analytical consideration.     

Reemission spectra and interference effects at the interaction of multiatomic targets with ultrashort electromagnetic pulses

The processes of reemission of ultrashort electromagnetic pulses by linear chains consisting of isolated multielectron atoms have been considered. The developed method makes it possible to accurately take into account the spatial inhomogeneity of the field of an ultrashort pulse and the momenta of photons in reemission processes. The angular distributions of reemission spectra have been obtained for an arbitrary number of atoms in a chain. It has been shown that the interference of the photon emission amplitudes leads to the appearance of characteristic “diffraction” maxima. The results allow standard generalization to the cases of rescattering from two-dimensional (graphene-like) and three-dimensional lattices, as well as to the case of the inclusion of thermal vibrations of the atoms of lattices.     

Generation of high-power ultrashort electromagnetic pulses on the basis of effects of superradiance of electron bunches

One of the promising methods for generation of ultrashort electromagnetic pulses (with duration of about ten periods of high-frequency oscillations) is radiation from spatially localized electron ensembles (bunches), which can be considered a classical analog of Dicke superradiance known in quantum electronics. In classical electronics, superradiance can be related to various mechanisms of stimulated radiation. Until now, cyclotron, undulator, and ?erenkov (in the case of interaction with both copropagating and counterpropagating waves) superradiance of electron bunches as well as superradiance during stimulated scattering of a pump wave have been studied theoretically and experimentally. As a result of these studies based on high-current RADAN and SINUS accelerators and their modifications, a new class of oscillators producing pulsed electromagnetic radiation has been created. They have such unique characteristics as pulses of high peak power (up to 1 GW and 3 GW in the millimeter-and centimeter-wave ranges, respectively) and ultrashort duration (from 300 ps to 1 ns, respectively). In this case, regimes with a peak radiation power exceeding the electron-beam power are experimentally realized. Regimes with high (kilohertz) pulse repetition rate and high average power (up to 2.5 kW) are obtained.     

Generation of ultrashort pulses of electromagnetic radiation in the mid- and far-infrared ranges

Generation of ultrashort pulses of electromagnetic radiation in the range of wavelengths from 3 to 17 μm is experimentally demonstrated using difference-frequency mixing of the signal and idler waves delivered through an optical parametric amplification of a week broadband seed by high-power near-infrared ultrashort laser pulses. Cross-correlation measurements demonstrate that pulses as short as a few cycles of electromagnetic field are produced in the mid- and far-infrared ranges by means of difference-frequency generation.     

Investigation of analytic wavefunctions of two-electron systems in dynamic interactions with multiply charged ions and electromagnetic field ultrashort pulses

Coherence of various simple analytic wavefunctions of two-electron systems is investigated in with electromagnetic field ultrashort pulses. The coherence of wavefunctions in dynamic processes, for which cross sections (and probabilities) can be expressed only in terms of the ground-state wavefunctions, is determined. Direct recommendations given for using specific analytic wavefunctions can be helpful in simple calculations and estimation of dynamic processes.