arXiv subjects

V. V. Strelkov

Publications and source records attributed to V. V. Strelkov.

17 recordsLinked to original sources

arXiv2026

Collective tunnel ionization in atomic systems

We present a theory of the collective tunnel effect in atomic systems subject to strong low-frequency laser fields. Using an analytic semi-classical method and numerical solution to the time-dependent Schrodinger equation we demonstrate the collective channel of the nonsequential double ionization in neutral xenon and in the negative bromine ion. Collective tunneling in the presence of the electron-electron repulsion comprises the joint sub-barrier motion of the two electrons along quantum trajectories transversely shifted in opposite directions with respect to the direction of the external electric field. Momentum distributions of the electron pairs carry clear signatures of the collective channel in the form of shoulders in the direction lateral to the field polarization. We propose and discuss potential experimental approaches to a search for collective tunneling in atomic systems using short circularly polarized laser pulses.

physics.atom-ph↗

arXiv2026

Ionization-free femtosecond UV pulse filamentation resulting from non-perturbative Kerr effect saturation due to transient photoexcitation of molecules

Our experimental and numerical study of filamentation of the 248 nm 100 fs pulses in air, nitrogen, and oxygen at atmospheric pressure shows that the densities of photoelectrons in the filaments are not sufficient to limit Kerr self-focusing. The simulations explain this result, showing that the nonlinear refractive index growth with the UV intensity is saturated due to transient photoexcitation of a molecule, not the photoionization. Correct simulation of the observed filament intensity ratio in different gases validates the suggested mechanism of the femtosecond UV pulses filamentation.

physics.atom-ph↗

arXiv2026

Interplay of the channel-closing and bound-bound transition resonances in multiphoton ionization and harmonic generation in intense laser pulses

In this paper, using a simplified model of the xenon atom, we numerically study the possibilities of efficient generation of coherent pulses in the XUV range through the resonant interaction of atoms with a moderate-intensity laser field, leading to the generation of its harmonics. We demonstrate the interplay of two systems of resonances affecting the harmonic generation efficiency. One is the channel-closing resonances, which arise when the sum of ionization and ponderomotive energies is equal to the energy of an integer number of laser photons. The second is the bound-bound transition resonances corresponding to an integer number of photons with a total energy equal to the energy gap between the Stark-shifted ground and excited states. The harmonic yields in the range of laser parameter values where both resonances occur exhibit a peculiar behavior, namely, near the intersection point of the resonances, a pronounced dip is observed, while the regions of increased generation efficiency due to the combined contribution of both enhancement mechanisms are slightly shifted from this point. We argue that this behavior, which is somewhat similar to the well-known phenomenon of 'avoided crossings', is associated with the formation of Fano-type resonant spectral lines. In contrast to 'avoided crossing' phenomena known in molecular physics, in the found interplay the contribution of one resonance system can be controlled, which is useful for experiments.

physics.atom-ph↗

arXiv2025

Nonperturbative regime of low-order harmonic generation in intense low-frequency laser field

We find the atomic response to the intense femtosecond laser pulse via solving numerically the three-dimensional non-stationary Schr\"odinger equation (TDSE) for a model atom and calculating its dipole moment. For weak quasi-static fields, the response is well described by a perturbation approach, but for intensities higher than about $0.6 \, \, 10^{14}$ W/cm$^2$ the accuracy of this description is unsatisfactory, regardless of the order of non-linearity taken into account. We suggest fitting the numerical TDSE solution results with a Pad\'e expansion, and show that this approximation describes the response well both in the perturbative regime and beyond it for intensities approximately up to $1.4 \, \, 10^{14}$ W/cm$^2$. To consider the non-perturbative nonlinearity beyond the quasi-static limit we use the model of nonlinear oscillator with the restoring force defined by the found Pad\'e expression. Our model fails to predict the behaviour of the nonlinear refractive index in the nonperturbative domain, but it describes well the nonperturbative growth of the efficiency with the laser intensity for other nonlinear optical processes, namely, the third and fifth harmonic generation in the IR field and the optical rectification in a two-color field.

