Interference stabilization of atoms is discussed. Raman-type transitions to Rydberg states with higher values of the electron orbital momentum l are taken into account in numerical calculations. These transitions are shown to change qualitatively theoretical predictions concerning the dependence of the time of ionization t_i on the light field-strength amplitude (epsilon) ₀: the time of ionization is shown to be a falling function of (epsilon) ₀ in the weak-field region and, then, in a rather large interval of (epsilon) ₀, to be approximately constant. The results derived are shown to be in a rather good agreement with the existing experiments.

Intense laser-atom interaction is modeled by placing an atom in a classical electromagnetic field. Computer simulations show new atomic responses to increasing laser intensities, among them are suppression of ionization and localization of the electron.

Multiphoton excitation with elliptically polarized light brings out more information in the photodetachment or photoionization processes than linearly polarized light. This information can be reached by recording photoelectron angular distributions for several values of the ellipticity angle, and fitting the whole set of data with an analytic formula. We give here the appropriate formula, valid for any number N of exciting photons, with the assumptions of (1) an initially even and momentumless (J equals 0) atom or ion and (2) that after excitation of an electron, the residual core be left in an odd state. These are the conditions met by multiphoton detachment from halogen negative ions. With elliptically polarized light of (lambda) equals 1064.2 nm, we have measured the angular distributions of three-photon detached electrons from the ion I^-, and four-photon detached electrons from Cl^- and F^-. The experimental results are quantitatively analyzed and commented on.

The recently observed generation, by samples of atomic rare gas, of high-order harmonics of the fundamental frequency of an infrared laser with orders N greater than 100, opens the possibility to have at one's disposal a table-top source of coherent and pulsed VUV, (or even soft-x-ray), radiation. The characteristics of such sources, (very high brightness, sub picosecond duration, . . .), as compared to conventional UV and soft x-ray sources, has recently attracted a considerable attention. Their usefulness has indeed been demonstrated in recent solid state inner-shell spectroscopy experiments, and also to determine the lifetimes of atomic autoionizing states in He. We discuss here, via a theoretical study involving the numerical resolution of the Time-Dependent Schrodinger Equation (TDSE), the main characteristics of a new class of two-color photoionization processes, dubbed Laser-Assisted Single-Photon Ionization (LASPI). The idea is to expose simultaneously an atomic target to the field of, on the one hand, a high order harmonic radiation with frequency (omega) _H, and on the other hand, the fundamental of the laser, with frequency (omega) _L, which has been used to generate the harmonic field. Then, the radiation pulses are highly correlated and one expects that they have similar durations, which conveniently ensures that the target experiences the two fields simultaneously. Then, if the harmonic frequency is high enough, i.e. is such that: (omega) _H greater than E_I, where E_I is the ionization energy of the atom, LASPI can be observed (atomic units will be used throughout, unless otherwise mentioned). As we show, our simulations indicate that one can find a regime for the respective intensities of the fields, in which the ionization of the target results in fact of the simultaneous absorption of one single photon from the high frequency field and of the exchange of a number of laser photons. The first experimental observation of this process has been reported at this conference. Note also that the observation of a similar process, namely laser-assisted Auger transitions, has been also recently reported.

A simple analytical approximation is obtained for ionization rates of atoms exposed to a strong, linearly polarized plane-wave field. Coulomb interaction between the ejected electron and the atomic core is taken into account in the frames of semi-classical perturbation theory. In the previous paper we introduced an analytical simple solution for an unbound electron in the simultaneous presence of a Coulomb potential and a circularly polarized plane-wave electro-magnetic field. Here we consider the Coulomb-Volkov correction in the case of a linearly polarized field using so called strong field approximation (SFA).

The interaction of the strong electromagnetic field with classical model system with short- range potential was studied. The phenomenon of ionization suppression was observed. The similarity between results of classical and quantum calculations was found.

The line contour of the autoionizing resonance of 6p7p configuration in barium atoms was experimentally studied in the range of energies 48,000 cm^-1 to 54,000 cm^-1 with the help of a two-step ionization via intermediate states 6snp(¹P₁), n equals 6, 7, 8. Using various combinations of polarization of the first- and second-step laser, both linear and circular dichroism were measured in the process of ionization from an excited state.

