V. V. Strelkov

Generation of isolated attosecond pulses by spatial shaping of a femtosecond laser beam

We present a new method for generating isolated attosecond pulses via high-order harmonic generation in gases. It relies on using collective effects to achieve transient phase-matching which provides both a high efficiency and a strong temporal confinement under specific conditions. By controlling the spatial shape of the fundamental beam and the geometry of the laser?gas interaction, this transient phase matching leads to the generation of isolated broadband attosecond pulses with long driving pulses (10?20?fs) even without controlling their carrier envelope phase. Such laser pulses are becoming available at high energy levels and our approach offers a route to increase the energy of isolated attosecond pulses by orders of magnitude as compared to existing sources.

Phase distortions of attosecond pulses produced by resonance-enhanced high harmonic generation

Resonant enhancement of high harmonic generation can be obtained in plasmas containing ions with strong radiative transitions resonant with harmonic orders. The mechanism for this enhancement is still debated. We perform the first temporal characterization of the attosecond emission from a tin plasma under near-resonant conditions for two different resonance detunings. We show that the resonance considerably changes the relative phase of neighboring harmonics. For very small detunings, their phase locking may even be lost, evidencing strong phase distortions in the emission process and a modified attosecond structure. These features are well reproduced by our simulations, allowing their interpretation in terms of the phase of the recombination dipole moment.

Spatio-temporal dynamics of electron density in femtosecond laser microplasma of gases

The formation and evolution of femtosecond laser plasma produced in microvolumes of gases at different pressures upon their multiply ionization by high intensity pulses of fundamental and second harmonics of a Ti:sapphire laser is studied. The interferometric technique for precise ultrafast optical diagnostics of such plasma was applied. The numerical technique of interferogram processing and reconstruction of instant spatial distribution of refractive index and free electron density in laser-induced plasma applied for this proposes is described. The spatiotemporal distribution of the refractive index and free electron density were studied with a spatial resolution of ∼1 μ m and a temporal resolution of ∼70 fs.

Phase characteristics of high-order harmonics in the cut-off region

The behavior of the phases of high-order harmonics generated upon interaction of intense laser radiation with matter is investigated. Some specific features typical of the harmonic phases in the high-frequency part of plateau (cut-off harmonic phases) are found. First, the phase difference between neighboring harmonics is a constant value. The width of the spectral range in which this regularity occurs determines the minimum duration of the attosecond pulse obtained from these harmonics by the so-called amplitude gating technique. Second, it is shown that the phase of each harmonic in the cut-off region depends linearly on the laser intensity. The proportionality coefficient is the same for all harmonics in this region and proportional to the cube of laser wavelength. Thus, this dependence is especially important for generating high-order harmonics by lasers with a wavelength of few micrometers.

Interferometric diagnostics of femtosecond laser microplasma in gases

The laser plasma formed in gaseous media due to their optical breakdown under tightly focused femtosecond laser pulses has been experimentally investigated. Pump-probemicrointerferometry is chosen to perform spatial and temporal diagnostics of the plasma. Time dependences of the laser plasma electron density are obtained. It is shown that in breakdown of different gases (air, nitrogen, argon, and helium) at different pressures (in the range from 1 to 10 atm) the electron concentration continues to increase during ∼1 ps when the laser irradiation is over. This effect is related to the impact ionization of the plasma by the hot electrons formed in interaction of intense femtosecond laser pulses with matter. The results of theoretical simulation of the post-ionization processes are presented.

S-matrix approach to the problem of high-harmonic generation in the field of intense laser wave

A method using expansion of the wave function in the basis of photonic and free atomic eigenstates is proposed for calculating the emission spectrum of an atom in a laser field. The wave function is constructed using the Kramers−Henneberger transformation so that the expression for the transition S matrix explicitly includes the nonlinear interaction with the laser field. The expansion coefficients are determined by the residual interaction, which depends on the coordinates of the classical free electron motion in the laser field. Resonances at the atomic transition frequencies explicitly arise in the emission spectrum when the residual interaction is considered in the first order. The numerical solution of the timedependent Schro¨ dinger equation for the hydrogen atom within the semiclassical approach is used to obtain emission spectra for laser pulses of different intensities and durations.

