An overview of theoretical and experimental results on first-overtone (FO) CO laser obtained for the period of approximately 20 years is presented. A special attention is paid to the quite new results jointly obtained at the Lebedev Physics Institute (Russia), TRINITI (Russia) and Air Force Research Lab (USA). Efficient multiwavelength pulsed FO CO lasing with output efficiency up to 11% and specific output energy up to 50 J/l Amagat is observed within the spectra range of 2.5 - 4.1 micrometers corresponding to the vibrational transitions V + 2 yields V from 6 yields 4 up to 37 yields 35. Single-line frequency tunable FO CO lasing on the wavelengths from approximately 2.7 up to 4.2 micrometers with the maximum output efficiency up to approximately 1% is also observed. Multiline and single-line lasing on 430 ro- vibrational lines is obtained. Theoretical calculations based on the experimental data predict that multiline FO CO laser efficiency can be enhanced up to 20%. FO CO laser with output efficiency and specific output energy comparable with those of CO₂ laser and spectral region of lasing covering that of HF and DF lasers can be considered as a potential source of high-power IR radiation for frequency selective laser ablation.

This paper reviews the Japanese activity of the laser application in space. The features of lunar energy network system) are shown. The design target is to get an innovative energy transfer system using laser. This project consists of four phases; fundamental technology development, high power laser, energy transfer from space and lunar energy park concept. The recent results of LOTV (Laser Orbit Transfer Vehicle concept) by advanced space propulsion investigation committee are shown. The target is to realize the low cost space transportation system using laser ablation. Due to this LOTV network, the transportation cost can be reduced to less than one twenty-fifth of the current cost.

This paper will discuss the laser conditions for producing efficient thrust on debris. In order to obtain high thrust for given laser energies, thermal energies dissipated into laser ablation and debris bulk have to be minimized. It was suggested that this minimization can be achieved by using ultrashort pulse interactions where laser pulse duration is too short for thermal conduction to take place deep into debris. The results of momentum coupling coefficients measured with ps and ns pulse duration will be presented.

Generally, the ABCD law for the complex parameter q of the TEM₀₀ Gaussian beam is not valid for high-order modes. The focussing of a Gaussian beam means imaging of the beam waist. Under this condition, the spot size in the virtual part of the parameter q should be substituted by the Rayleigh range of a certain resonator. Thus, the ABCD law of the parameter q can be used for the high-order modes or their superposition. The calculation shows that when the focus lens moves along the optical axis, the change of focal length and focal depth is not relative to the orders of the Gaussian beam modes for an ideal resonator without distortion. For high power gas lasers or solid lasers, the distortion of the resonator components can change the resonator parameters, and also the Rayleigh range. In this situation, for a certain resonator, its focus characteristics, including the change of focal length and focal depth, are relative to the output power. It's important for the applications of the laser space craft vehicle and flying optical processing.

On the basis of developed physical and mathematical model the mechanisms and tendencies of nonstationary focusing lens formation in the beam channel under propagating of a pulse laser radiation in resonance absorbing gaseous medium and in the atmosphere were studied. The dynamics of self- interactions and change in refractive index under propagating of CO₂-, HF-laser radiation under resonance absorption by H₂O molecules in moist atmosphere were considered. The new characteristics of change in refractive index of molecular gas due to excitation of rotation degree of freedom when spectral lines of absorbing transitions were overlapped (low troposphere) have been estimated.

The application of the pulsed laser deposition technology requires a better understanding of the material evaporation mechanisms, of the gas and plasma plume formation, of its expansion and of its deposition onto the substrate. Experimental and numerical studies revealed the importance of the gas-phase collisions on the formation of a peaked angular distribution of particles described by cos^p (theta) . Numerical calculations based on a Monte Carlo simulation of the desorption process were performed showing that the focusing effect of the angular distribution of particles increases with the number of collisions in the plume. This paper presents a 3D Monte Carlo simulation performed in vacuum and ambient gas in order to investigate the angular distribution of particles. Besides elastic collisions, the recombination-dissociation reactions proceed in the flow.

The regime of skin-effect interaction was proposed as the mechanism of ion acceleration, and the range of validity of the skin-effect mode was established. The results are illustrated on laser evaporation of graphite with Nd:YAG laser (1.064 micrometers ), KrF laser (248 nm) and ArF laser (193 nm). The UV lasers the interaction has a bi-modal nature: the interaction may proceed initially in the skin effect regime resulting in a few high-energy ions, until the hydrodynamics expansion begins at the later stage. The skin- effect interaction at the initial stage of UV laser pulse gives the first, to our knowledge, explanation for acceleration of ions up to approximately 100 eV at the low laser intensities 10⁸ - 10⁹ W/cm² and ns-range pulse duration.

