High-order parametric generation of coherent XUV radiation.

Extreme ultraviolet (XUV) radiation finds numerous applications in spectroscopy. When the XUV light is generated via high-order harmonic generation (HHG), it may be produced in the form of attosecond pulses, allowing access to unprecedented ultrafast phenomena. However, the HHG efficiency remains limited. Here we present an observation of a new regime of coherent XUV emission which has a potential to provide higher XUV intensity, vital for applications. We explain the process by high-order parametric generation, involving the combined emission of THz and XUV photons, where the phase matching is very robust against ionization. This introduces a way to use higher-energy driving pulses, thus generating more XUV photons.

Chirp-control of resonant high-order harmonic generation in indium ablation plumes driven by intense few-cycle laser pulses.

We have studied high-order harmonic generation (HHG) in an indium ablation plume driven by intense few-cycle laser pulses centered at 775 nm as a function of the frequency chirp of the laser pulse. We found experimentally that resonant emission lines between 19.7 eV and 22.3 eV (close to the 13th and 15th harmonic of the laser) exhibit a strong, asymmetric chirp dependence, with pronounced intensity modulations. The chirp dependence is reproduced by our numerical time-dependent Schrödinger equation simulations of a resonant HHG by the model indium ion. As demonstrated with our separate simulations of HHG within the strong field approximation, the resonance can be understood in terms of the chirp-dependent HHG photon energy coinciding with the energy of an autoionizing state to ground state transition with high oscillator strength. This supports the validity of the general theory of resonant four-step HHG in the few-cycle limit.

Harmonic Generation from Neutral Manganese Atoms in the Vicinity of the Giant Autoionization Resonance.

High harmonics from laser-ablated plumes are mostly generated from ionic species. We demonstrate that with ultrashort infrared (∼1.82 µm) driving lasers, high harmonics from laser-ablated manganese are predominantly generated from neutral atoms, a transition metal atom with an ionization potential of 7.4 eV. Our results open the possibility to advance laser-ablation technique to study the dynamics of neutral atoms of low ionization potential. Moreover, as manganese contains giant autoionizing resonance, intense and broadband high harmonics have been demonstrated from this resonance at energies from 49 to 53 eV. This opens the possibility to generate intense attosecond pulses directly from the giant resonances, as well as to study these resonances using high-harmonic spectroscopy.

Quasi-phase-matching of high-order harmonics in plasma plumes: theory and experiment.

We theoretically analyze the phase-matching of high-order harmonic generation (HHG) in multi-jet plasmas and find the harmonic orders for which the quasi-phase-matching (QPM) is achieved depending on the parameters of the plasma and the generating beam. HHG by single- and two-color generating fields is analyzed. The QMP is studied experimentally for silver, indium and manganese plasmas using near IR and mid-IR laser fields. The theory is validated by comparison with our experimental observations, as well as published experimental data. In particular, the plasma densities and the harmonic phase coefficients reconstructed from the observed harmonic spectra using our theory agree with the corresponding parameters found using other methods. Our theory allows defining the plasma jet and the generating field properties, which can maximize the HHG efficiency due to QPM.

High-order harmonic generation from the dressed autoionizing states.

In high-order harmonic generation, resonant harmonics (RH) are sources of intense, coherent extreme-ultraviolet radiation. However, intensity enhancement of RH only occurs for a single harmonic order, making it challenging to generate short attosecond pulses. Moreover, the mechanism involved behind such RH was circumstantial, because of the lack of direct experimental proofs. Here, we demonstrate the exact quantum paths that electron follows for RH generation using tin, showing that it involves not only the autoionizing state, but also a harmonic generation from dressed-AIS that appears as two coherent satellite harmonics at frequencies ±2Ω from the RH (Ω represents laser frequency). Our observations of harmonic emission from dressed states open the possibilities of generating intense and broadband attosecond pulses, thus contributing to future applications in attosecond science, as well as the perspective of studying the femtosecond and attosecond dynamics of autoionizing states.

Spatio-spectral structures in high-order harmonic beams generated with Terawatt 10-fs pulses.

