Trends of development of ultra-short pulse solid-state lasers go towards new (infrared) wavelengths and shortest pulses, or towards high-average powers in the subpicosecond regime generated with high efficiency by diode-pumping. Avenues of pulse quality improvement are carrier-envelope phase control and amplitude shaping. Applications may be focused on shortest pulse durations, broadest spectra or high-average powers allowing innovative approaches.

A number of factors that influence spectral position of the femtosecond pulse in a Kerr-lens modelocked Cr:LiSGaF laser have been identified: high-order dispersion, gain saturation, reabsorption from the ground state, and stimulated Raman scattering. Using the one-dimensional numerical model for the simulation of the laser cavity, the relative contributions of different factors have been compared. The Raman effect provides the largest self- frequency shift from the gain peak (up to 60 nm), followed by the gain saturation (approximately 25 nm), while the high-order dispersion contribution is insignificant (approximately 5 nm). Comparison with the experimental data confirm that the stimulated Raman scattering is a main cause of the ultrashort pulse self-frequency shift observed in Cr:LiSGaF and Cr:LiSAF lasers.

Numerical simulation of ultrashort pulse generation in the laser with a composite active medium and additional Raman active element in a cavity has been done. It was created that for some laser parameters the optimization of a Raman gain and a frequency shift values was resulted in additional shortening of pulse duration.

The generation of individual high-intensity attosecond pulses by stimulated electronic Raman self-scattering of femtosecond optical pulses is investigated through numerical simulation. The shortest and most intense attosecond pulses are found to occur for 4(pi) - pulses, which are associated with previously predicted self-induced transparency in the stimulated Raman self-scattering of fs-pulses. The simulations show that an initial 10 fs laser pulse (4 optical cycles at the full width at half maximum with 780 nm wavelength) is transformed into a pulse of width 0.33 fs having a peak intensity significantly greater than the initial pulse.

The potentiality to increase light pulse conversion efficiency into pulses of the millimeter and submillimeter (GHz - THz) range using the waveguide partially filled with a nonlinear crystal is suggested. This technique has been theoretically substantiated and experimentally studied. Phase matching is defined by the degree of partial filling. The calculated data as well as the experimental results of the difference frequency generation in LiNbO₃, ZnTe, GaAs and DAST crystals are presented. DAST refractive index as well as its tn(delta) have been measured in the 70 - 900 GHz frequency range.

An objective of the present work is considering of the unidirectional propagation and interactions of linearly polarized extremely short pulses in a nonlinear dispersive medium modeled by an anharmonic oscillator combining quadratic and cubic nonlinearities. Two families of exact analytical solutions (with a positive or negative polarity) are found for the moving solitary pulses. Direct simulations demonstrate that the pulses are very robust against weak perturbations, and collide nearly elastically, irrespective of their relative polarity.

Propagation and amplification of an ultrashort laser pulse in a two-level medium is numerically investigated with the use of the finite-difference time-domain (FDTD) procedure simultaneously solving Maxwell and Schrodinger equations with no assumptions characteristic of slowly varying envelope approximation. Comparison of the results of FDTD numerical simulations with the predictions of the pulse area theorem has demonstrated that the developed numerical procedure provides an adequate description of pulse evolution in a two-level medium. Amplification of ultrashort light pulses in a two-level medium is analyzed. Two methods of improving the gain of ultrashort pulses in a two-level medium by modulating the spatial distribution of dipole moments of resonant transitions and using initially chirped frequency-detuned pulses are explored.

A three-dimensional interference fringe structure containing only a small number of fringes is considered. The diffraction and geometrical-optical regimes of interaction of radiation with the structure are investigated.

We describe how spectral analysis of an ultrashort pulse after propagation in a single mode fiber can be used to characterize picosecond laser pulses. The technique is demonstrated for a cw mode locked Nd-YVO₄ laser.

