Semiconductor microcrystals (quantum dots) embedded in another crystal, glass matrix, colloidal solution, pores of zeolites or obtained through etching and ion implantation of a quantum well, ranging in dimensions between 1000 angstrom and 10 angstrom represent a system with challenging new physical properties. Although the simple interpretation in terms of confined particles is relatively old and successful, theorists struggle further for a better understanding of the many startling aspects. After reviewing the already classical representations, a peculiar attention is devoted to the strong interaction with the LO-phonons and its relation to possible surface states. Among the new topics the theory of quantum dots of indirect gap materials gets special attention.

The increasing interest in low-dimensional semiconductors structures is mainly motivated by the search for materials with tunable optical properties of evident technological relevance. The discovery of visible photoluminescence in porous silicon a few years ago has given momentum to this field of research. The quantum confinement effects of charge carriers in silicon nanostructures seem to be the clue to explain the optical properties of porous silicon. We study hydrogenated silicon quantum wires by first principles electronic structure calculations based on Density Functional Theory. We use the ab initio molecular dynamics (Car-Parrinello) method in order to determine the equilibrium geometry for these nanostructures. We compute the energy gap and the imaginary part of the dielectric function, which is related to the absorption coefficient, as a function of the diameter of the wire. We investigate also the possible relevance of the orientation of the wire in determining the optical and electronic properties of the porous material. Another promising direction for the search of optically active systems, obtained by self-limiting processes, is the growth of small semiconductor clusters in host materials. We show ab initio molecular dynamics results for III-V hydrogenated microclusters with the purpose of identifying stable and optically active structures suitable for inclusion into zeolite cages.

Simple preparation technique of nanoporous semiconductors by anodization has opened new ways to form and investigate quantum and surface effects in nanosized objects. It has also allowed technologists to extend the range of possible practical applications of well-known semiconducting materials. The bright visible photoluminescence (PL), in particular, of porous silicon has made this material very promising in the technology of light-emitting devices. The visible PL of nanosized silicon particles is supposed to be connected with a quantum size effect which transforms the indirect gap material into a direct gap one with a simultaneous strong increase of E_G. This type of band gap engineering approach may be useful when applied to indirect gap semiconductors, other than Si. Porous GaP is such an example; it was fabricated recently and has exhibited intense green PL and a broadened LO phonon Raman peak. In this communication we present detailed experimental results on Raman scattering (RS) spectra of porous GaP layers obtained by electrochemical anodization of (100) and (111) substrates in hydrofluoric acid solution at different current densities.

One promising approach for the development of silicon-based-light-emitting devices is the epitaxial growth of Si nanostructures. In this context, we propose the lattice matched system CaF₂/Si/CaF₂ as a prototype of a well controlled and ordered Si-based system with known microscopic structure. We present here a combined theoretical and experimental investigation of ultra-thin silicon (111) layers embedded in CaF₂. Our all electron calculation predicts the band gap opening and the presence of confined and interface states leading to a quasi-direct band gap. We have synthesized, by molecular beam epitaxy, Si/CaF₂ multilayers which efficiently photoluminesce at room temperature. The photoluminescence spectra show a strong resemblance to those of porous silicon. There is a critical dependence of the photoluminescence efficiency on the thickness of the Si layers and a blue shift for decreasing Si layers thickness. Our results allow us to conclude unambiguously that quantum confinement is a necessary condition for visible luminescence in our Si-based structures.

The effect of arc plasma jet treatment (APJT) on the porous silicon (PS) structure, surface, and photoluminescence (PL) has been studied. The investigation of PS FTIR absorption spectra indicate that the Ar/air APJT induces the decreasing of the concentration of Si-H_n, C-H_n, (n equals 1, 2, 3) bonds and corresponding increasing of Si-O_x bonds more than an order of magnitude. This transformation of surface conditions resulted in not enough large decreasing (about 2 - 3 times maximum) of the PL intensity with red shifting of the peak from 665 nm to 700 nm. These results suggest that the change of PL occurs as a result of APJT induced replacement of the Si-H_n bonds with Si-O_x bonds. We have analyzed the effect of plasma conditions on the PL characteristics. The luminescence mechanism is also discussed.