A large international effort is nowadays devoted to increase the energy of the extreme ultraviolet pulses by using high-peak power ultrashort fundamental pulses (Terawatt level). Using such fundamental pulses brings specific constraints that need to be addressed. Here we study high-order harmonic generation in gases with 10 fs pulses at Terawatt peak power and demonstrate that extreme ultraviolet beams can be highly structured and complex in various conditions. We use a single-shot spatially resolved spectral detection and demonstrate direct observation of the spatio-temporal coupling occurring in the generating medium. Clear and reproducible complex spatio-spectral structures are observed in the far field. Similar structures are reproduced with simulations and we show that they are intimately associated to the high nonlinearity of high-order harmonic generation. Those findings are of prime importance for the generation of high-energy attosecond pulses and reveal important issues for their applications.

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.

Generation of two attosecond pulses with tunable delay using orthogonally-polarized chirped laser pulses.

We investigate theoretically the high-order harmonic generation (HHG) by two orthogonally-polarized linearly chirped laser pulses. We show that such generating field has a specific temporal variation of the ellipticity which provides generation of two XUV attosecond pulses with tunable delay between them. This delay is controlled by the delay between the two generating pulses. Perspectives of application of this technique for the attosecond pump - attosecond probe experiments are discussed.

Origin for ellipticity of high-order harmonics generated in atomic gases and the sublaser-cycle evolution of harmonic polarization.

We investigate numerically and analytically the polarization properties of high-order harmonics generated by an atom in intense elliptically polarized laser field. The offset angle of the harmonic polarization ellipse can be well described with the semiclassic "simple-man" high-harmonic generation model. The harmonic ellipticity itself, however, can be hardly understood within this model. We show that this ellipticity originates from quantum-mechanical uncertainty of the electron motion. We develop a theoretical approach describing this ellipticity and, more generally, the time evolution of the high-harmonic polarization state within the laser cycle. The analytical results are verified with the exact numerical solution; to make the comparison accurately, we develop a specific technique for separating the contributions of quantum paths in the numerical calculation.

[The "near zone" effect in dynamic chaos]. / Effect "blizhnei zony" v dinamicheskom khaose.

The dynamics of the system with a determined chaotic behavior (Lorentz system) was studied by comparing the histograms. It was shown that the dynamics of the system exhibits phenomena similar to those observed in studies of fluctuations in physical systems. In particular, upon comparison of histograms constructed from different time intervals, the "near zone" effect makes itself evident. It was shown that a very slight modulation of only one parameter of the system leads to a change in behavior.

[Moscow Research Institute of Viral Preparations tissue-culture smallpox vaccine in a coded control experiments of adult revaccination by scarification]. / Izuchenie tkanevoi ospennoi vaktsiny MNIIVP v shifrovannom kontroliruemom opyte pri revaktsinatsii vzroslykh metodom skarifikatsii.

Comparative studies of tissue culture and dermal smallpox vaccines were carried out in a strictly controlled coded trial by revaccination of adults by scarification. Two lots of tissue culture and one lot of dermal vaccines with a similar infectious titer (8.0 lg PFU/ml) were used. All the lots tested "took" in 100%. In the group revaccinated with tissue culture vaccine seroconversion was observed in 94.2 +/- 2.8%--95.0 +/- 2.8%, with a 4.6 +/- 5.3-fold rise in geometric mean antibody titer; the dermal smallpox vaccine produced seroconversion in 85.7 +/- 4.4% with 4.3-fold rise in geometric mean antibody titer. Febrile reactions in revaccines with the tissue culture vaccine were observed in 23.1%--26.1% which did not exceed this parameter in the group revaccinated with the dermal vaccine (25.2%); in all the groups febrile ractions were of mild degree (37.1 degrees--38 degrees C). Local reactions of the type of confluent erythema were observed 1.8--5.3-fold more frequently among those revaccinated with the dermal vaccine than among those revaccinated with the tissue culture vaccine. Thus, the tissue culture vaccine used epicutaneously was as good as the dermal one by the take rate and antigenic activity, but had less remarked reactogenic properties which allows to recommend it for public health practice.