Interaction of femtosecond (30 fs-5 ps) and intense (up to 10¹⁷ W/cm²) laser pulses with Ar clusters (180 to 350 Å radius) has been studied. The laser absorption and the cluster heating have been measured using different diagnostics, demonstrating the production of very hot and dense plasmas, in the keV range. A special attention has been devoted to the K-shell x-ray emission spectra (2.9-3.3 keV). X-ray emission has been observed from ions in very high charge states (Ar¹⁶⁺). Time-resolved measurements have been performed, giving evidence, for the first time, of extremely short x-ray pulses down to the sub-picosecond. Simulations based on both an hydrodynamic model and collisional-radiative atomic physics reveal an extremely brief x-ray emission burst consistent with measurements.

High-resolution X-Ray spectromicroscopy methods were used for investigations of fs laser interaction with N₂O cluster media. Elongated (up to 8.5 mm) femtosecond laser self-channeling in N₂O cluster media under sufficiently low laser intensity (0.5-4x10¹⁷ W/cm²) was observed. Results are revealing a strong macroscopic effect on laser beams owing to their interaction with a gas of clusters. This has occurred at moderate pulse intensities, so the effect is unrelated to either relativistic self-focusing or ponderomotive filamentation. Enough homogeneous multicharged ions plasma with bulk electron temperature around 100 eV was observed along the plasma channel. The spectral lines shapes of the H- and He-like Oxygen ions demonstrated the presence of strong "blue wings", which are caused by Doppler-shifted lines radiation from the essential fraction of ions (~10^-2 - 10^-3) with energies 0.1 - 1.5 MeV. The slope of Doppler-shifted lines radiation is good approximated by ~300 KeV ion temperatures.

The energy and angular distributions are obtained for electrons at the rear surface of thin foils irradiated by an oblique relativistic laser pulse. Vacuum heating at the front surface in the summary field of incident and reflected laser waves is considered as a main mechanism of electron heating up to relativistic ponderomotive values.

By focusing an intense femtosecond, high temporal contrast, laser on ultra-thin foils (100 nm) in the 10¹⁸W/cm² intensities range, we demonstrate that we create instantaneously a hot solid-density plasma. The use of highorder harmonics generated in a gas jet, providing a probe beam of sufficiently short wavelengths to penetrate in such media, enables to study the dynamics of this plasma on the picosecond time-scale. The comparison of the transmission of two successive harmonics permits to determine the electronic density and the temperature with an accuracy better than 15% never achieved up to date in relativistic regimes.

The experimental results of investigation of X-ray radiation provided by interaction of intense femtosecond laser pulses with Fe-clusters are presented. A new method of iron particles formation at room temperature by the photo-dissociation of Fe(CO)₅ vapor is applied. The X-ray radiation was studied using photodiodes and high efficiency focusing crystal von Hamos spectrometer with a CCD linear array as X-ray detector. The results of investigation of clusters formation, spectral measurements in photon energy range E=1÷13 keV and estimation of X-ray radiation yield are presented.

Experimental and numerical investigations of femtosecond laser plasma at the cleaned surface of W target are carried out. The constructed picture of ionization, acceleration and recombination of ions in plasma well explains main features of atomic, charge and energy spectra of fast and slow ions. It was shown that experimentally measured maximum charge of W ions exceed charge predicted by impact ionization and can be reached due to ambipolar field ionization.

Laser plasma produced with high-intensity picosecond laser pulse like proton source for radiography was investigated. It was found that maximum particle output and best possible spatial uniformity of proton beam took place for two-layer target when the front layer was the high-Z film. It was shown that the ion radiography of the convenient objects with using the two-layer targets allow to get the projecting pictues with high spatial resolution that was about one micron. The explanation of such high spatial resolution is in laminar motion of ion flow. Threshold spatial sensitivity of proton radiography is estimated.