We present the results of our investigations on the surface damage/ablation threshold and processing morphology for sapphire after single and multiple laser pulse irradiation at 800 nm in the picosecond and sub-picosecond duration range. The threshold for ablation drops sharply for multiple laser shot irradiation, due to material dependent incubation effects. We observe two distinctively etch phases: `gentle' and `strong'. Monitoring the mechanism and dynamics of the ion expulsion using combination of time-of-flight mass spectroscopy and femtosecond pump-probe technique, we identified Coulomb explosion as the dominant mechanism for ion emission in the `gentle' etch phase on a time scale of 1 ps. The momenta of the emitted ions are equal under these conditions. After sufficient incubation the damage threshold decreases and the ablation is shifted towards the `strong' phase. The velocity distributions shift to lower values, evidence for `phase explosion' is seen and the ions tend here to equal kinetic energies.

To investigate the optimum laser condition for transmyocardial laser revascularization, ablation characteristics of porcine myocardium tissue have been measured in vitro in the spectral region from UV to near IR (230 - 1064 nm). In experiments, a nanosecond, tunable optical parametric oscillator and a Q-switched Nd:YAG laser (the fundamental band and its 2nd, 3rd, and 4th harmonics) were used for ablation. Measurement of ablation hole depth and diameter, and histological analysis with an optical microscopy were performed. It was found that the ablation efficiency increased and the thermal damage threshold of the tissue decreased with decreasing the wavelength. However, at relatively high fluences (approximately 5 J/cm²), unexpected deep ablation was obtained with a 1064-nm laser light, where not thermal but acoustic damage was observed near the walls of the ablation holes. Because intense laser- induced plasma was observed in this case, the shock wave would contribute to removal and/or damage of the tissue.

Initial results for a new laser based procedure to make ceramic coatings on ferrous metals are described. The procedure is performed at ambient temperature and pressure to avoid the use of a vacuum chamber. An Nd:YAG laser beam (1064 nm) coupled to a mechanical scanner is used to produce coating. The coating precursor materials are sprayed onto the metal sample before the laser-generated heat treatment. A jet of argon gas is used to avoid oxidation of the metallic substrate. The principal ingredients of the coating precursor are sodium tetraborate and a natural clay mineral. The product is a glassy ceramic. The product has been characterized by scanning electron microscopy, optical microscopy and hardness and adhesion tests. The results indicate that the surface material is a micrometric, single layer which adheres to the metal surface.

The excimer laser nitriding process reported is developed to enhance mechanical properties of aluminum alloys. An excimer laser beam is focused onto the alloy surface in a cell containing 1 bar nitrogen gas. A vapor plasma is expanding from the surface and shock wave dissociates and ionizes nitrogen. Then vapor and gas species stay several hundreds of microsecond(s) in contact with the surface and nitrogen diffuses in depth. Thus it is necessary to work with a sufficient laser fluence to create the plasma, but this fluence must be limited to prevent from a too large laser-induced surface roughness. The nitrogen concentration profiles are determined from RBS and SEM coupled to EDX probe analysis. The roughness and surface state are studied to define the best irradiation conditions corresponding to a smooth and homogeneous surface.

Experiments on laser-rock-fluid interactions have been carried out by using pulsed CO and CO₂ lasers which irradiated rocks typical for oil field: sandstone, limestone, shale and granite. Energy fluence and laser intensity on rock surface were up to 1.0 kJ/cm² and 10⁷ W/cm², respectively. The dependencies of specific energy consumption (i.e. energy per volume needed for rock excavation) on energy fluence, the number of pulses, saturated fluid, rock material and irradiation conditions have been obtained for various rock samples. The dependencies of momentum transferred to the rock on energy fluence for dry rocks and rocks with surface saturated by water or mineral oil have been measured. High-speed photography procedure has been used for analyzing laser plasma plume formation on a rock surface. Infrared spectra of reflectivity and absorption of rocks before and after irradiation have been measured.

In former laser removal experiments of 20 micrometers anodized aluminum layer in air, we found at 1064 nm and 532 nm wavelength a mechanical breaking of the film. In this paper, we present our photoacoustic measurement with a piezoceramic. The experiments were carried out with the aim to investigate the ablation mechanism in dependence on the energy density for 1064 nm and 248 nm laser wavelength. The main result is a strong change of the acoustic signal and shock wave velocity for the first and second laser pulse in amplitude and temporal behavior. To verify this, the experiments are repeated with an ultra fast camera, which allows to record the shock wave in the surrounding air and to visualize the breaking particles of 100 micrometers size. We found a changing of the ablation mechanism with the pulse number.

