The parametric down conversion of femtosecond pulses at high intensity is discussed. An injection-locked parametric oscillator based on a nonlinear LBO crystal pumped by XeCl- and dye-laser systems is developed. The nonlinear refractive index n₂ approximately equals 1.7 10^-16 cm²/W and the optical surface breakdown threshold Ibr approximately equals 3.8 10¹³ W/cm² of LBO crystal for approximately 400 fsec pulses were measured. The spatial dispersion of effective nonlinearity deff2/n3 for parametric amplification in LBO crystal is calculated. The design and performance of femtosecond dye- laser system and XeCl-excimer-laser system capable of producing a focused intensity in excess of 10¹⁶ W/cm² are described.

Amplification of high-power ultrashort pulses in gas-discharge amplifiers is investigated theoretically. The most essential features of amplification in this case are connected with an electron density wave (EDW) generated by laser pulse in gain medium due to the avalanche ionization. Traveling with amplified pulse, EDW results in the pulse amplitude and phase modulation. Also, it determines the ultimate pulse parameters (energy, peak intensity, duration) and features of pulse evolution in long amplifier. The laser operation with a variety of CO₂ multi-isotope mixtures is investigated also. Such operation enables amplification of laser pulses with duration down to 100 fs. Moreover, in a particular spectral domain, the picosecond pulse amplification at reduced gas pressure becomes possible.

The experimental results on efficient coherent harmonic generation and four-wave Raman and hyper-Raman scattering in a laser-produced and electric discharge plasma are presented. It has been shown that in the four-wave mixing processes, spectra resonances appear due to the Raman and hyper-Raman scattering on atomic and ionic excited states. The temporal behavior of the scattered signal has been found to be connected with the population relaxation of atomic and ionic excited states. We have observed for the first time the resonance in the spectrum of coherent hyper-Raman scattering in electric discharge plasma connected with the electron transition from the excited to autoionizing state of a copper atom.

Recent progress in powerful subpicosecond laser generation technology has stimulated interest in interaction of ultrashort electromagnetic pulses with nonlinear media. Being a classical example of a nonlinear medium and a traditional object of laboratory investigations, the plasma demonstrates an extreme variety in behavior under the action of the electromagnetic fields. Characteristic rise and relaxation times of nonlinear response can be varied by a few orders of magnitude depending on particle concentration, wavelength, external magnetic field, etc. Thus, the ultrashort pulse duration should be defined as a particular notion with respect to the phenomenon in question. In this paper we take interest in electromagnetic wave-plasma interaction processes faster than the relaxation of the nonlinear plasma response. In this context, we treat the interaction as essentially dynamical and the plasma as a medium with long-term memory properties. The nonsteady state of the nonlinear coupling and the plasma wake generation are two main features essential to the basic physical effects and applications.

The interaction of the relativistic charged particle beams with the superintense interference laser fields is studied. New methods for laser control of the relativistic particle motion are discussed.

The problem of interaction of short laser pulse (light frequency (omega) o, pulse duration (tau) < 1_s/V_Ti, 1_s is the skin depth, V_Ti is the ion velocity) with dense ((omega) _o << (omega) _pe) semi-infinite plasma was solved. We formulated the self- consistent problem of obtaining the electron distribution function and space dependence of the electric field in the skin layer, and solved the problem for several cases analytically and numerically. On the basis of the obtained solution, we have calculated mean electron energy, absorption coefficient, Bremsstrahlung radiation, and time-dependent skin depth. We have also found dependence of the absorption coefficient on the wave incidence angle and its polarization.

It is shown that in a strong circularly polarized laser field, classical electron motion around the ions can occur. The non-relativistic scattering by these electrons in plasma has a certain (Thomson) cross-section only in the limit of a very strong field (it is practically the case of relativistic motion of electrons). In a circularly polarized field with an amplitude on the order of the inneratomic one, the cross section of this process is less. In the spectrum that the scattering of this field gives in plasma, there are non-ion satellites along with the basic frequency.

The conditions for shock-wave generation by UV light pressure are examined. It is shown that in the nonrelativistic regime of laser intensities 7 10¹⁸ W/cm² < I < 7 10²⁰ W/cm² the radiation pressure can exceed thermokinetical pressure of hot electrons. In a time scale less than 10 fs, the formation of shock-pulse with Gbar pressure can be achieved.