The status of present understanding of collisionless absorption of intense laser pulses out of linear resonance is discussed in Sec. 1. In Sec. 2 skin layer absorption is modelled in a test particle picture and it is shown that the energy gained by a single particle depends strongly on the injection phase relative to the laser field and that electron reflection is not localized at the target boundary. To provide for adequate absorption an energetic component must be present. The main achievement of this paper is contained in Sec. 3. We show that most of collective (collisionless) absorption is by anharmonic resonance of electron bunches in their own space charge field. In Sec. 4 basic considerations on electron acceleration by laser beams in vacuum are outlined and a summary of the main achievements by wake field acceleration in tenuous plasmas is presented.

It was shown that liquid gallium heated up to 270°C can be used as a target for 10 Hz femtosecond laser plasma highly stable x-ray source. The decreasing of hard x-ray yield during 50000 laser shots is less then 25% and can be easy compensated by additional focusing of objective or temperature tuning.

We have studied the high harmonic generation and attosecond pulse production in a plasma or gas under conditions when the single-atom response is affected by neighboring ions of the medium. We solve numerically the three dimensional Schroedinger equation for a single-electron atom in the combined fields of the parent ion, the neighboring particles and the laser, and average the results over different random positions of the particles using the Monte-Carlo method. We observe a change of the harmonic properties due to a random variation of the harmonic phase induced by the field of the medium, when the medium density exceeds a certain transition density. The transition density is found to depend on the harmonic order, but it is almost independent of the fundamental intensity. It also differs for the shorter and longer quantum paths. The latter effect leads to a narrowing ofthe harmonic lines and a shortening of the attosecond pulses generated using a group of harmonics. The effect of the medium might be important even for much lower densities in the case of XUV generation using radiation in the micron wavelength regime.

A simple semiclassical approach has been proposed recently to describe the strong-field phenomena in molecules beyond the Born-Oppenheimer approximation. We use this approach to study high harmonic generation in molecules with moving nuclei. We study how the competition between rotational, vibrational, and electronic excitation, as well as dissociation and ionization, influence the harmonic spectra from light molecules. The possibilities to use the nuclear motion to control the spectral shape and temporal confinement of harmonic radiation are discussed.

A Multi-Petawatt High-Engergy laser coupled to the LIL (MPWHE-LIL) is under construction in the Aquitaine Region in France. This facility will be open to academic community. Nd:glass laser chain and Chirped Pulse Amplification (CPA)technique makes possible to deliver high energy. Optical Parametric Chirped Pulse Amplification (OPCPA) for pre-amplification and new compression scheme will be implemented. The MPWHE-LIL will deliver output energy of 3.6 kJ in 500 fs on target corresponding to more than 7 PW. The PW laser facility linked to UV-60kJ-ns beam from LIL, will give new scientific research opportunities.

At present there are two main problems in the ICF program. The first one is to define laser facility parameters and target design which can provide thermonuclear ignition. The second problem is to create such a facility with compact architecture and reasonable cost. The facilities existing at the national laboratories over the world cannot answer the first question. The reason is a too low level of laser energy. These facilities provide investigators with experimental data that serve for verification of the basic theoretical models for numerical simulations of thermonuclear targets. Figure 1 presents results of the study of the direct drive target compression using one-dimensional code SNDP [1,2]. These calculations show that a polymeric target could be ignited by the tailored laser pulse ofthe energy of 500 kJ at the wavelength of 0.35 μm. Gain factor in calculations is G=10 at the initial target radius of 1.5 mm, polymeric shell thickness of 33 μm and DT-ice thickness of 23μm. The essential fact is that volumetric compression necessary to achieve ignition is about 10⁴.

