A. A. Silaev

Ionization-Induced Multiwave Mixing: Terahertz Generation with Two-Color Laser Pulses of Various Frequency Ratios.

Ultrafast strong-field ionization is shown to be accompanied by atypical multiwave mixing with the number of mixed waves defined by the dependence of the ionization rate on the field strength. For two-color laser pulses of various frequency ratios, this results in the excitation of a free-electron current at laser combination frequencies and possibly in the excitation of the zero-frequency (residual) current responsible for terahertz (THz) generation in a formed plasma. The high-order nature of ionization-induced wave mixing may cause THz generation with uncommon laser frequency ratios (such as 2:3 and 3:4) to be virtually as effective as that with the commonly used frequency ratio of 1:2.

High-order harmonic generation by atoms in a few-cycle laser pulse: Carrier-envelope phase and many-electron effects

Analytic formulas describing high-order harmonic generation (HHG) by atoms in a short laser pulse are obtained quantum mechanically in the tunneling limit. These results provide analytic expressions of the three-step HHG scenario, as well as of the returning electron wave packet, in a few-cycle pulse. Our results agree well with those of numerical solutions of the time-dependent Schroedinger equation for the H atom, while for Xe they predict many-electron atomic dynamics features in few-cycle HHG spectra and significant dependence of these features on the carrier-envelope phase of a laser pulse.

High-order-harmonic generation in an elliptically polarized laser field: analytic form of the electron wave packet

An analytic expression for the electron wave packet (EWP) describing high-order harmonic generation (HHG) by atoms in an intense laser field with small ellipticity is derived quantum mechanically in the tunneling limit within the time-dependent effective range theory. This result is valid over a wide interval of returned electron energies in the HHG plateau region and generalizes the previous result for HHG rates obtained by Frolov M V et al (2012 Phys. Rev. A 86 063406) only for the HHG plateau cutoff region. It is shown that the most important difference from the case of a linearly polarized field originates from a nonzero electron energy at the moment of ionization in an elliptically polarized field that, in turn, results in the dependence of ionization factor in the EWP on the returning electron energy. Our analytic result for the EWP averaged over interference oscillations in the HHG spectrum is applied for analysis of the laser wavelength scaling of the HHG yield induced by an elliptically polarized midinfrared laser field as well as for the improvement of the recently suggested method of elliptic HHG spectroscopy for retrieving both the energy and angular dependence of the photorecombination cross section of the target atom (see Frolov M V et al 2016 Phys. Rev. A 93 031403).

Ionization Mechanism of the Generation of Tunable Ultrashort Pulses in the Mid-Infrared Range

The generation of pulses in the mid-infrared range during the ionization of a gas by an intense two-color femtosecond field containing quasimonochromatic components with the frequency ratio close to 2 has been studied. The generation occurs at the detuning frequency equal to the difference between the higher frequency and the doubled lower frequency. It has been shown that the efficiency of the generation is determined both by the magnitude of the detuning frequency and by its sign and is higher at negative detunings. The duration of the generated pulse is determined by the ionization duration, which is much smaller than the duration of the ionizing field, which ensures obtaining few-cycle and subcycle pulses in the mid-infrared range.

Excitation of low-frequency residual currents at combination frequencies of an ionising two-colour laser pulse

We have studied the processes of excitation of low-frequency residual currents in a plasma produced through ionisation of gases by two-colour laser pulses in laser-plasma schemes for THz generation. We have developed an analytical approach that allows one to find residual currents in the case when one of the components of a two-colour pulse is weak enough. The derived analytical expressions show that the effective generation of the residual current (and hence the effective THz generation) is possible if the ratio of the frequencies in the two-colour laser pulse is close to a rational fraction with a not very big odd sum of the numerator and denominator. The results of numerical calculations (including those based on the solution of the three-dimensional time-dependent Schrodinger equation) agree well with the analytical results.

Suppression of the contribution of short trajectories into above-threshold ionisation spectra by a two-colour laser field

We have studied spectra of above-threshold ionisation of atoms by a two-colour laser field with collinear linearly polarised components. We have found a sharp (gap-like) dependence of the length of the high-energy plateau in above-threshold ionisation spectra on the relative phase of the two-colour field at comparable intensities of the field components. Using the quasi-classical analysis we have shown that this effect results from the suppression of partial above-threshold ionisation amplitudes, associated with closed classical trajectories of an electron in the laser field, within a certain range of relative phase values.

Ellipticity dependence of high harmonic yield in intense laser field: case of s-valence electron

Having solved numerically the time-dependent Schrodinger equation, we have analysed the dependence of the high harmonic generation yield on the ellipticity of an intense laser field. For the case of a zero angular momentum of an initial state, it has been shown that the ellipticity dependence of the HHG yield is affected by the harmonic number. The numerical results are interpreted in the framework of our recently developed quasi-classical analytical model for HHG. In the quasi-classical approximation, the difference in the ellipticity dependence of the HHG yield for different harmonics is shown to be caused by the interference effects of quantum orbits.

Quantum-mechanical calculations of residual current density excited during gas ionisation by an intense two-colour laser pulse

By solving analytically and numerically the three-dimensional time-dependent Schrodinger equation, we have studied the excitation of a residual current density during gas ionisation by a two-colour laser pulse containing a field at the fundamental frequency and an additional field at the doubled frequency. We have found the dependences of the residual current density on the phase shift between the components of the field and on the intensity of the fundamental harmonic. It is shown that the strong-field approximation taking into account the interaction of freed electrons with the parent ion yields a good quantitative agreement with the results of direct numerical simulation.

Broadband terahertz emission from laser-produced plasmas

The review of our studies of the phenomenon of terahertz waves generation caused by gas ionization with intense femtosecond laser pulses is presented.