physics.optics↗

arXiv2025

Role of plasma waves in rescattering processes in intense laser fields

Rescattering of the photoelectron at its parent ion underlie a number of phenomena in intense laser field interaction with matter, such as high harmonic generation, attosecond pulse production, non-sequential double ionization, and others. These processes are unavoidably accompanied by the medium photoionization. The interaction of the laser pulse with the photoionization-induced plasma excites wakefield waves, which are self-consistently coupled plasma density and Langmuir waves. We study theoretically the effect of the electric field of the plasma wave on the rescattering processes. We show that this field can compensate for the magnetic drift of the rescattering electron, which otherwise dramatically suppresses the rescattering efficiency in intense low-frequency laser fields. Moreover, the presence of the plasma wave field leads to new lines in the spectrum emitted due to the XUV free-induction decay (XFID). Observation of these lines can allow, in particular, the detection of forbidden transition frequencies, thus providing new perspectives for XFID spectroscopy.

physics.optics↗

arXiv2025

Fano resonance in XUV generated by helium with few-cycle intense laser pulses and its classical analogy

We integrate numerically the Schrödinger equation for the model helium atom irradiated by intense few-cycle laser pulse and find the emitted XUV spectra. They demonstrate resonant peaks at the frequencies of transitions from the doubly-excited autoionizing states (AISs) to the ground state. We study the properties of these peaks depending on the laser pulse duration and find that the decay of the AISs due to photoionization by the laser field affects them. Moreover, we consider the classical system of two coupled oscillators and find that both the quantum (the atom with AIS in the field) and the classical (the coupled oscillators with friction) systems demonstrate Fano-like resonant peak described by an essentially complex asymmetry parameter. We find a remarkable similarity in the behavior of these systems and conclude that the classical system of coupled oscillators with friction is an analogy of the AIS having an extra decay channel in addition to the autoionization one.

physics.optics↗

arXiv2025

Breakdown of sequential tunnel ionization in ultrashort electromagnetic pulses

We consider double ionization of negative bromine ion in intense low-frequency electromagnetic fields. By solving numerically the two-electron time-dependent Schr{\" o}dinger equation we demonstrate that while for pulses of a few tens of femtoseconds duration and longer the sequential single-electron approximation perfectly describes the ionization dynamics, for pulses as short as a few femtoseconds this picture breaks down entirely, and the electron-electron interaction suppresses the rate of ionization by roughly one order of magnitude. We also show clear signatures of the collective tunneling effect in the photoelectron density distribution. This counter-intuitive channel of ionization opens up due to the electron-electron repulsion in the direction lateral to the applied electric field.

physics.atom-ph↗

arXiv2023

Macroscopic effects in generation of attosecond XUV pulses via high-order frequency mixing in gases and plasma

We study the generation of attosecond XUV pulses via high-order frequency mixing (HFM) of two intense generating fields, and compare this process with the more common high-order harmonic generation (HHG) process. We calculate the macroscopic XUV signal by numerically integrating the 1D propagation equation coupled with the 3D time-dependent Schrödinger equation. We analytically find the length scales which limit the quadratic growth of the HFM macroscopic signal with propagation length. Compared to HHG these length scales are much longer for a group of HFM components, with orders defined by the frequencies of the generating fields. This results in a higher HFM macroscopic signal despite the microscopic response being lower than for HHG. In our numerical simulations, the intensity of the HFM signal is several times higher than that for HHG in a gas, and it is up to three orders of magnitude higher for generation in plasma; it is also higher for longer generating pulses. The HFM provides very narrow XUV lines ($δω/ ω= 4.6 \times 10^{-4}$) with well-defined frequencies, thus allowing for a simple extension of optical frequency standards to the XUV range. Finally, we show that the group of HFM components effectively generated due to macroscopic effects provides a train of attosecond pulses such that the carrier-envelope phase of an individual attosecond pulse can be easily controlled by tuning the phase of one of the generating fields.

physics.optics↗

arXiv2022

Dark and bright autoionizing states in resonant high harmonic generation: simulation via 1D helium model