Two new schemes of chirped pulse amplification (CPA) lasers are demonstrated, which are specially designed to overcome some of the bottlenecks of the current Ti:sapphire lasers. In particular, the issues of temporal contrast and spectral narrowing of the pulse during amplification are considered. The proposed schemes are double CPA (DCPA) and the negatively- and positively-chirped pulse amplification (NPCPA). Both of them are an extension of the conventional CPA scheme to Ti:sapphire lasers of petawatt and higher power levels. The double DCPA laser includes two CPA stages with intermediate non-linear temporal pulse filtering. The method reduces substantially the background, including amplified spontaneous emission as well as pre- and post-pulses. The former is demonstrated by achieving a temporal contrast of at least 10¹⁰ with pulses of 2O mJ output energy and 50fs pulse duration. The NPCPA laser scheme implements a combination of negatively- and positively-chirped pulse amplification. This method of amplification suppresses gain related spectral narrowing, typical to CPA lasers, thus supporting pulse spectrum much broader than a conventional CPA. With a NPCPA Ti:sapphire laser we have achieved laser pulses of 50 nm spectral width and 150 mJ energy without any additional spectral correction. The scheme appears as an easy and reliable solution to preserve spectral bandwidth in Ti:Sapphire lasers, especially at high power levels up to the Petawatt regime.

Optical parametric amplification of broad-band pulses in large-aperture KD*P (DKDP) crystals has been studied in experiment. A procedure of adjusting the amplifier for single-shot operation of a pump laser has been developed. Peak power of compressed signal radiation (pulse duration 45 fs at energy 9 J) was 200 TW. This considerably exceeds the record level obtained so far in lasers based on optical parametric chirp pulse amplification.

We have observed in experiment the effect of strong dispersion of thermally induced depolarization in ceramics, which we earlier predicted theoretically. This effect is specific to ceramics only and has no analogs either in glasses or in single crystals. The consequence of this effect is that both the polarized and depolarized radiations always have a small-scale intensity modulation with a characteristic transverse size of the order of a grain size. Depth of this modulation increases with increasing heat release power and decreasing ratio of sample length to grain length.

LIGO (Laser Interferometer Gravitational-wave Observatory) is a trio of sensitive Michelson interferometers designed to detect extremely relativistic astrophysical processes by the ripples they produce in spacetime. For best sensitivity, these interferometers are kilometers long, contain nearly unstable cavities, and operate at high optical power, making them uniquely susceptible to thermal aberrations and radiation-pressure-derived instabilities. We describe the LIGO interferometers, and their high power lasers and input optics, and described how thermal aberrations have been successfully controlled using adaptive corrective heating. The Advanced LIGO detectors, an upgrade to LIGO planned for installation in the year 2010, will operate with even higher optical power. We detail the additional challenges in construction and thermal compensation for Advanced LIGO, and detail how radiation-pressure derived instabilities influence the design, operation, and sensitivity ofAdvanced LIGO.

In this paper we describe adaptive compensation of thermal lens in a Faraday isolator using a DKDP crystal. Thermal lens measurements were made with a modification of the Hartmann sensor -a 2D scanning Hartmann sensor. This device proved to be convenient and adequate for thermal lens measurements. Our experiments showed that a DKDP crystal does not influence the isolation ratio of Faraday isolator and efficiently compensates thermal lens. The negative effect produced by the thermal lens is estimated as the quantity of power losses from the original beam mode (Gaussian mode in our experiments). Without compensation the losses were measured to be about 5% for 50W of radiation power and were compensated to less than 1% by a negative thermal lens in a few millimeters thick DKDP crystal. Numerical extrapolation of experimental data to a higher powers range showed that for the power up to 200W, power losses can be made less than 5%.

In this work the application of Thermal Lens (TL) and Z-scan techniques to the study of thermo-optical and spectroscopic properties of Solid-State Laser Materials (SSLM) is described. The theorectical basis for quantitative measurements is discussed together with the advantages and limitations of the methods. We discuss applications of the TL technique to the study of thermal diffusivity, the temperature coefficient of optical path length (ds/dT), heat efficiency (the fraction of absorbed energy converted into heat) and the florescence quantum efficiency (η). Several approaches to determine η and the study Energy Transfer mechanisms in SSL materials doped with Nd³⁺, Y³⁺, Tm³⁺ and Er³⁺ are presented. The electronic contribution to the nonlinearity was investigated using the Z-scan technique in the time-resolved mode. The measurements were performed spectroscopically allowing the determination of the lineshapes of real and imaginary parts of the nonlinear refractive index (n₂) in resonance with laser transitions. The results are interpreted considering resonant and nonresonant contribution to n₂. We also compare the magnitude of electronic and thermal refractive index changes in SSL.