There are number of methods to detect formation of nanoclusters produced by ns-laser ablation, during the expansion of vapor: optical time of flight (TOF), photoluminescence, Rayleigh scattering, etc. For typical conditions of expansion into the vacuum, formation of nanoclusters is completed at the moment of quenching (when the collisions are terminated) which occur after a few microsecond(s) . During the expansion of vapor into the background gas, the condensation process is terminated due to mixing of vapor with background gas. The characteristic mixing time can be estimate from the characteristic increment of the Rayleigh-Taylor instability; this time is around a few microsecond(s) for typical pressures of background gas. Fast formation of overcritical clusters is confirmed by TOF experiments.

Time-of-flight mass spectrometry has been used to probe neutral and positive ionic species produced by laser ablation of a graphite target at 266, 355, 532, and 1064 nm. The arrival time distributions of major carbon species (C₃, C⁺, and C₃⁺) at a deposition substrate, 45 mm from a target, were fitted with a thermal Maxwell-Boltzmann (MB) distribution or a shifted MB distribution with a narrower width. The characteristics of laser ablation with shorter-wavelength laser light are the formation of atomic carbon species with high kinetic energies and the promotion of their ionization. We discuss the generation process of C⁺ and the formation of a tetrahedral amorphous carbon film with a high fraction of sp³ bonded carbon atoms.

The distribution function of free electron gas in metals and insulators for the case of irradiation with a laser pulse of moderate intensity is calculated. A microscopical description on the basis of time-dependent Boltzmann equations is used. For the metal, photon absorption by free electrons, electron-electron collisions and electron-phonon collisions are considered each by a corresponding collision integral. In dielectrics, additional terms for two ionization processes (strong-electric-field ionization and impact ionization) are included. We choose aluminum as a representative of a metal and SiO₂ for modeling an insulator. The results show the sequence of excitation and relaxation of the electron gas. Due to photon absorption the occupation number of electron gas differs significantly from Fermi distribution. For metals we show that electron thermalization to a Fermi distribution occurs rapidly within less than hundred femtosecond after irradiation ended. For dielectrics we find that for pulses shorter than about 100 fs, impact ionization is negligible in comparison with strong-electric-field ionization. We shown that in this case impact ionization can not be described by simple rate equations.

This paper describes a performance of 100 J-class KrF laser system `GARPUN' intended for target irradiation experiments by 100 ns pulses. A controllable space-time distribution in a focal spot reaching 5*10¹² W/cm² produced megabar ablation pressure, which irradiated conical shock wave in solids. It propagated in a quasi-steady manner together with an ablation front that resulted in anomalous high penetration rate of laser radiation throughout the matter. Long-time sample loading together with strong tangential shear flow of compressed layers produced favorable conditions for pressure-induced transformation of the pyrolytic graphite into a diamond-like phase by martensitic mechanism.

Transient processes, capable of improving the coupling efficiency of specific targets exposed to laser radiation, are subjects of numerous investigations, both concerning fundamental research and technological applications. The present paper refers to recent studies, related to laser- target effects on optronic materials, carried out at the German-French Research Institute of Saint-Louis. The availability of a unique repetitively pulsed CO₂-laser with pulse energies up to 150 J and repetition rates up to 100 pps, allowed experiments to be performed in a large range of fluences up to hundreds of J/cm² with pulse durations of several microseconds, corresponding to peak power densities up to more than one hundred MW/cm². Ablation processes were studied under various irradiation conditions (ratios of peak power densities to average power densities of more than 10⁴:1. The flexibility of the laser allowed to optimize energy transfer rates according to specific material properties, by varying pulse energies, pulse shapes, repetition rates and lengths of pulse trains. Experiments investigations were aimed at improving the knowledge both on surface effects in case of optically opaque materials and on volume processes for materials which are transparent or translucent for the incident laser wavelength. Special high-speed diagnostic techniques have been used and are discussed. Particular interest was to analyze nonlinear processes at peak power densities close to and above surface plasma ignition thresholds.