The new unknown resonances which occur in the case of multiphoton ionization of Ba atoms at simultaneous influence of two laser fields were discovered. The character of the display of these resonances depends on mutual orientation of polarization vectors of these fields.

At three-photon ionization of aligned Ba atoms by radiation of laser on color centers (8300 - 9100 cm^-1) resonant maxima caused by two-photon excitation of bound states were observed. Moreover, maxima which can't be explained by known processes were also observed.

A single-color, two-step, three-photon ionization is examined in a point of view of the dynamic shift of two-photon resonance within the framework of the Bloch equation. It is suggested that a formula well-describes the dynamic shift even beyond usual quadratic behavior and it corresponds to the laser-frequency detuning causing a complete population transfer to the third-bound level.

Photoionization signals are obtained by using the Schroedinger equation for the case where a two-level atom interacts simultaneously with the linearly and the circularly polarized lasers. Both double-peak and double-dip structures due to the optical Autler-Townes effects are examined.

The differential cross-sections of electron-atom collisions in a radiation field are different for right and left circular polarization of field. Simple expression for the dichroism in one-photon electron-atom scattering and spontaneous bremsstrahlung is presented. The numerical estimation of dichroism for one-photon Coulomb scattering shows that the effect may be observed experimentally.

With the help of Lyapunov exponents different routes to quantum chaos are demonstrated numerically in two semiclassical models of the interaction of N two-level atoms with their own radiation field in a resonant cavity. The usual transition to chaos is observed when varying the external control parameter b which describes the interaction of the atomic ensemble with the field. After reaching its critical value b approximately equal to 1 the system becomes more and more chaotic as b increases. As the second control parameter a (the dimensionless individual Rabi frequency) increases the system undergoes transitions to chaos by a completely different way. We observe a complex sequence of order-to-chaos transitions that resembles those for the classical Lorenz attractor.

The nonlinear theory is presented for light-induced drift (LID) in the region where the drift velocity has an anomalous temperature dependence and change sign with increasing temperature. It is shown for the first time that the anomalous temperature dependence of the LID velocity leads to self-excited oscillations in the concentration of the absorbing gas and the temperature. Nonlinear effects caused by a dependence of the LID velocity on radiation intensity are investigated too.

Results of the experimentation observation of changing the profile of Rydberg sodium atomic beam at strong resonance multiphoton interaction with microwave radiation are presented. The influence of kinetic phenomena and of rapid damping of Rydberg states in the resonance standing wave on the beam profile is discussed Theoretical estimates for the process of deflection of Rydberg atoms have been done.

Computer simulations carried out using the developed method permit us to reveal new effects, such as the increasing of angle of rotation of plane of polarization of the probe field in the presence of magnetic field and linearly polarized pump field resonant to adjacent transition and modifications in self-rotation spectra and resonant Raman spectra in the presence of magnetic field.

Three different experimental approaches for the generation of coherent VUV and XUV radiation are studied and discussed. With a 400 fs KrF excimer laser system tunable VUV radiation around 155 nm with a peak power of approximately 0.5 GW has been generated by near-resonant four-wave difference-frequency mixing in xenon. A 150 fs, 0.7 TW Ti:Sapphire laser system has been used for investigations on high-order frequency mixing with linear and circular polarized laser fields. With the same system soft x-ray lasing in low charged oxygen ions has been demonstrated.

HHG in a system 'ion with internal degrees of freedom plus outer electron' is discussed quantum-mechanically and classically. Resonance effects can be strongly manifested in such a system. Spectral amplitude of the resonant harmonic of the dipole moment of the system can be estimated multiplying such amplitude evaluated under single-electron approximation by some correcting factor. The last depends on polarizability of the ion and of a free electron at the harmonic frequency.

A novel mechanism for efficient high harmonics generation is proposed based on the self- scattering of powerful femtosecond light pulse in the Raman-active medium. Within the framework of the closed self-consistent model the pulse field electrodynamics are investigated, a simple analytical formula for the spectrum generated is derived. The 'plateau' effect and the sharp cutoff of harmonic spectrum are predicted. Unlike the case of rare gases, the spectrum generated is found to be tuned by the variation of input ultrashort pulse intensity.