Vibrational coherence of the condensed-phase polyatomic molecules was studied by several types of femtosecond time- resolved spectroscopy. The Raman-active vibrational coherence in the ground-state was observed through optically-heterodyned impulsive stimulated Raman scattering (ISRS) spectroscopy. Transient ISRS experiment was also undertaken for the measurement of the low frequency vibrations of the excited state. In transient absorption spectroscopy carried out with a 40-fs time resolution, vibrational coherence was created by ultrashort pump pulses and the wavepacket motion in the excited state was observed for trans-stilbene and diphenylcyclopropenone.

The relaxation times ((tau) ) of the first excited state of cationic symmetric polymethine dyes absorbing in the spectral range of 750 - 850 nm have been determined by a direct pump and probe method. The influence of the dye structure and solvent nature on the value of (tau) is discussed. The role of different intra- and intermolecular interactions in deactivation of the excited state of the dyes is considered. The perspective polymethine dyes ((tau) equals 11 - 75 ps) for passive mode locking of lasers on LiGaAlF₆:Cr³⁺, KZnF₃:Cr³⁺, and LiSrAlF₆:Cr³⁺ crystals are proposed from the analysis of relaxation times.

A new method for determination of the mobility edge in disordered semiconductors by femtosecond pump-supercontinuum probe spectroscopy is presented. The method is based on the determination of the spectral dependence of a stretched exponential relaxation in a wide spectral range of probing, h(omega) _probe equals 1.6 - 3.2 eV. The method is demonstrated for porous silicon. It is shown that the relaxation parameters for porous silicon have essential spectral dependence. The spectral dependence of stretched exponential index (beta) ((omega) ) give unique information about existence and position of the mobility edge in disordered materials, and thus may be used as effective tool in manifestation of the transition from localized to delocalized relaxation regime on the femtosecond time scale.

Investigation of the photoinduced processes in pure C₆₀ films was performed by means of sub-100 fs resolution spectrometer. Excitation by 100 fs laser pulses with photon energy above and below the mobility edge revealed that both charged and neutral components appear during the excitation pulse. At both excitation wavelengths electrons and holes are generated by direct optical excitation and not due to singlet-singlet annihilation. Anions are created with a delay of 10^-13 - 10^-11s in a result of electrons capture by C₆₀ molecules.

To interpret the data of transient nonlinear spectroscopy of cooper-oxide high-temperature superconductors, a phenomenological model, describing magneto-dipole self- organization of holes in CuO₂ planes and kinetics of the corresponding phase transition, has been developed. It has been shown that, for the exchange energy approximately 100 meV and the doping level <n> approximately 0.1 hole/cell, the temperature T decreases below the critical point T* approximately 150 K leads to formation of a spatially non-uniform distribution of holes (the stripe- structure) and to appearance of corresponding energy gap in the electronic structure. Calculated steady-state dependencies of T* on <n> and of the gap width on T agree with the known experimental data. The predicted phase transition kinetics depends on the initial temperature T. When T* < T < T_m approximately equals (1.4 divided by 1.5) T*, the stripe-structure decay is rather slow (approximately 10^-9s and more) two-stage process. During the first stage, a large-scale fluctuation of an average dipole moment is formed and after that the spatial region, entrained in the fluctuation, broadens as the phase switching wave.

On the basis of experimental data of degenerate four-photon spectroscopy of Ni, Au and Pt ultra-thin (with the thickness approximately 10 divided by 20 nm) films, a conclusion about a predominant role of inter-band electronic transitions under formation of a nonlinear response of such films in the visible range has been made. It has been shown that, when a metal film thickness is smaller than the mean-free path, thermalization and cooling of optically excited electronic subsystem result from an ultra-fast inelastic scattering of excess free carriers on the film surface. Numerical estimations, performed for ultra-thin Ni films, have shown that, thanks to this scattering process, up to 10% of picosecond (duration approximately 20 ps) pump pulse energy can be transformed to the surface deformations. In case of spatially non-uniform optical excitation, an efficient direct generation of the surface acoustical waves can be realized.