The words of this title may at first seem incompatible. We review a range of experiments where dynamic structures have been created. We show that it is possible to construct gas and plasma shapes using colliding shocks cigars and cylinders, curved waveguides, and even waveguides with rectangular cross sections. The colliding plasma lens/isolator lead to the colliding shock lens. We now suggest that colliding shock waveguides could find application in laser acceleration and soft X ray schemes. Colliding shock waveguides can be as long as necessary unlike gas jets.

In the present paper, a model is presented aiming to provide a physical basis for the theoretical estimation of both the plasma expansion dynamics following laser irradiation and the shock wave propagation into the treated material with a specific consideration of its constitutional properties (i.e. taking into account its real mechanical and E.O.S. parameters). Although initially limited to a 1D description, the proposed model aims to overcome the difficulties existing for an accurate theoretical estimation of the process due to the treated material behavior not directly amenable to analytical solutions and sometimes modeled through empirical approaches, and to provide a detailed treatment of the plasma behavior (i.e. ionization, breakdown, etc.). Additionally, and as a direct consequence of its analytical-numerical character, the model can provide a fully time dependent representation of the processes, including the laser pulse temporal profile, what is a real advance over previous theoretical studies.

Laser ablation of metals by femto- and picosecond pulses is analytically and numerically studied within the framework of different models for the ablated material. Within the plasma model ablation is initiated by high-power thermal and hydrodynamic waves which propagate into the irradiated material. Analytical expressions for the thermal ablation and for the ablation by the shock wave are obtained. Numerical simulations with the computer code RAPID are in a good agreement with analytical results.

Sol-gel coatings are finding increased applications in fields as diverse as traditional optical coatings (eg anti- and high-reflectance) and microelectronic devices. Following the deposition process heat treatment is required to densify or crystallize the films. The disadvantage of conventional heat treatment, by hot plate or oven, is the limit on temperature and time set either by the thermal stability of the materials or by interactions between film and substrate. A study has been made of the use of excimer lasers for heat treatment of PZT (PbZrO₃:PbTiO₃) sol-gel films, the main application being pyroelectric detector arrays.

The new method of surface shaping for optical plastics have been investigated using UV laser ablation on acrylic resin (PMMA) or ultraviolet (UV)-cured-resin. The surface figuring of optical flat was obtained with direct beam irradiated ablation with ArF laser. The depth of the figuring was determined with the measurement of wave front distortion on the optics using interferogram. The ablated surface had small surface roughness. The miniature size optics such as the collimate lens for a diode laser (LD) was fabricated on flat PMMA at using the focusing beam irradiation. The wave front of LD light through the LD lens was measured with Shach-Hartmann sensor. The wave front control for LD light was carried out with the measured wave front data by the laser ablation on the LD lens surface. The collimated LD light has been obtained.

Refractive index modification in multimode optical fibers is first demonstrated using the plasma channeling excited by a high-intensity femtosecond (110 fs) Ti:sapphire laser ((lambda) _p equals 790 nm). The refractive index modification induced in a pure silica multimode step-index fiber with a 100/110 micrometers core/cladding diameter reaches the length of approximately 9 approximately 10 mm from the input face of the optical fiber with the diameters ranging from 5 to 8 micrometers at input intensities more than 1.5 X 10¹² W/cm². The graded refractive index profiles are fabricated to be a symmetric form from the center of a multimode fiber and a maximum value of refractive index change ((Delta) n) is measured to be 2.1 X 10^-2. According to the electron spin resonance spectroscopic measurement, it is found that the defect concentration of SiE' center increased significantly in the modified region in relation to that of the region without modification. The plasma self-channeling would induced the refractive index modification with the defects. The intensity profile of the output beam transmitted through the modified multimode fibers shows that the bulk modification produces a double cladding structure. The fabrication method of the double cladding structure in optical fibers can be a useful tool for a variety of applications such as mode converters and single-mode connectors in the fields of optical communication and optical sensor.

We describe the pulsed-laser deposition and characteristics of bismuth silicon oxide (BSO, Bi₁₂SiO₂₀) thin films and Ce-doped yttrium iron garnet (Ce:YIG, Ce_xY_3-xFe₅O₁₂) thin films for the electric and magnetic field sensor application. It was found that BSO films deposited on yttria-stabilized zirconia substrates heated at 400 degree(s)C or higher in an oxygen ambient gas were crystallized and the (310) plane was perpendicular to the substrate normal. The highly (310) oriented crystallized films were also deposited even on SiO₂ glass substrates. On the other hand, crystallized Ce:YIG films were obtained by the deposition in an Ar ambient gas, and not in an oxygen ambient gas, indicating the importance of the control of the charge state of Ce ions. The magneto-optic data of Ce:YIG films are also reported.