In this paper we report the first results obtained in the study on the properties of porous GaAs (P-GaAs) produced by electrochemical etching in electrolytes on the basis of hydrofluoric acid. As the initial material we used monocrystalline n- and p-type (100)GaAs substrates Te- and Zn-doped to 2*10¹⁸ cm^-3 and 6*10¹⁸ cm^-3, respectively. The substrates were subjected to chemical-mechanical and diamond-paste polish. Etching was performed in an electrolytic cell with a platinum cathode in the galvanostatic regime with anode current densities ranging from 5 to 150 mA/cm². We were interested in P-GaAs layers with thicknesses from 0.5 to 50 micrometers. The methods used in the study of P-GaAs samples included x-ray diffractometry, electron microscopy, x-ray microanalysis, secondary ion-mass spectroscopy, electrochemical C-V profiling and photoluminescence.

Large interest to porous silicon (PS) has arisen since the discovery of its intensive luminescence in the visible range under ultraviolet radiation or charge injection excitation. A lot of experimental and theoretical works have been devoted to the investigation of PS microstructure, its electrical, electro-optical, linear and non-linear-optical properties. This research was stimulated by the prospects of creation of integrated electro-optical devices on a silicon base. Formation of diffractional grating on the porous silicon layer may be interesting for the planar technology optoelectronic devices on the porous silicon. This work is also interesting from the point of view of powerful pulse radiation interaction with PS and influence on its optical and structural properties.

The photosensitivity of germanium-doped silica glass fibers and related phenomena, namely photoinduced second harmonic generation and refractive index change, attract considerable interest. In the present paper the microscopic mechanisms of the photosensitivity, which are based on photoionization and structural transformation, are considered. The recent experimental results, which reveal the role of excited triplet state of germanium oxygen deficient defect in the photosensitivity of germanium-doped glass, are discussed.

Two novel modifications of the MCVD process extended with the 'solution doping' technique are presented and used for reproducible preparation of preforms with a high content of P₂O₅ (up to 20 mol.%) and rare-earth ions (up to 36000 ppm) in the Al₂O₃-P₂O₅-SiO₂ core. Erbium-doped and ytterbium-sensitized silica optical fibers have been fabricated by this method and used for the investigation of fiber lasers emitting at 1550 nm when pumped at 1064 nm. The relation between the core composition, conditions during the stages of the preform fabrication and the final fiber properties are discussed resulting in a conclusion that lasing of the fibers is influenced by central perturbations of the refractive- index profile. Fibers which can be characterized by a slope efficiency of up to 20% and laser threshold of 400 - 500 mW measured under Fresnel reflection from the fiber end have been fabricated.

The results of investigation of the optical properties of vanadium dioxide are reported. Thin VO₂ films exhibited sharp and reversible optical switching due to the metal-insulator transition at T approximately equal to 70 degrees Celsius. The effect of laser irradiation on amorphous films of anodic vanadium oxide was also studied using a YAG Nd³⁺ laser at wavelengths of 1.06 and 0.53 micrometer. Modifications of the structure and physical properties are reported, and the application of such films for recording and storage of optical information is demonstrated.

A ferroelectric liquid-crystal spatial light modulator (FLC-SLM) has been developed that can be applied to real-time holography. The paper reviews the devices and their holographic applications.

UV-laser irradiation of polymers leads to different types of surface modifications, including the formation of structures with sub-micrometer dimensions. These structures may influence the adhesion of surface coatings. In this paper we investigate the temperature dependence of the growth of branched dendritic structures. Growth rates in excess of 1 nm/s were measured near the glass transition temperature of poly(ethylene terephthalate). Periodic surface structures are investigated with respect to physical formation processes.