The recording geometry and recording media for the method of achromatic wavefront reconstruction are discussed. The femtosecond recording on the thick slabs of dichromated gelatin and the samples of silver-containing porous glass was obtained. The applications of the method to ultrafast laser spectroscopy and to phase conjugation were suggested.

The production of short-pulse, coherent, XUV radiation by High Harmonic Generation (HHG) has become a routine operation in many laboratories equipped with an intense femtosecond Titanium-Sapphire laser. The required intensity of 10¹⁴ to 10¹⁵ W.cm^-2 is easily reached with 1 - 2 mJ, 40 - 100 fs pulses focused by a long focal length lens (1m). The most usual medium for HHG is a noble gas. Xenon or argon are the most efficient ones (with efficiencies of the order of 10^-5) while neon and helium allow for the generation of the shortest wavelengths (2 - 4 nm) albeit with a reduced efficiency (10^-9 - 10^-8). For symmetry reasons, only the odd harmonics of the fundamental frequency are generated and a typical spectrum like the one in Fig. 1 consists of narrow lines separated by twice the fundamental photon energy (1.55x2 eV in the case of Ti:Sap lasers). The harmonic pulses are naturally synchronized with the pump pulse and usually much shorter. This latter property combined to the high brightness, coherence and directivity of HH make them ideal for pump-probe experiments and particularly for multicolor- multiphoton transitions requiring a spatio-temporal overlap of the IR and XUV pulses. Such applications have been carried out in atomic and molecular and solid state Physics. The present work is about recent studies of multiphoton ionization of rare gas atoms by a superposition of HH and IR pulses and their applications to the metrology of femto and attosecond XUV pulses produced by HHG.

Spatial distributions of above-threshold photoelectrons produced during the tunnel ionization of multicharge ions by laser radiation of relativistic intensity are theoretically investigated. It is shown that an essential modification of these distributions should be due to simultaneous manifestation of relativistic effects and Coulomb interaction of photoelectron with its parent ion. For short laser pulse, the distribution becomes composed of two peaks. Positions of these peaks depend on the radiation intensity and ellipticity, and also, on the charge of the parent ion. The last dependence can be used for the control of ionization process of multielectron atoms. A new mechanism has been found that increases the multicharge ion stability in regard to ionization by superintense ultrashort pump pulse. The gradient forces on the trailing edge of the pulse able to compensate the relativistic drift of photoelectron. And due to Coulomb attraction, it is possible to electron be captured back to the closed orbit after the pump pulse is over.

We present results of analytical and numerical calculations of the angular spectrum and the power of high-order harmonics generated with a limited laser beam in an extended medium. It is shown that the spectrum of harmonics generated in a thick target dramatically differs from that of the single-atom response. In particular, in this spectrum there are two plateaus due to off-axially phase-matched generation of some harmonics (phase-matching condition holds only for a part of the harmonic angular spectrum). The origin that makes the off-axially phase-matched generation effective is the amplitude modulation of the high-frequency single-atom response in the laser beam cross section that increases the divergence of the harmonic beam. In Gaussian beams the off- axially phase-matched generation in extended medium (i.e. under target thickness close to the confocal length) is effective under any value of geometrical dispersion (i.e. under arbitrary hard focusing). Our treatment explains features of the high harmonic spectra obtained experimentally using extended targets as well as high conversion efficiencies achieved in these experiments. It is shown that under self-guiding of a laser beam in a noble gas off-axially phase-matched high harmonic generation is possible if some easily ionizable gas is added to the main generating gas. In calculations of high-order harmonic generation in such mixtures conversion efficiency as high as approximately10^-3 - 10^-2 for 33-rd harmonic of Titanum-Sapphire laser and about 10^-4 for 121-st one is obtained.

We present results of computer simulations showing dramatic increases in the production and intensity of high-order harmonics (in the cutoff region) in rare gases interacting with counter-propagating pulses. Using two different models of medium, classical anharmonic oscillators (for Ne, Ar, Kr) as well as quantum oscillators (for Ne⁺, Ar⁺, Kr⁺), we demonstrate the increase of efficiency by four and by six orders in these models, respectively. This is at least by two orders of magnitude more than obtained with other methods.