The single-shot pulse laser-induced damaging thresholds (LIDTs), an important laser-optical constant of GaN material, were determined to approximately 34 and 65 nJ upon the irradiation of 400 and 800 nm wavelengths, 150 fs duration laser pulse focused by 40X magnification of dry objective lens (a lateral size of focal spot roughly at 1.22 (lambda) /NA, where NA equals 0.65). The critical energy of sub- threshold pulses was determined for multi-shot optical damaging. The factors that influenced the LIDTs, optical properties of damaged GaN material and the possibility of laser processing of nitride devices were also discussed.

In this work, hydrogen-free DLC (Diamond Like Carbon) films have been deposited by XeCl (308 nm) pulsed laser ablation of pure graphite target with the base pressure of 1 X 10^-3 Pa at room temperature. The films obtained show remarkable properties similar to those of diamond. Raman spectroscopy analysis gave a broad Raman peak centered at 1523 cm^-1, which indicated that the film had an amorphous structure. FTIR analysis showed that the film head a transmittance above 90% from 2.5 micrometers to 25 micrometers without any absorption peak. Microhardness test showed that the film had a mechanical hardness up to 23 GPa. The film also had chemical inertness in various acids and organic solutions. The effects of experimental parameters on the film properties have been investigated. The optimum dc bias of -500 V and the threshold of laser (308 nm) power density of 3 X 10⁸ W/cm² which is necessary to obtain DLC film have been observed. The emission spectra of laser induced plasma have been obtained to study the ionization state of particles in laser-induced plasma. Considering the experimental results, we suggest that the energy and ionicity of the particles in laser-induced plasma should be important factors in the formation of DLC film. Particles with higher energy and higher ionicity will result in DLC films with excellent properties.

Electron trapping thin films CaS:Eu,Sm have been deposited by pulsed laser deposition in vacuum and simultaneous sulfur coevaporation. The films prepared by doubly rare-earth ions doped alkaline earth sulfides CaS have the characteristics of infrared upconversion and optical storage. Its phases and microstructures were identified by X-ray diffraction. The chemical composition of the films was determined by secondary ion mass spectrum. The studies on the optical properties of the films show they can convert the infrared light (800 - 1600 nm) to red light (approximately 672 nm). Infrared upconversion efficiency of CaS:Eu,Sm thin films with different thickness has been investigated by using the ultrashort infrared laser with different FWHM from microsecond(s) to ps. It is shown the upconversion efficiency of CaS:Eu,Sm thin films not only depends on the growth conditions and the post annealing process, but also has the `exhaustion' phenomenon. By means of measuring transmittance and spatial resolution of CaS:Eu,Sm thin films, the post annealing process was found to promote grain growth which could obviously improve upconversion efficiency of CaS:Eu,Sm thin films, even though it had negative influence on transmittance and spatial resolution of CaS:Eu,Sm thin films.

The Nd³⁺,Cr⁴⁺ co-doped GGG epitaxial thin films for self Q-switched waveguide laser has been fabricated by a two-target pulsed laser deposition method. Concentrations of Nd and Cr ion in Nd,Cr:GGG thin films are well adjusted by changing laser ablation fluences independently for Nd:GGG and Cr,Ca:GGG sintered targets. The structure of Nd,Cr:GGG thin films on YAG substrate shows a planar waveguide structure. It is confirmed that Nd³⁺ and Cr⁴⁺ ions are optically active as laser active ions and saturable absorbers around 1.06 micrometers .

There is considered formation and propagation of shock electromagnetic waves (SHEW) of visible spectral range as possible nonlinear optical phenomenon taking place at laser intensities characteristic of femtosecond laser interaction with transparent isotropic solids. Main regularities of SHEW formation are studied on the basis of 1D model of plane-wave propagation in isotropic dielectric with Kerr nonlinear optical response. Necessary conditions for formation of SHEW are obtained, in particular, threshold amplitude is estimated. There is presented a model for numerical simulation of SHEW formation and propagation influenced by dispersion of linear and nonlinear parts of refractive index. Using the simulation, we studied dynamics of SHEW formation on several first optical cycles of femtosecond laser pulse in transparent medium. Important observed features of SHEW of optical frequency are discussed. Obtained results are considered from the viewpoint of experiments on femtosecond laser interaction, in particular, laser damage and ablation.