The dynamics of high-intense laser fields at coherent Raman self-scattering are investigated by the use of numerical simulation. The processes under investigation take place at the Raman scattering on electronic states in atoms with transition to be forbidden in electro-dipole approximation, on vibrational or rotational states in molecules, on states connected with the spin flip and other. It is found that the threshold intensities exist provided that the pulse duration is smaller than dephasing time of the Raman transition in the medium. Under this, the following processes occur: an effective generation of the high-order harmonics comes up; the plateau forms in the harmonic spectrum. The maximum which arises on this plateau shifts to the higher-harmonic numbers in case of the higher threshold intensities of the pulse. It is shown how the transformation of the initial pulse intensity into high-order harmonics intensity can be considerably increased with the different initial parameters of the pulse and the medium.

In the frame of classical mechanics the analytical expression has been obtained for the radiation emission spectrum, consisting of a large number of the odd harmonics. It has been shown that the high frequency boundary of the spectrum is determined by the ratio of the electron oscillation velocity to the atom size. The oscillations of the electron in a laser field take steady-state form for a time about of optical cycle. The field amplitude should be sufficient for electron tunneling through the variable in the time potential barrier created by Coulomb attractive force of a nucleus and by an alternate extracting electrical field radiation E(t). The quantum tunneling process will be exponential depending on the absolute value of E(t) and will be quasistatic under certain conditions. This statement corresponds to experimental fact of the sharp occurrence and the fast increase of high harmonics radiation intensity in the range of laser radiation intensity 10¹⁴ less than I less than 10¹⁶ W/cm², corresponding to subatomic values of field E_o less than E_a. The electrons appear outside of the barrier at the peaks of the sinusoidal field. Consequently they have zero drift velocity and hereinafter make the forced oscillations near their 'parent' atoms. At the large laser field strength E_o greater than or equal to E_a the electrons can tunnel during a considerable part of every optical period, when field time derivative differs from a zero, and therefore they will have drift velocity V_d less than eE_om(omega) . In this case the electron drift displacement for a time of laser pulse (tau) very much greater than 1/(omega) can exceed the oscillation amplitude and interatomic distance. This can result in turn in appearance of the fully ionized plasma. But in the present paper only the case E_o less than E_a is considered.

Simple classical models of optical harmonic generation in excited atomic systems with Coulomb and Debye interaction are considered. The influence of the effective potential on the shape of the spectrum and properties of optical harmonics is studied. Polarization effects in coherent four-wave mixing are analyzed.

The influence of phase-matching conditions on the generation of the third and fifth harmonics of Nd:YAG-laser radiation in the plasma of optical breakdown produced on surfaces of metal targets is experimentally investigated. It is demonstrated that an appropriate choice of phase- matching conditions provides the opportunity to considerably increase the efficiency of optical frequency multiplication via harmonic generation. We determine optimal conditions of focusing incident radiation for the efficient generation of the third and fifth harmonics in a laser-produced plasma.

A non-stationary kinetic model of a plasma laser with wavelength lambda equals 26.5 nm on transition 3s yields 2p of Li-like ion of nitrogen N⁴⁺ is created. It is shown, that lasing is possible at the leading edge of pump pulse. As a result of simulation there are optimum lasing conditions.

The electromagnetic pulses generated by the spike pulse of radiation are discussed for two physical models: excitation of the elementary dipoles by quanta of radiation and formation of the macroscopic current due to the anisotropic distribution of the photoelectrons.

The formation of the amplified spontaneous emission (ASE) in the laboratory x-ray laser with the Markov fluctuations of the dielectrical permittivity (epsilon) is studied by the method of paraxial equation for the transverse correlation function of the radiation field. The fluctuations (epsilon) are found to impose a limit on the ASE spatial coherence and to decrease the power of the coherent ASE. In the case of the weak regular refraction, the ASE scattering by (epsilon) is analogous to the linear absorption and decreases the gain observed. At the strong defocusing refraction the effect of (epsilon) on the ASE has a 'latent' character: an essential fall of the coherence length is accompanied by a relatively small decrease in the axial intensity and by a small broadening of the ASE beam as a whole.