Two-component pump of atoms with strong low-frequency field and ultrashort pulse of high-frequency radiation is proposed for high-order harmonic generation (HHG). At specific relative phase between pump fields HHG will occur with a selection of narrow frequency domain that is important for applications. In addition, the total number of photoelectrons can be essentially less in this case compare to traditional HHG scheme that improves phase-matching condition. Interaction of atoms with such two-component field can also restrict HHG efficiency owing to harmonic absorption while the atom tunnel ionization. Reversing this effect is proposed for harmonic amplification with additional high-frequency pump radiation.

A formalism is developed to treat the (T-1D) dynamics of the interactions between relativistically intense laser pulses and cold underdense plasma electron component oscillations. Asymptotic solutions to the Maxwell and electron fluid dynamics equations are developed in the Compton limit, where the ratio of the plasma and the optical field frequencies plays the role of a small parameter. The strong electromagnetic waves are described by applying the envelope approximation to the Akhiezer-Polovin waveforms. A theory of plasmon generation by the propagating laser pulse and of the corresponding nonlinear self-modulation of its envelope is established.

A SiO₂ aerogel with absorbed deuterium is suggested as a target for the fusion reaction d + d yields He³ + n by a super-intense ultra-short laser pulse. The multiple inner ionization of oxygen and silicon atoms in the fibers of this fractal aerogel takes place in super-intense laser field. The Coulomb explosion of the deuterated aerogel skeleton propels deuterons up to kinetic energies of ten keV and higher. The neutron yield is estimated up to 10⁵ neutrons per laser pulse during approximately 500 ps if the peak intensity is 10¹⁸ W/cm² and pulse duration is 35 fs.

The soft x-ray generation properties for both flat targets and structured targets such as nanohole-alumina and Au- nanocylinder are evaluated. The experimental results for flat metal targets have revealed the fundamental properties of soft x-ray such as broadband continuum spectra and short pulse duration of less than 3 ps. By adopting structured targets such as nanohole-alumina and Au-nanocylinder targets, a more than 30 fold enhancement of x-ray generation yield is achieved compared with that for flat targets of the same materials with a slight increase of pulse duration, less than 20 ps. Then, the duration of soft x-ray pulse from femtosecond (fs) laser produced W plasma was measured by using cross-correlation method. In the experiment, we measured the transmission spectra of short soft x-ray pulse produced by main fs optical pulses through ionized Kr gas by probe fs optical pulses as a function of the time delay between main and probe fs optical pulses. The pulse duration of 4 ps was observed for soft x-ray at 15.6 nm of W plasma. The time-resolved measurement of the inner-shell absorption change of Si during the irradiation with a high-intensity fs optical pulse is achieved by using a picosecond soft x-ray pulse as a probe pulse in pump-probe experiments. A more than 5% increase in the absorption of Si membrane at near the L_II,III edge (around 100 eV) was observed. The recovery time of the absorption change was measured to be about 20 ps. From these experimental results, this absorption change is assumed to be the bandgap renormalization of Si.

The properties of internal electronic conversion of nucleus excited in hot dense laser plasma are considered. The important role of dynamics of plasma charge state in residual gas is shown.

It is shown that in superstrong electromagnetic field the ponderomotive forces are not potential and depend on the field polarization. A new technique for charge acceleration with high-intensity two-polarized laser beams is proposed and investigated.

We report on the experimental observation of neutrons released from d(d,n)³He fusion reaction, taking place in plasma created by femtosecond (200 fs) laser pulse with moderate intensity (10¹⁶ W/cm²) on the modified surface of a solid-state deuterium enriched titanium target. Target surface was modified by preceding laser pulse with the same parameters.

The damage process of crystal aluminum surface by powerful pulse beams of laser radiation has been investigated with the help of the molecular dynamics method. A direct transformation of beam energy into an energy of mechanical atomic motion is shown to be possible owing to a change of the Coulomb interaction screening in an ionic subsystem upon excitation of valence electrons.