Short pulse carrier-envelope phase absolute single-shot measurement by photoionization of gases with a guided laser beam.

We present an all optical approach to measure the value of the carrier-envelope phase (CEP) of a short intense laser pulse. This method relies on photo-ionization of gases with a guided laser beam. This approach that provides the absolute value of the CEP, is compatible with single shot characterization, is scalable in wavelength, does not suffer from bandwidth limitation and is largely intensity independent. It has also the potential to provide a full characterization of the pulse profile via high order autocorrelation on a single shot basis.

Ultrafast Space‐time and Spectrum‐time Resolved Diagnostics of Multicharged Femtosecond Laser Microplasma

We present the review of the recent experimental studies of fundamental mechanisms of femtosecond laser plasma formation and evolution performed in A. M. Prokhorov General Physics Institute of Russian Academy of Sciences. The main object we dealt with was the micro‐sized plasma produced in gases with high intensity (up to 5×1017 W/cm2), tightly focused (to a few microns in diameter) IR and UV femtosecond laser pulses. The main attention in the experiments was paid to the initial stage of microplasma formation and evolution characterized by strong laser‐plasma coupling resulting in efficient ionization and heating of the medium, distortion of laser beam, and nonlinear spectral conversion of laser radiation to the continuum and laser harmonics. Using a precise pump‐probe micro‐interferometric technique the dynamics of plasma was studied with femtosecond time‐resolution in a wide density range—from a minimal detectable electron concentration (1019 cm−3) to the almost total (down to nuclei) ionization of ions...

Femtosecond laser breakdown of gases and transparent solid states: ultrafast space-time and spectrum-time resolved diagnostics of multicharged microplasma

We present the results of experimental studies of formation and evolution of multiply ionized (multicharged) laser micro-size plasma produced in gases (air, nitrogen, argon and helium) and inside the transparent solids (fused silica) with high intensity (up to ~ 1017 W/cm2), ultrashort (τ ~100 fs), 800nm/400nm laser pulses tightly focused in a region of ~1.5 μm in diameter. The measuring techniques and experimental setups for generation and precise optical diagnostics of laser-induced plasma - pump-probe microinterferometry and ultrafast spectroscopy are described. The measured spatiotemporal distributions of plasma refractive index/electron density and plasma spectra are demonstrated. In the experiments, the main attention was paid to the most intriguing initial stage of ultrafast plasma formation and evolution characterized by strong laser-matter and laser-plasma coupling resulting in efficient photoionization of material and plasma heating. We found out that the almost complete ionization (down to nuclei) of the initial gas occurs even at the initial stage of plasma formation. Besides, it was observed, for the first time, that a characteristic time of laser plasma formation considerably (in times) exceeds the duration of the pump laser pulse. This postionization process is attributed to impact ionization of plasma by hot electrons heated due to inverse bremsstrahlung. A theoretical model describing the mechanism of plasma postionization by hot photoelectrons was proposed. We compare the results of the experiments with what the theory predicts - the results of electron density calculations are in good agreement with the experimental data. The dynamics of plasma emission (spectral continuum and spectral line formation) in UV-visible spectral range was investigated with a picosecond time resolution applying the developed ultrafast streak-camera-based spectrometer. The spatiotemporal distributions of refractive index of laser irradiated fused silica were recorded with pump-probe microinterferometry. It was demonstrated that the induced refractive index of laser-matter interaction area changes its sign from the positive to the negative during the laser irradiation and again to the positive one after the laser pulse ends.

Sub-100 attosecond XUV pulses

For the first time, we observe unambiguous signature of broadband (50 eV) XUV harmonic radiation temporally confined down to an isolated attosecond pulse by applying polarization gating to phase-stabilized-few-cycle laser pulses.