The rate of tunneling ionization as a function of laser field is obtained taking into account subbarrier electron tunneling as well as overbarrier ones. It is shown that local ionization suppression, i.e., stabilization of atoms or ions in regard to ionization, may occur in a strong laser field due to the Stark effect. Also, there may be a suppression of (4n-1)- harmonic generation of laser radiation in nonlinear plasma created in such ionization.

It is well known that the interaction of laser beams with periodically corrugated surfaces (e.g., with diffraction gratings) can have a resonant character and can be accompanied by the strong local field's enhancement, suppression of the specular reflection, and anomalous high absorption of light. These effects are known as Wood's anomalies. Previously we have theoretically investigated the peculiarities of these anomalies, when the ultrashort (pico- and femtosecond) regular laser pulses are incident upon a corrugated surface. In this paper, we consider the interaction of the frequency-modulated (chirped) laser pulses with gratings. The effect of the resonant excitation of surface electromagnetic waves (SEW) with a high amplitude and a duration less than that of the incident pulse is predicted. It can be considered as a method for the ultrashort powerful SEW generation. Different specific features of the Wood's anomalies, such as transformation of the specular reflected pulse and anomalous high ultrafast input of the radiation energy into the surface, are analyzed.

The x ray emission from femtosecond laser-driven plasma has been investigated using spherical multilayered mirrors. Different laser pulse widths have been used. The conditions required to produce high intensity x ray emission (up to 10¹¹ W cm^-2) from H-like excited ions are discussed.

The role of multiphoton resonances in the formation of the `plateau,' characteristic of high- order harmonic generation, is examined by a series of experiments on both above-threshold ionization and harmonic generation in the rare gases. The shift of intermediate states in and out of resonance by ac Stark effect plays an essential role in the production of high-order harmonic radiation and is found to be responsible for the initiation of the plateau.

New experiments based on laser heating of a target for creation of a nonequilibrium (supercooled) plasma are discussed. Estimates show it is a promising method for construction of the recombination x ray laser on the 3 - 2 transitions in hydrogen-like ions.

In this work a further investigation of characteristics of the LPP-MLM combination at (lambda) approximately equals 48 angstrom has been performed. A system, consisting of the LPP source generated by Nd-glass laser ((lambda) equals 0.53 micrometers ) and a focusing MLM, was evaluated from the standpoint of reaching the highest power density of the focused SXR radiation. A dependence of the conversion coefficient for the laser-SXR energy transformation was studied, with spatial and time parameters of the source determined for low-Z and high-Z targets.

There are a variety of vacuum ultraviolet and soft x ray radiation sources. They may be stationary (as a synchrotron) and of pulse nature (x-ray tubes with rotating anode, z-pinches, plasma focus, etc.). Present-day investigations prove that the ideas of laser technology may be used for producing sources over this range of photon energies. Here we should note x-ray lasing, laser driven plasma x ray sources, and nonlinear conversion of visible laser radiation. Though this variety gives researchers a wide latitude in the use of such sources, careful examination and optimization for certain experimental conditions must be done. From our point of view, a laser driven x ray source (LDS) has good scientific and commercial potential. LDS has the advantages of small size, short duration, sufficiently high energy, and an easily changeable spectral range. These performances are very suitable for lithography, high resolution microscopy of wet biological samples, and many applications in inertial confinement fusion researches.

A two-dimensional hydrodynamic code for modeling an x-ray laser is developed. Modeling of a CO₂ laser-produced plasma is performed. The uniformity of plasma column in the magnetic field is also discussed.

Deposition possibility of the small d-spacing (d approximately 1 - 2 nm) multilayers on the basis of the material combinations W/Sb, W/B4C, Cr/Sb, Cr/Sc, Fe/Sc, and their utilization as dispersive and focusing elements for the photon energy range E > 0.3 keV have been investigated. The employment of the normal incidence spherical multilayers W/Sb and Cr(Fe)/Sc for imaging of a high-temperature laser-produced plasma within the `water window' spectral range (0.3 < E < 0.5 keV) are presented.