Mechanisms of refractive index changes in intensively pumped Yb-doped crystals (Yb:YAG, Yb:KYW, Yb:KGW, and Yb:YVO₄) were studied using both a polarization interferometer and the transient grating method at the testing wavelength of 633 nm. The electronic component strongly predominated over thermal index changes in the experimental conditions. The polarizability difference of the excited and unexcited Yb³⁺ ions (of the ground state and the excited metastable level ²F_7/2) was determined.

In this paper we review of our recent results in different aspects of fundamentals and applications of continuous wave (CW) stimulated Brillouin scattering (SBS) in optical fibers. We show that with increasing pump strength the temporally stable component emerges from initially stochastic Stokes emission slightly above the SBS threshold to become the dominant contribution at higher pump powers. The effect is independent of fiber characteristics. These findings are shown to be a manifestation of spectral self-phase conjugation in SBS providing first experimental evidence of this phenomenon in optics. In application domain a considerable power scaling for the MOPA system comprising a CW Nd:YAG master oscillator, a quasi-CW diode pumped Nd:YAG slab power amplifier and a fiber based SBS phase conjugate mirror is reported. A twelve-pass amplifier configuration is employed to achieve high gain in a small length slab amplifier. Residual and pump induced optical inhomogenities in the slab are corrected by the fibre phase conjugator to achieve diffraction limited output beam quality. 300 W quasi-CW output power and a maximal attainable gain of ~150 for the system are obtained. We also considered theoretically SBS in high power fiber amplifiers. We show that for an end-pumped rare-earth doped double-clad fiber the inhomogeneous distribution of temperature, which is caused by absorption of pump radiation, may result in total suppression of SBS even for output powers of CW single-frequency radiation well above 1000 W.

The group velocity of an optical pulse in an optical amplifier in the presence of population oscillations was studied both theoretically and experimentally. The superluminal propagation of harmonically modulated light was measured in a diode-pumped Nd:YVO₄ crystal. The resonant phase shift was found to have resonance at the frequency of population relaxation.

We introduce several types of terahertz- (THz) wave parametric sources. THz-waves can be generated by optical parametric processes based on laser light scattering from the polariton mode of nonlinear crystals. Using parametric oscillation of MgO-doped LiNbO₃ crystal pumped by a nanosecond Q-switched Nd:YAG laser, we have realized broadband sources as well as coherent (narrow band) and widely tunable THz-wave sources. The THz-wave Parametric Generator (TPG) generates a broadband THz wave using a simple configuration; the THz-wave Parametric Oscillator (TPO) and the injection seeded THz-wave Parametric Generator (is-TPG) are two sources that generate coherent, widely tunable THz radiation by suitably controlling the idler wave. We report the characteristics of the oscillation and the radiation including linewidth and tunability. Further, we show the recent progress about these THz-wave parametric sources. We developed two new kinds of TPG by using compact pump sources. One TPG includes a flash-lamp-pumped multimode Nd:YAG laser with a top-hat beam profile, that allows generating high energy, broadband THz waves. Fitting in a space as small as 12 cm × 22 cm (including the pump source) this TPG outputs more than 100 pJ/pulse, which is about 100 times higher than the best results previously reported for TPG. The other has a potential to be a narrow-linewidth injection-seeded TPG, based on an laser-diode-pumped single-mode microchip Nd:YAG laser. The pump laser linewidth is below 0.009 nm and its size is 105×30×32 mm³. This allowed us to achieve a narrow-linewidth compact injection-seeded terahertz-wave parametric generator.

Zinc oxide is a promising material for creation of novel ultraviolet light sources. In this work we study random laser action in a thin ZnO nanocluster film under two-photon pumping. The results are compared with the case of single-photon pumping. A theoretical model is developed, which shows the effect of boundary conditions on lasing in the film. Measurement results of nonlinear transmission are presented and compared with classical two-photon absorption model.