We study the role of dark and bright autoionizing states (AIS) in photoionization and high harmonic generation (HHG) using a 1D helium model. This model allows numerical integration of the time-dependent Schrödinger equation beyond the singe-electron approximation completely taking into account electronic correlation. We find the level structure of the system and the spatial distribution of the electronic density for several states including AIS. Studying the HHG efficiency as a function of the detuning from the resonances with AIS we find the HHG enhancement lines. The shapes of these lines are different from the corresponding Fano lines in the photoelectronic spectra, in agreement with the experimental studies in helium. Moreover, we simulate HHG under the conditions when the fundamental frequency is close to the even-order multiphoton resonance with the dark AIS. We find the enhanced generation of the neighbouring odd harmonics. The details of the enhancement lines for these harmonics can be understood taking into account the temporal delay between the emission of the non-resonant and resonant XUV; this delay is defined by the AIS lifetime. Finally, our simulations show that the HHG enhancement due to the dark and the bright AIS is comparable in the studied system.

physics.atom-ph↗

arXiv2022

Generation of Attosecond Pulses with Controllable Carrier-Envelope Phase via High-order Frequency Mixing

Advancing table-top attosecond sources in brightness and pulse duration is of immense interest and importance for an expanding sphere of applications. Recent theoretical studies [New J. Phys., 22 093030 (2020)] have found that high-order frequency mixing (HFM) in a two-color laser field can be much more efficient than high-order harmonic generation (HHG). Here we study the attosecond properties of the coherent XUV generated via HFM analytically and numerically, focusing on the practically important case when one of the fields has much lower frequency and much lower intensity than the other one. We derive simple analytical equations describing intensities and phase locking of the HFM spectral components. We show that the duration of attosecond pulses generated via HFM, while being very similar to that obtained via HHG in the plateau, is shortened for the cut-off region. Moreover, our study demonstrates that the carrier-envelope phase of the attopulses produced via HFM, in contrast to HHG, can be easily controlled by the phases of the generating fields.

physics.optics↗

arXiv2021

Polarisation control of quasi-monochromatic XUV produced via resonant high harmonic generation

We present a numerical study of the resonant high harmonic generation by tin ions in an elliptically-polarised laser field along with a simple analytical model revealing the mechanism and main features of this process. We show that the yield of the resonant harmonics behaves anomalously with the fundamental field ellipticity, namely the drop of the resonant harmonic intensity with the fundamental ellipticity is much slower than for high harmonics generated through the nonresonant mechanism. Moreover, we study the polarisation properties of high harmonics generated in elliptically-polarised field and show that the ellipticity of harmonics near the resonance is significantly higher than for ones far off the resonance. This introduces a prospective way to create a source of the quasi-monochromatic coherent XUV with controllable ellipticity potentially up to circular.

physics.optics↗

arXiv2020

High-order parametric generation of coherent XUV radiation

We observe a new regime of coherent XUV radiation generation in noble gases induced by femtosecond pulses at very high intensities. This XUV emission has both a reduced divergence and spectral width as compared to high-order harmonic generation (HHG). It is not emitted at a moderate intensity of the driving pulses where only high-order harmonics are generated. At high driving intensities, the additional XUV comb appears near all harmonic orders and even exceeds the HHG signal on the axis. The peaks are observed in several gases and their frequencies do not depend on the driving intensity or gas pressure. We analyze the divergence, spectral width and spectral shift of this XUV emission. We show that these specific features are well explained by high-order parametric generation (HPG) involving multiphoton absorption and combined emission of an idler THz radiation and an XUV beam with remarkably smooth spatial and spectral characteristics.