A new model describing initiating of laser-induced damage and ablation of transparent materials by femtosecond pulses is presented and discussed. The model is based on properties of shock electromagnetic wave (SEW) formed at laser fluences characteristic of femtosecond interactions with transparent media. It is presented simplified description of SEW-induced processes within approach of classical mechanics and electrodynamics: atoms are described as dipoles with certain ionization energy interacting with SEW in potential valley formed by crystal lattice. There are shown possibilities of laser-induced ionization, point-defect formation and delocalization of crystal points by SEW. Obtained results and predictions of presented theoretical model are compared with experimental results and shown to be capable of explaining many observed regulations and peculiarities of femtosecond laser interactions with transparent media.

Using a pulsed CO₂ laser test-bed the properties of a laser lightcraft are investigated with respect to the thrust generating mechanisms. New optical diagnostics allow to measure the mechanical impulse imparted to the lightcraft by the laser pulse with high accuracy. Density variations are measured with a HeNe probe laser and identified as shock waves originating from the laser-induced air breakdown. Wire guided indoor vertical flights are demonstrated with accelerations of about 1 g. Satisfactory agreement between theory and experiment in the time-space domain is found using a strong explosion model. Prediction of mission requirements suggests that lightcrafts of 1 - 10 kg can be placed into orbit with current technology.

A mechanical impulse generated from laser-illuminated targets is investigated. Solid material, liquid-droplet and gas-puff targets are compared. A free expansion of explosion products and a forced trajectory change using a solid deflector are studied. A momentum transfer into the deflector was measured. The optimal combination of the target and deflector material is also examined. The deflector can be used in a hybrid laser propulsion system.

This paper describes the widows for laser transport in the atmosphere. The limitations caused mainly by Rayleigh scattering, stimulated Raman scattering, thermal blooming and so on are discussed here. From these limitations, windows for laser space applications are found out. The widows have enough area in laser parameters for several applications of laser in the space.

Ordinary Free-Electron Lasers (FEL's) can be found in successful operation in the spectral range from millimeters to ultraviolet wavelengths. However the operation of the common FEL's in the extreme ultraviolet and X-ray wavelength regimes faces certain adverse effects. Some of the main obstacles in the way of the realization of X-ray FEL are electron momentum spread and angular divergence. Another point to keep in mind is that ordinary FEL's work on the principle of `momentum population inversion.' By this one means that electrons with momenta larger than the resonant value contribute to the gain whereas electrons with momenta smaller than the resonant value contribute to the loss. Thus to ensure a net gain we need more electrons with momenta lying in the upper momentum domain than in the lower one, i.e. a `momentum population inversion.' To bypass these difficulties the idea of Lasing Without Inversion to achieve the successful operation of short-wavelength (extreme UV and X-ray) FEL's has been suggested by Scully and co-workers, which is an ingenious suggestion.

A new device used for tilted wavefront correction in adaptive optical cavity of CO₂ laser was presented. The devices are composed of a pair of microlens arrays in the Galilean telescopic geometry, which is designed using binary optical technology. The microlens arrays pair is used for correction one order aberration of the laser resonator. The analyses show that the device has the advantages.

There are presented results of theoretical investigation of self-depolarization effect resulting in variation of space distribution of polarization-ellipse parameters of high- power focused laser beam. In particular, both linear and circular initial polarizations are shown to change and turn into elliptic polarization with inhomogeneous distribution of polarization-ellipse parameters in focal area. Detailed results and calculations are presented for particular case of Gaussian beams of low order (TE₀₀, TE₀₁, TE₁₀ and TE₁₁). Bearing in mind obtained results, we discuss specific symmetry structure of self-depolarization effect allowing experimental checking of described phenomenon. Possibility of observing polarization dependence for damage and self-focusing thresholds in transparent materials is also considered.

The laser ablative figuring of the silica glass has been investigated experimentally. F2 laser and ArF excimer lasers are used as the laser light source for efficient ablation of silica glass material. The output beam of F2 and ArF laser were focused on to the surface of silica glass plate. The ablation rate were measured by a surface profilometer. The process on the surface was done by scanning in X-Y direction and a uniform ablation was observed. However the surface roughness was large as compared with the case of PMMA. The waveform of incident and transmitted laser light was measured by high speed photo-tubes to observe the time dependence of the absorption. The measured waveform indicates that the absorption was small at the leading edge of the laser pulse, and a strong ablation was induced at the latter part of laser pulse due to the excited state absorption. These phenomena are quite similar to both in F2 and ArF laser light. We have developed a simple model in which the instantaneous absorption is defined by the absorbed energy prior to the moment.