We report the experimental study of the ultra-fast modification of the dielectric function of pure water by an intense femtosecond laser pulse. Using a time-resolved optical interferometric technique, we measured the variation of the phase shift, which is proportional to the modification of the real part of the refractive index, as well as the variation of the fringes contrast, proportional to the modification of the absorption coefficient. We first observe a positive phase shift due to Kerr effect and immediately followed a negative one. After 200 fs, the phase shift becomes positive and remains so for at least 3 ps. Using the simple Drude - Lorentz model, we interpret this evolution as the result of electron self-trapping.

The interaction of intense femtosecond laser pulse with model samples containing gold nanoparticales embedded in dielectrics is studied in order to understand the role played by nanodefects in optical breakdown of dielectrics. A theoretical study of the conduction electrons dynamics in the laser field predicts an efficient injection of carriers from the metallic inclusion to the conuction band of the dielectric, which leads to a strong local increase of the optical absorption in the initially transparent matrix. This prediction is tested experimentally by using time -resolved spectral interferometry to measure excitation densities as a function of the laser intensity in silica and samples doped with gold nanoparticles, which are compared with similar measurements in pure silica.

A general model of a typical adaptive optics system is described in the report. The model includes all main elements of a real system: the path of a beam propagating in the atmosphere, wavefront sensor, and adaptive mirror with continuous surface. Solution to the some of adaptive optics problems was obtained with the model. These results were included in the report as illustration ofthe model utility.

We demonstrate how closed-loop adaptive optical system can be used to obtain a good focused beam. The optimal correction of the high-power beam aberrations can be found by use of genetic and hill-climbing algorithms. A bimorph mirror is used as a wavefront corrector and CCD camera at the focal plane of the lens is a sensor. This adaptive system can correct for the low-order slow changing aberrations without any measurements of the wavefront.

We present experimental and numerical studies of pulse propagation in continuous and periodically modulated nonlinear waveguides, made of Silica glass. When intense femtosecond pulses are passed through this χ₃ material, a positive Kerr nonlinearity is formed. The unique characteristic of glass is accessibility to all domains of possible temporal dispersion (normal, zero and anomalous) in the spectral range of currently available femtosecond pulse sources. In particular, the anomalous dispersion regime enables simultaneous self-focusing in space (X) and time (T), yielding complex dynamics of the beam involving several mechanisms that couple between the X and T dimensions. We show that under certain circumstances, the combination of these mechanisms can lead to simultaneous spatial and spectral filtering in the continuous sample as well as steering of the point of break-up, and beam trapping in the periodic sample, using near-field microscopy and conventional spectroscopy.

Dissipative localized structures, also known as cavity solitons, arise in the transverse plane of several nonlinear optical devices. We present two general mechanisms for their formation and some scenarios for their instability. In situations of coexistence of a homogeneous and a pattern state, we characterize excitable behavior mediated by localized structures. In this scenario, excitability emerges directly from the spatial dependence since it is absent in the purely temporal dynamics. In situations of coexistence of two homogeneous states, we discuss localized structures either due to the interaction of front tails (dark ring cavity solitons) or due to a balance between curvature effects and modulational instabilities of front solutions (stable droplets).

We consider the process of low-power light scattering by optical solitons in a slab waveguide with homogeneous and inhomogeneous refractive index core. We observe resonant reflection (Fano resonance) as well as resonant transmission of light by optical solitons at certain incident angles. The resonance position can be controlled experimentally by changing the soliton intensity and the relative frequency detuning between the soliton and the probe light beams.

We consider an experimental setup, modelling the FitzHugh-Nagumo equation without recovery term and composed of a 1D nonlinear electrical network made up of discrete bistable cells, resistively coupled. In the first place, we study the propagation of topological fronts in the continuum limit, then in more discrete case. We propose to apply these results to the domain of signal processing. We show that erosion and dilation of a binary signal, can be obtained. Finally, we extend the study to 2D lattices and show that it can be of great interest in image processing techniques.

It was found in experiments that characteristics of spatio-temporal chaos of topological defects arising in a tetragonal spatially periodic structure on the surface of a vertically oscillating liquid layer (Faraday ripples) may be controlled by periodically modulating the frequency of the oscillations. It was revealed that such a control is possible because of changes in the motion and interaction of individual topological defects.