physics.optics↗

arXiv2019

Spectral caustic in two-color high harmonic generation: role of Coulomb effects

The high-order harmonic spectrum generated in two-color intense laser field under certain conditions has a pronounced maximum caused by the so called spectral caustic. Using a numerical solution of the 3D time-dependent Schrödinger equation we study the width of the maximum and the degree of the generation enhancement due to the caustic as a function of the fundamental intensity, wavelength, and atomic ionization potential. It is shown that the degree of this enhancement can be well quantitatively characterized by a single parameter: the ratio of the radius of the free electronic oscillation in the laser field to the atomic size. This behavior can be explained by the Coulomb attraction of the photoelectron to the parent ion, whereas the effect of this attraction on the width of the maximum is negligible. Choosing the field parameters providing a pronounced enhancement, we calculate the high harmonic macroscopic response taking into account the transient phase-matching. We show that the caustic feature can be used to provide a quasi-monochromatic XUV source with an almost linear frequency vs. time dependence. Such source can be used, in particular, for the time-resolved single-shot XUV imaging.

physics.optics↗

arXiv2016

Attosecond pulse production using resonantly-enhanced high-order harmonics

We study theoretically the effect of the giant resonance in Xe on the phase difference between the consecutive high order resonantly-enhanced harmonics and calculate the duration of the attosecond pulses produced by these harmonics. For certain conditions resonantly-induced dephasing compensates the phase difference which is intrinsic for the off-resonance harmonics. We find these conditions analytically and compare them with the numerical results. This harmonic synchronization allows attosecond pulse shortening in conjunction with the resonance-induced intensity increase by more than an order of magnitude; the latter enhancement relaxes the requirements for the UV filtering needed for the attosecond pulse production. Using a two-color driving field allows further increase of the intensity. In particular, a caustic-like feature in the harmonic spectrum leads to the generation efficiency growth up to two orders of magnitude, however accompanied by an elongation of the XUV pulse.

physics.atom-ph↗

arXiv2015

Phase properties of the cut-off high-order harmonics

The cut-off regime of high-order harmonic generation (HHG) by atoms in an intense laser field is studied numerically and analytically. We find that the cut-off regime is characterized by equal dephasing between the successive harmonics. The change of the harmonic phase-locking when HHG evolves from the cut-off to the plateau regime determines the optimal bandwidth of the spectral region which should be used for attosecond pulse generation via amplitude gating technique. The cut-off regime is also characterized by a linear dependence of the harmonic phase on the fundamental intensity. The proportionality coefficient grows as the cube of the fundamental wavelength, thus this dependence becomes very important for the HHG by mid-infrared fields. Moreover, for every high harmonic there is a {\it range} of laser intensities providing the generation in the cut-off regime and the atomic response magnitude in this regime can be greater than that in the plateau regime. Thus the cut-off regime substantially contributes to the harmonic energy emitted under typical experimental conditions where the laser intensity varies in time and space.

physics.atom-ph↗

arXiv2015

High-order optical processes: towards nonperturbative nonlinear optics

We develop an approach describing nonlinear-optical processes in the strong-field domain characterized by the nonperturbative field-with-matter interaction. The polarization of an isolated atom in the external field calculated via the numerical solution of the time-dependent Schrödinger equation agrees with our analytical findings. For the practically important case of one strong laser field and several weaker fields we derive and analytically solve propagation equations describing high-order (HO) wave-mixing, HO parametric amplification and HO stimulated scattering. These processes provide a way of efficient coherent XUV generation. Some properties of HO processes are new in nonlinear optics: essentially complex values of the coefficients in the propagation equations, the super-exponential (hyperbolic) growing solutions, etc. Finally, we suggest conditions for the practical realization of these processes and discuss published numerical and experimental results where such processes could have been observed.

physics.atom-ph↗

arXiv2013

High-order harmonic generation and Fano resonances

We present a high harmonic generation theory which generalizes the strong-field approximation to the resonant case, when the harmonic frequency is close to that of the transition from the ground to an autoionizing state of the generating system. We show that the line shape of the resonant harmonic is a product of the Fano-like factor and the harmonic line which would be emitted in the absence of the resonance. The theory predicts rapid variation of the harmonic phase in the vicinity of the resonance. The calculated resonant harmonic phase is in reasonable agreement with recent measurements. Predicting the phase-locking of a group of resonantly-enhanced harmonics, our theory allows to study the perspectives of producing attosecond pulse train using such harmonics.

physics.atom-ph↗