The model of the defect formation wave and cluster nucleation wave, propagating jointly in transparent dielectrics, activated by multiphoton electronic excitation is developed in close analogy to combustion wave. A new criterion of multipulse laser damage of transparent dielectrics is introduced and compared in detail with experimental results.

A phenomenological analysis is carried out of novel nonlinear optical processes taking place in macroscopically noncentrosymmetric isotropic solutions of chiral (lift-ring mirror asymmetric) macromolecules, which are the primary elements of living organisms and their metabolic products. Among the most interesting and potentially useful for spectroscopic purposes are: optical rectification/photogalvanic effects consisting in electrostatic field/direct electrical current generation in such liquids under irradiation with the intense circularly polarized laser beam and the five-wave mixing phase-matched process of BioCARS to selectively record, background-free, vibrational spectra of chiral molecules.

The photoinduced change of transmittancy of chalcogenide glasses was investigated under exposure to the laser radiation at 1.064 and 1.318 micrometer. It was established that two- photon absorption in these glasses arises under exposure to radiation with irradiances of greater than or equal to 10⁸ W/cm². It results in irreversible structural transformation of the glasses in the interaction region. This process has threshold character over a narrow range of radiation irradiance (plus or minus 0.1I_thr) and results in appearance of a small-dispersion phase in the bulk glass. Formation of such regions has fluctuating character and causes the appearance of strong light scattering. The effect observed is similar phenomenologically to the known effect of photo-structural transformations in the thin films under exposure to near bandgap light. However, in contrast to the last, the emerged changes of the glass properties are not erased completely by the annealing at the glass transition temperature.

The processes arising in glasses as an example of solid dielectrics under high-power femtosecond laser radiation have been investigated. In the investigation samples of fused silica were used as well as a wide selection of commercial borosilicate glasses K8 (Russia) and BK7 (USA) which have the band gap near 6 eV. The glass samples were irradiated with the output of a laser system allowing the production of laser pulses at 0.85 micrometer wavelength and duration approximately 10^-13 s. It is found that color centers formation and intrinsic luminescence of borosilicate glasses are observed under high-power femtosecond radiation at 0.85 micrometer. These processes result from supercontinuum generation in the bulk glass and are followed by two-photon absorption of the short-wavelength part of this supercontinuum. These same processes lead to the damage of glasses under conditions of intensive ionization of material matrix, and show that the damage process is not connected with avalanche ionization.

The chi⁽²⁾-grating recording in barium-borosilicate glasses doped with Tb or La ions was investigated. Correlation between BaO content and PSHG efficiency was established. The comparison of the absorption spectra with the data of PSHG efficiency measurements allows us to assume the role of resonance transition in Tb³⁺ in the rising of PSHG effectiveness due to resonance increase of chi⁽³⁾.

The glass structures formed by ionic and covalent bonds are considered. We suggest a new mechanism of these structures ordering by the irradiation of YAG:Nd laser's first and second harmonics. The equivalent Si-O^-Pb²⁺O^--Si equivalent structures of lead-silicate glasses have been studied by computer simulation. It is found that the suggested mechanism can play an important role in the photoinduced second harmonic generation in these glasses.

Carbon-dioxide-lasers operating in the pulsed mode with energy densities up to several tens of J/cm² and peak power densities in the multi-MW/cm²-range may cause fast heating and melting. Eventually quasi-explosive ejection, decomposition or vaporization of material can be observed. Surface plasmas are strongly influencing the energy transfer from the laser radiation field to any target. For optically transparent plastics, such as PMMA for example, only slowly expanding plasmas (LSC-waves) are ignited at fluences around 20 J/cm², with a low level of self-luminosity. High brightness, supersonically expanding plasma jets (LSD-waves) are generated at the same fluences on glasses. Similar conditions were found for metals as well. From recordings with a high speed CCD-camera, interesting features concerning the initial plasma phases and temporal evolution were deduced. Additionally, information was obtained concerning the quasi explosive ejection of material for PMMA.