Spatio-temporal dynamics of waves propagating in resonators in opposite directions is investigated for media with anomalous dispersion. Numerical experiments demonstrated that stationary sources may exist during interaction of the counterpropagating waves in media, where group and phase velocities have opposite signs. We have found regimes when additional spatial periods referred to as "phase slipping" are formed periodically in time in the counterpropagating waves against the background of source.

We apply methods based on nonlinear dynamics to several problems in biosignals analysis. We concentrate mainly on EEG-signals, but the methods are applicable also to other biosignals used in medical diagnosis. In particular, we use two methods - Higuchi fractal dimension method and our own symbolic dynamics method. Both methods measure signal complexity and may be applied as medical diagnostic tools for characterizing changes induced by different pathologies, for computerized classification of sleep stages, for quantitative assessing of influence of electromagnetic fields, of photo-therapy, of administered drugs, of the depth of anaesthesia.

Current conceptualisations of brain function implicate synchronization of neuronal activity as an important factor determining brain activity and its relation to behaviour. Many obstacles arise in the assessment of synchrony in neuronal recordings, especially those derived from electroencephalography (EEG) or magnetoencephalography (MEG), as the experimental time series thus obtained are nonstationary, and are characterised by important noise and fluctuations. We studied phase synchronization during generalized epileptiform activity, in attempts to address the apparent "hypersynchrony" seen in visual inspection of EEG/MEG recordings, and to determine possible differences depending upon the specific epileptic syndrome. To this end, we used MEG recordings from epileptic patients with generalized seizures, as well as control subjects, in order to address the extent of phase synchronization (phase locking) in local (neighbouring) and distant cortical areas, and to explore the time scales (frequencies) at which synchrony occurs. All seizures were characterised by enhanced local - although not necessarily stable - synchrony as compared with distant phase locking. There were fluctuations in the synchrony between specific cortical areas that varied from seizure to seizure in the same patient, but in most of the seizures studied there was a constant pattern in the dynamics of synchronization indicating that seizures proceed by a recruitment of neighbouring neuronal networks in the neocortex. Taken together, these data indicate that the concept of widespread "hypersynchronous" activity during generalized seizures may be misleading and valid only for specific neuronal ensembles and circumstances.

A new third-order solution for bichromatic bi-directional water waves in finite depth is presented. Details of the complete solution can be found in a companion paper. In the present paper, we give the form of the solution, and focus the attention on the nonlinear dispersion relation, which incorporates the effect of an ambient current, with the option of specifying zero net volume flux. The dispersion relation is verified against classical expressions from the literature, and limitations and problems with these classical expressions are identified. Finally, third-order resonance curves for unidirectional and short-crested finite amplitude waves and their three-dimensional perturbations are calculated. Dominant class I and class II wave instabilities (determined by classical stability analyses) are shown to be located close to these resonance curves.

The maximum electric fields typically measured in thunderclouds are reviewed with a view toward lightning initiation mechanism. Initial breakdown processes in both cloud-to-ground and cloud discharges are considered, and characteristics of associated electric and magnetic field pulses are given. Narrow bipolar pulses attributed to the so-called compact intracloud discharges are characterized. Proposed mechanisms of lightning initiation in thunderclouds are presented and discussed.

The equatorial waveguide in the vicinity of the Earth equator in the ocean and atmosphere is a semi-transparent one since the systems contain both the wave modes trapped in this waveguide and the free modes freely propagating across the equator. Non-linear interactions between the trapped and free Rossby waves are studied using two-layer model of the ocean, or the atmosphere. It is shown that the free wave can resonantly excite a pair of the trapped ones with amplitudes much greater than its proper amplitude. In turn, the interaction between the excited trapped modes results in a non-linear scattering of the free wave on the wave guide. The envelopes of the baroclinic waves obey Gmzburg-Landau type equations and exhibit non-linear saturation and formation of characteristic "domain wall" and "dark soliton" defects.