In this paper, we study the ablation behavior of polyetheretherketone polymer samples of 100 micrometers thickness when irradiated with visible laser light of 445 nm wavelength and 200 fs pulse width at 82 MHz pulse repetition rate. Etch depths corresponding to a particular laser power were measured at room temperature in open air, by counting the number of pulses required to perforate the sample. The laser focal spot size has been measured to be 0.03 mm² and was kept constant during the experiment. The surface topology of the polymer samples was investigated by Atomic Force Microscopy in the continuous contact mode. The obtained Atomic Force Microscopy images revealed no mechanical damage in the inner ablation crater wall. Plots of the ablation rate as a function of the laser power, both in a linear and a logarithmic scale, show a large deviation from a straight line. These findings along with the high average power and suggest that Multiphoton Dissociation is the predominant mechanism responsible for ablation in this experiment.

Wave of Change of Reflection and Conduction (WCRC) is the new reversible IR laser induced phenomenon, which can present solitonic mechanism of heat transfer in solids. WCRC appears in answer to one exciting pulse as the consequence of the following each another pulses (wave-components) and could be important in many laser-surface interactions. One of the most interesting features of WCRC is the observed discrete (in 2 times) change of the velocity of each following component. Present work gives one more experimental proof of this statement.

Different regimes of heat propagation in metals irradiated by subpicosecond laser pulses are studied on the basis of two-temperature diffusion model. New analytical solutions for the heat conduction equation, corresponding to the different temperature dependences of the electron thermal conductivity (formula available n paper), are found. It is shown that in case of a strong electron-lattice nonequilibrium, the heat penetration depth grows linearly with time, l_T varies direct as t, in opposite to the ordinary diffusionlike behavior, l_T varies direct as t^1/2. Moreover, the heat propagation velocity decreases with increasing laser fluence.

Thermal ablation of a metal surface by low-energy ultrashort laser pulses is considered theoretically. The temporal dynamics of the surface electron and lattice temperatures is studied within the framework of the two-temperature model and for different temperature dependencies of the characteristics of the metal (electron-relaxation time, heat capacity, thermal conductivity). The approximation of evaporation into a vacuum is used to determine the ablation depth. Analytical expressions for the ablation-threshold fluence F_th as well as the threshold values of the lattice temperature T_th(F_th) and the characteristic time of lattice temperature decay t_d(F_th) are obtained. This analytical description is in satisfactory agreement with particular numerical calculations.

Metal ablation taking into account hydrodynamics of a dense ablated material with ion temperature close to the critical one is considered. The extended two-temperature model taking into account the hydrodynamic plasma expansion, degeneracy of the electron gas, cold pressure of ions and interaction between the electron and ion subsystems (nonideality of the metal plasma) is developed. The new version of the RAPID code is used to perform calculations of ablation rates for several metal targets under conditions where the electron degeneracy is important.

We present results of single shot ablation experiments for a variety of metal samples (In, Al, Cu, Mo, W, Ti) using visible, nanosecond lasers at fluences up to approximately 10⁴ J cm^-2. At low fluences, usually less than 10² J cm^-2, small amounts of material were removed and removal was approximately uniform across the ablation crater. As the fluence increased above approximately 10² J cm^-2, substantially more material was removed and a conical pit developed in the center of the ablation crater. The appearance of these conical pits is consistent with material removed by phase explosion mechanisms. In this paper, this ablation phenomenon will be investigated by presenting the crater morphology as a function of fluence. Consequences for micromachining with visible, high repetition rate, nanosecond lasers will be discussed.

In the field of laser processing, it is important to monitor the beam quality such as the spatial- and time-distribution. A novel time-resolved imaging technique named FTOP (Femtosecond Time-resolved Optical Polarigraphy) for visualizing the ultrafast propagation dynamics of intense light pulses in a medium has been proposed and demonstrated. FTOP is used to monitor the 3D intensity distribution of the pump pulse focused in a medium by the probe pulse. Femtosecond snapshot images can be created with a high spatial resolution by imaging only the polarization components of the probe pulse; these polarization components change due to the instantaneous birefringence induced by the pump pulse in the medium. Ultrafast temporal changes in the 2D spatial distribution of the optical pulse intensity were clearly visualized in consecutive images by changing the delay between the pump and probe. We observe that several filaments appear and then come together before the vacuum focus due to nonlinear effects in air. We also prove that filamentation dynamics such as the formation position and the propagation behavior are complex and are strongly affected by the pump energy. The results collected clearly show that this method FTOP succeeds for the first time in directly visualizing the ultrafast dynamics of the self- modulated nonlinear propagation of light.

