Streaking camera in the high intensity regime.

The laser-induced decay of an atomic system in an intense infrared and perturbative extreme ultraviolet (XUV) pulse is considered within Keldysh and streaking ionization channels. The streak camera method is discussed for two cases corresponding to different ranges of photoelectron momentum: i) the streaking channel significantly dominates the Keldysh channel and ii) the Keldysh channel of ionization is dominant, while two channels may interfere. The retrieval of XUV pulse parameters for these two cases is discussed and supported by numerical calculations.

Channel separation of secondary generated radiation induced by orthogonal XUV and IR pulses.

The secondary generated radiation induced by orthogonal linearly polarized extreme ultraviolet (XUV) and infrared (IR) pulses is analyzed for the spectral region of the second XUV harmonic. The polarization-filtering-based method is utilized to separate two spectrally overlapping and competing channels, which are the XUV second harmonic generation (SHG) by IR-dressed atom and XUV-assisted recombination channel of high-order harmonic generation in the IR field [Phys. Rev.

Using the generation of Brunel harmonics by elliptically polarized laser pulses for high-resolution detecting lower-frequency radiation.

We propose to use the generation of even Brunel harmonics (BHs) by optical laser pulses for high-resolution gas-biased coherent detection of lower-frequency radiation in terahertz and mid-infrared ranges. BHs arise due to the acceleration of electrons liberated in the tunneling ionization process, and BHs pulses are much shorter than the laser ones. The latter makes it possible to significantly increase the temporal resolution of sampling detection compared to the use of cubic response of bound electrons generating the second harmonic of the gating pulse.

Contribution of the collective electron dynamics to the polarization response of an atom subjected to intense IR and weak XUV pulses.

We analyze the polarization response of a single Ne atom in an intense infrared (IR) laser field and weak extreme ultraviolet (XUV) isolated attosecond pulse (IAP). The analysis is based on the numerical solution of the time-dependent Kohn-Sham equations and the recently developed perturbation theory in the XUV field for an atom subjected to an intense IR field. In our numerical results, we observe a significant increase in the magnitude of the atomic polarization response at the frequencies near the carrier frequency of the IAP and associate it with XUV-induced collective dynamics contributing to the polarizability of Ne.

Waveform retrieving of an isolated attosecond pulse using high-order harmonics generation of the superimposed infrared field.

An all-optical method is suggested for the metrology of an isolated, pulse-to-pulse stabilized attosecond pulse. It is shown analytically that high-order harmonic generation (HHG) yield for an intense IR pulse and time-delayed attosecond pulse keeps encoded waveform of the attopulse, which can be decoded by the time delay measurements of the HHG yield. The retrieval method is demonstrated by modeling HHG from Ne atom within time-dependent Kohn-Sham equations.

Time-frequency analysis of high harmonic generation using a probe XUV pulse.

Interpretation of strong-field phenomena is mostly based on the analysis of classical electron trajectories in an intense laser field, whose specific properties determine general features of nonlinear laser-matter interaction. Currently, the visualization of closed electron trajectories contributing to high harmonic generation (HHG) of the laser field is the prerogative of a theoretical analysis based on the time-frequency spectrogram of the induced dipole acceleration. Here, we propose a method for direct reconstruction of the HHG time-frequency spectrogram using a time-delayed probe XUV pulse.

Generation of tunable mid- and far-infrared pulses during gas ionization by a chirped two-color laser field.

We propose and investigate a method for generating tunable and phase-controllable mid- and far-infrared pulses in gas ionized by an intense two-color laser field composed of the chirped fundamental and its second-harmonic pulses with group time delay. The generation frequency equals the difference between the second-harmonic and the doubled fundamental frequencies and is continuously tunable by varying chirp or time delay. The duration of the generated pulses is determined by the ionization duration, which is much shorter than the ionizing pulse and is controlled by laser-pulse stretching or changing its intensity.

Control of Harmonic Generation by the Time Delay Between Two-Color, Bicircular Few-Cycle Mid-IR Laser Pulses.

We study control of high-order harmonic generation (HHG) driven by time-delayed, few-cycle ω and 2ω counterrotating mid-IR pulses. Our numerical and analytical study shows that the time delay between the two-color pulses allows control of the harmonic positions, both those allowed by angular momentum conservation and those seemingly forbidden by it. Moreover, the helicity of any particular harmonic is tunable from left to right circular without changing the driving pulse helicity.

Generation of Few- and Subcycle Radiation in Midinfrared-to-Deep-Ultraviolet Range During Plasma Production by Multicolor Femtosecond Pulses.

Our closed-form analytical formulas and numerical calculations show that the plasma production by a two-color (or, more generally, multicolor) femtosecond pulse leads to generation of strong few- and subcycle radiation. The spectral composition of the radiation is defined by the numerous combination frequencies of the ionizing pulse. The radiation duration is equal to the ionization duration, which is much shorter than the multicolor pump.

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.

Two-color laser-plasma generation of terahertz radiation using a frequency-tunable half harmonic of a femtosecond pulse.

We investigate for the first time, both experimentally and theoretically, low-frequency terahertz (THz) emission from the ambient air ionized by a two-color femtosecond laser pulse containing, besides the fundamental-frequency main field, a weak additional field tunable near the frequency of the half harmonic. By controlling the mutual polarization and the powers of the main and additional fields, we determine the dependences of the THz power and polarization on the parameters of the two-color pulse. We also discover the resonantlike dependence of the THz yield on the frequency detuning of the additional field.

Ionization-induced conversion of ultrashort Bessel beam to terahertz pulse.

We examine the conical terahertz emission from the superluminous ionization front created in air by an axicon-focused femtosecond laser pulse. We develop the theoretical model that explains the experimental results and predicts new possibilities to control terahertz pulse parameters.

Residual-current excitation in plasmas produced by few-cycle laser pulses.

Along with the generation of extreme-ultraviolet and soft x-ray radiation, gas ionization by an intense few-cycle laser pulse can also induce the generation of low-frequency terahertz waves. The latter is caused by the excitation of a residual quasi-dc current in the produced plasma by the electric field of the laser pulse. We describe this phenomenon using the quantum-mechanical approach based on solving the 3D time-dependent Schrödinger equation.

Optical-to-THz wave conversion via excitation of plasma oscillations in the tunneling-ionization process.

A new ("linear-parametric") mechanism of a direct conversion of an ultrashort laser pulse into terahertz radiation is suggested. The conversion is due to the ionization-induced excitation and the subsequent electromagnetic emission of the superluminous polarization wave created by the axicon-focused laser pulse. For a few-cycle pulse with an optimum carrier-envelope phase, the considered mechanism is found to be much more effective than the alternative one based on the excitation of plasma oscillations in the laser wakefield by the ponderomotive force and able to provide THz radiation of the gigawatt power level with the use of moderate optical intensity (approximately 10(14)-10(15) W/cm2).