The plasma plume created during the interaction of a KrF laser beam (248 nm) with a graphite target under high-vacuum conditions has been investigated using temporally and spatially resolved spectroscopy and time-of-flight charged particle detection. Measurements lead to information on nature and kinetic energy of ejected particles. Hard carbon films are deposited on various substrates located in front to the target and their properties are investigated. These properties are found close to those of diamond (hardness equals 80 GPa, infrared transparency greater than 90%) when deposition temperature is low (less than 100 degrees Celsius).

A high vacuum pulsed laser deposition system is described where an intersection of two ablation plumes from twinned simultaneously irradiated targets is used. This system allows thin film and multilayer deposition of a wide variety of materials (including low melting point metals like tin) practically without droplet contamination. The intersection region acts as a filter for droplets and high energy plasma particles. The use of twinned targets of different materials facilitates preparation of artificially mixed supersaturated thin film solid state solutions used as a media for sub-micrometer and nanometer-scale surface processing. Special design of the target holder that can carry simultaneously up to 24 targets and computer control of the deposition process make it possible to easily change targets without venting the deposition chamber and to deposit arbitrary multilayer combinations of various materials.

Thin metal films (Cu, Au, Ni) and thin Si films with different thicknesses on fused silica were irradiated with a ns-pulse from a Nd:YAG-laser (lambda equals 532 nm, FWHM 7 ns). In this paper we want to focus on the optical properties of an undercooled metallic liquid and the observed dewetting of the thin films. The ongoing processes were monitored in situ with ns- time-resolved reflectivity measurements. We determined the thresholds for partial and compete melting of these films. Due to the high cooling rates (up to 10¹¹ K/s) different 'frozen' stages of the dewetting phenomenon can be studied in detail with ex situ microscopic investigations, e.g. optical microscopy and scanning near field acoustic microscopy. We present measurements which show that we have observed spinodal dewetting of these thin films.

We report a study of the characteristics of thin films deposited at room temperature on Si and KBr substrates by XeCl laser ablation of graphite in low pressure (0.25-2.5 mbar) nitrogen and ammonia atmospheres. Very hard films, with a very high electrical resistivity were obtained. The deposition rates decrease with increasing ambient pressure. N/C atomic ratios up to 0.6 were calculated from backscattering measurements. Different diagnostic techniques (XPS, IR absorption spectroscopy, etc.) demonstrate the formation of carbon nitride with a prevalent graphite-like structure. Films deposited in NH₃ are thinner and present a lower quantity of N atoms bound to C atoms than films deposited in N₂ at the same ambient pressure.

The present paper is devoted to revealing specific kinetic features of laser-induced solid-phase acidifying-restoration (reduction-oxidation) reaction capable of occurring in two ways: photo- chemical and thermo-chemical. There are observed and explained phenomena of changing of one way of reaction occurring for the other under action of laser radiation as well as connected with them variation of the optical properties of the substance reacting. Thin films of silicon- molybdic acid (SMA) dissolved in polyvinyl alcohol (PVA) were chosen as the object of investigations.

In this paper some advantages of using thin freely suspended films as a target in femtosecond laser-plasma interaction are discussed. It is shown by numerical simulation for carbon film, that for 10 - 30 nm films significant increase in plasma temperature can be achieved due to strongly limited thermoflux deep into the target. This increase in temperature is accompanied by increase in x-ray yield. The other task of this work is to investigate whether it is possible to excite surface plasmon-polariton for the thin film target.

Investigations devoted to the silicate glass film deposition from gas discharge initiated by laser plasma are presented. Tetraethylorthosilicate (TEOS) is introduced in the gas composition. It was the source of the SiO₂. Addition of the other components of the deposited coatings comes from the laser plume of the ablated solid state target.

Comparative study of laser patterning of YBaCuO thin films has been accomplished using XeCl, N₂, Cu, CO₂, and Nd:YAG lasers. Various tracks of different dimensions and shapes have been generated using a computer-controlled X-Y stage. A complete removal of the film material has been achieved. Issues relating to the limit of resolution, annealing of films, and degradation of superconducting properties are discussed for the different lasers used. The cw carbon-dioxide laser has been used for irradiation of the films thus creating 'ex situ' and 'in situ' annealed regions with different film orientation and morphology. The films have been characterized using EDAX, SEM, and RBS.