The ablation of metallic targets from Al, Cu, Nb, Ta has been studied under irradiation with 308 nm excimer XeCl laser. A single-wire electron Langmuir probe has been used in time-of-flight measurements of velocity of plume ions scatter. An investigation has been pursued of the dependence of ion current shape at the probe both on laser radiation flux in the range from 0.2 to 2 J/cm² and on the probe- to-target interval 23 to 113 nm. The velocities of ions scatter and their energy spectrum have been found with regard for the time of charge reaching the probe. The dynamics of energy spectrum of the plume ions has been studied. Multimode distributions of ions by scatter velocities have been revealed. The dependence of amplitudes of plume energy components on plume scatter time and on radiation flux has been investigated. The experimental time- of-flight data have been compared with Maxwell distribution by velocities, and a good agreement has been obtained. The threshold values of laser radiation flux have been determined for different modes of ion energy spectrum. Measurements have been taken of ion energy spectrum dependence upon scatter angle in the planes of orthogonal cross-sections of radiation focused spot on the target.

The absorption coefficient of silicon carbide plasmas was calculated at temperatures 5,000 K and 7,000 K, thickness from 0.0 mm to 7.5 mm, and pressure from 0.1 MPa to 1.0 MPa. The silicon carbide plasmas were assumed to be isothermal and in local thermodynamic equilibrium. The radiations included molecular bands, atomic lines, and continuum processes. It was found that silicon carbide ablation materials were effective in reducing the radiative flux.

The present study concerns the cleaning of materials using pulsed laser irradiation and it summarizes the most recent results obtained by the collaborative research of different European groups, within the framework of a European program for training and mobility of researchers. A series of pulsed lasers, which emit at various wavelengths (from UV to IR) with short duration of pulse (few nano-, pico- or femto- seconds), is used for the removal of metallic, ceramic and organic pollutants from contaminated solid surfaces of different natures. The scientific results obtained so far are focused on the laser cleaning of silicon wafers from sub-micrometer particles, the theoretical modeling of particles removal mechanism during dry laser cleaning, the removal of oxide layers from oxidized metals and alloys, as well as on the development of laser imaging as a diagnostic tool for the estimation of the efficiency of the proposed cleaning technique.

Ultrashort pulse laser ablation has been investigated for noble metal targets: gold, silver and copper by use of commercially available titanium sapphire laser and chirped pulse amplifier. The pulse duration dependence of the ultrashort laser ablation characteristics in sub-picosecond regime has been shown for micro-machining application purpose. The application of two different ablation regimes is proposed. In order to know the obvious laser requirements, experimental results as well as theoretical description are discussed.

The interaction of a Transversely Excited Atmospheric (TEA) CO₂ laser, pulse duration less than 100 ns, with austenitic stainless steel-AISI 316 and low thickness coating of titanium-nitride (TiN) deposited on steel are considered. The results have shown that steel as well as coating had been modified by TEA CO₂ laser beam. Compared to steel, TiN have shown more diverse changes in the bombarded area: crater like form, solidified boundary and droplets. On austenitic stainless steel--wall material of the vacuum vessel for a fusion reactor--different mechanism of surface degradation was found. A qualitative description comprises: surface corrugation, cracking and resolidified.

The mechanism of original crack formation on cylindrical shells with inner-pressure and under laser beam irradiation was investigated. By high-speed photographer, it was found that the crack formed at the central area of the surface irradiated by laser beam. The mechanism of crack formation was examined and discussed on the base of fractography, metallographic inspection and stress analysis. A simple criterion is setup to judge the original crack formation on the shell irradiated by laser beam. All information provided here is conductive to a better understanding of this problem.

Wavefront control is a key issue for developing different systems including: lasers (for various applications: isotopic separation, femto second solid state lasers, micro lithography...), imaging through turbulent media, ophthalmology... For these purposes, we have developed an adaptive optics system that can achieve wavefront very high precision correction. Subsystems will be described including the Wavefront Sensor (Shack-Hartman type), the Deformable Mirror (Bimorph type) and the Real Time Computer (PC type). Emphasis will be made on results that can be obtained using such a system: ultimate quality of the corrected wavefront, measurement range (important during capture phases).

Super high power laser systems have been pursued for the nuclear fusion research in these 30 years. GEKKO XII glass laser, LEKKO VII CO₂ laser, diode pumped solid state laser and PW cpa Nd glass laser are developed at Osaka University. They have proved to be very effective to the laser fusion research and also to the various applications in the wide fields of science and technology.