During recent years the interest in media with strong nonlinear response is growing. These media allow the user to observe different nonlinear optic effects using small intensities of light. It is well known that liquid crystals are rather promising media for this research. This paper is devoted to the experimental research of the photoinduced conduction of a mixture of a nematic liquid crystal and a dye. Dependence of the conduction on the intensity of light was studied for different concentrations of a dye added to the nematic crystal. Also the problem of the optimum type of a dye for observing the photorefractive nonlinearity using Ar⁺- ion laser was considered. We made the experiments using the following available laser dyes: rhodamine '6G,' rhodamine 'G,' rhodamine 'C' and two ocsasine-type dyes also. The mixture of the nematic crystal 5CB and a dye was placed in a cell of 100 micrometer width, with the plates filmed with the transparent electrodes of SnO₂. The dc voltage on the order of magnitude 1 V was applied to decrease the influence of the cell capacity on the conduction measurements of the samples. We used the light of two wavelengths: lambda₁ equals 488 nm, lambda₂ equals 514.5 nm. The best dyes for these wavelengths were the rhodamine- type dyes. Taking the other two dyes we observed much smaller effect of influence of the laser radiation on conduction of the samples. Maybe the reason was that the pump wavelength of ocsasine dyes is too far way from the wavelength of the radiation used. So the optimum dye must have the wavelength of the pump near to the wavelength used. Using rhodamine 'C' we obtained the dependencies of the induced conduction on laser light intensity for three different concentrations of the dye.

Giant increase (up to 10¹⁰ times) of aggregation rate of silver colloids to the fractal structures under influence of optical radiation is found. Spectral, temporal and energy characteristics of the process are investigated. The instantaneous spectral changes are explained by the influence of laser-induced far-ranged electrostatic forces on the fractal aggregation and by the subsequent fractal photomodification.

The renormalization of the energetic spectrum of paraexcitons in the presence of the Bose- Einstein condensate of orthoexcitons, or at least of their saturation, is studied. Due to the conversion of a pair of orthoexcitons into a pair of paraexcitons an absolute instability in the energy spectrum of paraexcitons appears. The paraexcitons that appeared can be downscattered by optical phonon (Gamma) ₅^-(88 cm^-1) in a state above the bottom of the paraexciton band. This could be the reason for the observed peculiarities in the paraexciton luminescence band. On this basis a new explanation of the extra peak appearance in the luminescence spectrum of the uniaxially stressed Cu₂O crystal is suggested. It has the same origin as the bump of the luminescence band in unstrained crystal. The spectral positions of the extra peak and of the bump differ due to the diminution of the ortho-para splitting in uniaxially stressed crystals. The difference of their widths is due to different densities of the condensed orthoexcitons in stressed and unstrained crystals.

The reflection and conduction wave excited by the IR laser pulse was found in crystals of all four main lattice types. The peculiarities of the observed wave founded in the materials transparent in the visible region of spectra are very important for the phenomenon model development.

The sum frequency generation from the surface of an isotropic gyrotropic medium is investigated in the reflection geometry for the plane elliptically polarized incident waves. Modified boundary conditions for the electro-magnetic field are employed, which are still correct if the spatial dispersion of the optical response of the medium is taken into account.

A dielectric microsphere possesses natural modes of oscillation at characteristic frequencies, corresponding to specific size to wavelength ratios. These structure resonances are known to cause extremely large field intensities and have extremely high quality. We investigate here systematically the characteristic equation, defining the natural frequencies, as a function of the change in the asymptotic behavior of spherical Bessel functions which reveals the fact that there exists resonances with very narrow linewidth. This characteristic has very important implications to nonlinear optics.

Nonlinear optical absorption in heavily doped n-GaAs and n-InP has been studied experimentally in a nanosecond time scale. Theoretical description of the nonlinear absorption is given that takes into account band-filling, band-gap renormalization and Coulomb screening effects. The results of experimental investigation of self-diffraction effect observed in these semiconductors are presented.