The growth and characterization of YBaCuO thin films and related buffer layers (BaTiO₃, ZrO₂, BaTiO₃, LiNbO₃) grown by laser ablation on Si and GaAs substrates, are described. Both buffers and YBuCuO layers have been deposited using XeCl excimer laser (308 nm). The morphology and structure of the films have been determined using XRD and SEM analyses. A good quality textured YBaCuO film could be grown on single crystal as well as polycrystalline buffer layer deposited on Si and GaAs. The key to the successful film deposition is the low processing temperatures involved, which minimized the interface reactions as observed by EDAX spectroscopy, as well as direct cw carbon-dioxide laser irradiation of growing film.

The authors report about the laser method of manufacturing Josephson bridge junctions on the surface of bulk YBa₂Cu₃O_7-δ ceramic. The characteristics of the junction are given. The nonmonotonical dependence of differential resistance on the value current through the junction, which was experimentally observed, is determined by the energy gap for Cupper pairs. This technology permits us to fabricate Josephson interferometers and devices containing them.

Laser chemical vapor deposition (LCVD) method was used for silicon films formation. Results on Raman scattering and x-ray induced photoelectron spectroscopy (XPS) of these polycrystalline and amorphous films are presented. The same LCVD methods for the case of quasi-resonant molecules were studied too. Theoretical criteria for nonthermal and thermal reactions in laser fields are given. The frequency dependence of laser chemical reaction ignition threshold was experimentally registered for the silicon tetrafluoride decomposition reaction in carbon-dioxide-laser field. It was proved that the laser radiation intensity needed for maintaining the reaction is substantially lower than the threshold ignition intensity.

Results of the latest research of the record characteristics of diode-laser pumped solid-state laser and physical properties of crystals for these lasers are discussed. Crystals, possessing one optical axes and having strong polarization dependent physical properties, and, first of all, absorption coefficient on a length of a diode pump wave and emission cross sections on a length of waves of generation, permit us to maximally effectively use polarized radiation of a laser diode.

In this work, we consider the different methods of populating the initial and final working levels of laser transitions in TR-doped crystals under the selective 'up-conversion' and 'avalanche' diode laser pumping. On the basis of estimates of the probabilities of competing non-radiative energy-transfer processes rates obtained from the experimental data and theoretical calculations, we estimated the efficiency of the up-conversion pumping and selfquenching of the upper TR³⁺ states excited by laser-diode emission. The effect of the host composition, dopant concentration, and temperature on the output characteristics and up-conversion processes in YLF:Er; BaY₂F₈:Er; BaY₂F₈:Er,Yb and BaY₂F₈:Yb,Ho are determined.

Passive mode-locking of a cw lamp pumped Nd:YAG laser with the nonlinear mirror technique is reported. Light pulses with 2W of average power and pulse duration shorter than 100 ps at 1064 nm have been obtained. The nonlinear mirror consists of a 3.5 mm long KTP frequency doubling crystal and a dichroic mirror with high reflectivity for the second harmonic and lower reflectivity for the fundamental frequency. The mode-locking process is self-starting.

We have investigated theoretically and by computer simulation the formation of ultra-short pulses in our laser system using the idea of description of ultra-short coherent optical pulses as temporal Gaussian beams analogous to complex Ermit-Gaussian beams. We have analyzed the laser system with Kerr lens feedback in the phase trajectory of five-dimensional space: the pulse intensity, the width of the pulse, the 'chirp' of the pulse, the phase-front radius of curvature, and the beam size. The investigation of the structure of the phase space transformation shows that the transformation possess an asymptotically stable stationary point and more complicated structure. The analysis of the solutions in our model reveals that chaotic instabilities can be reached through increasing of nonlinear interaction temporal and spatial Gaussian beam.

The analysis of the problems arising when the KTP crystals are used for the second harmonic generation in the case of the high peak and high average power laser radiation is presented. The main attention is paid for the investigation of the radiation absorption including the induced one.