S. I. Tsypina

Inhomogeneous nature of UV absorption bands of bulk and surface oxygen-deficient centers in silica glasses

Photochemical and photoluminescence studies of oxygen-deficient centers stabilized in the bulk and on the surface of silica glasses clearly demonstrate the inhomogeneous nature of the absorption and luminescence spectra of oxygen-deficient centers. The conclusion is drawn that the inhomogeneity of the absorption spectra is due to the dispersion of the energy of the S0-S1 transition, while the inhomogeneity of the luminescence spectra is due to the dispersion of the energy barrier of intersystem crossing. The inhomogeneous nature of the oxygen-deficient centers in silica glasses is assumed to be caused by a small dispersion in the geometrical parameters of different groups of centers with similar chemical properties.

The fibre optic reflectometer: A new and simple probe for refractive index and phase separation measurements in gases, liquids and supercritical fluids

A fibre optic reflectometer (FOR) has been developed to monitor refractive index (n) and density (ρ) changes in gases, liquids and supercritical fluids (SCFs). The key operating principle is the measurement of the intensity of light from a light emitting diode, LED, which is reflected from the end of a fibre immersed in the medium. The amount of reflected light depends on the difference between the refractive indices of the fibre and the medium. The refractive index of the medium is related in turn to its density. The FOR is particularly sensitive to phase separation, a key aspect of supercritical fluid systems, because of the density discontinuity which accompanies the separation. This device allows the collection of data with a high spatial (9 μm-fibre diameter) and time (∼0.02 s) resolution. The small size of the fibre optic probe means that it can be put into operating reactors and even catalyst beds. Experiments are described with pure CO2 and CHF3, and mixtures CO2 + cyclohexane and CO2 + MeOH over wide pressure and temperatures ranges. The measurements are in excellent agreement with published data for these systems.

Laser fabrication of periodic microstructures from silver nanoparticles in polymer films

The fabrication of photoand thermostable periodic structures from silver nanoparticles in polymer plates (cross-linked oligourethanemethacrylate impregnated with silver precursors Ag(hfac) and Ag(fod) dissolved in the supercritical carbon dioxide) is studied. The process is based on the local (depending on the irradiated spot size) photochemical decomposition of the silver precursors in the polymer matrix that initiates the atomic aggregation and creation of silver nanoparticles with the plasmon resonance in absorption in the spectral range 420–430 nm. The third-harmonic radiation of a Nd:YAG laser (355 nm) and the Kr+-laser (521 nm) radiation are employed for the recording of periodic structures with submillimeter and micron resolutions. The photosensitivity of the polymer matrices impregnated with the silver precursors to the UV and visible radiation is discussed.

Direct observation of ultraviolet laser induced photocurrent in oxygen deficient silica and germanosilicate glasses

UV laser induced inonization of silicon oxygen deficient centers (SODC) in silica glasses and germanium oxygen deficient centers (GODC) in germanosilicate glasses has been studied by direct displacement photocurrent measurements. For both SODC and GODC the two‐photon (two‐step) nature of photoionization has been established. The cross sections of transitions from first excited to upper (ionizable) states were estimated as σ12=6×10−18 cm2 for SODC and (0.5–1.0)×10−19 cm2 for GODC. The effect of pulse‐to‐pulse degradation of the photocurrent signal caused by the screening effect is applied to estimate the lifetimes and ranges of the free carriers. The conclusion was drawn, that ionized SODC and GODC play the key role for free charge trapping.

Oscillator strengths of UV absorption and luminescence for oxygen-deficient centers in germanosilicate fibers.

The time-resolved measurements of triplet and singlet luminescence from germanium oxygen-deficient centers (GODCs) in germanosilicate fibers excited by short UV laser pulses are used to determine the cross section of UV absorption of GODCs at lambda = 248 nm (sigma = 5 x 10(-18) cm(2)), the oscillator strength of UV absorption (f(abs) = 0.027), and UV luminescence (f(lum) = 0.031). The basis for these measurements is the direct luminescence detection of lifetimes of electronically excited states of GODCs, the saturation of triplet luminescence signals, and the analysis of the rate equations that govern the laser photoexcitation and relaxation processes in GODCs.

Kinetics of water transfer and stress relaxation in cartilage heated with 1.56 μm fiber laser

In this work we have used for the first time 1.56 micrometer fiber laser to study mechanisms of IR laser induced stress relaxation in cartilage. We have applied several in-situ monitoring techniques: local temperature measurements (IR radiometry and thermocouple), IR-light absorption, direct stress measurements, micro-balancing, visible light scattering and optical coherent tomography. We have measured temporal behavior of 1.56 micrometer laser light transmission through the cartilage sample at different intensities with synchronous temperature and stress monitoring. The observed bleaching effect (self-induced transparency) is caused by water release from irradiating zone, water evaporation from the cartilage surface and, also, by temperature shift and decrease of intensity of water absorption bands.

Reflectometric studies of the etching of a silica fiber with a germanium silicate core in sub- and supercritical water

Fiber optic reflectometry (FOR) and scanning electron microscopy (SEM) were used to study the regularities of the etching of a single-mode optical silica fiber with a germanium silicate core in subcritical and supercritical water. It was demonstrated that the rate of etching of the germanium silicate core, being higher than that of etching of the silica cladding, was responsible for the formation of a well at the fiber end face, the depth of which increased with the time of etching. The temporal behavior of the FOR signal was of oscillatory character, an observation that accounted for the interference effects that accompany the reflection of radiation (from the photodiode used in the FOR) from the fiber end face during its etching (well deepening). The interference-controlled character of the FOR signal made it possible to directly measure the rate of etching of the fiber end face in water in its different phase states (gaseous, liquid, and supercritical) at various temperatures and pressures. The lowest measured rate of etching of the germanium silicate core (at 200°C and 54 atm) was 10−3 nm/s, whereas the highest measured rate was 30 nm/s (at 400°C and 246 atm). The temperature dependence of the etching rate was demonstrated to obey the Arrhenius law, with an activation energy of 58 ± 3 kJ/mol. At later stages of etching, the FOR signal changed from regular oscillatory to irregular noisy due to the formation defects of various sizes, as could be clearly seen in SEM images.

Supercritical fluid micronization of risperidone pharmaceutical substance

A comparative study of the supercritical fluid micronization of risperidone pharmaceutical substance with an initial particle size of 50 to 100 μm by the RESS and SAS methods aimed at increasing the bioavailability of risperidone as a drug was performed. Both methods makes it possible to prepare risperidone particle of various forms, 5–20 μm in size. However, the SAS method is preferable, because in contrast to RESS, it does not cause contamination of risperidone with organic solvents used in both processes or any other impurities and also makes it possible to vary the shape and size of particles. It is shown that, during SAS micronization, the polymorphic form of risperidone changes from triclinic to monoclinic.

One photon and two photon process in photo-decomposition of germanium oxygen deficient centres

UV photon-induced transformation of germanium oxygen deficient centres (GODC) in germanium-doped silica glass have been studied using photocurrent measurements, absorption and fluorescence bleaching. It has been identified that the photocurrent are generated via a two photon effect. Evidence have been found suggesting that the UV photon-induced destruction of GODCs is achieved via two reaction pathways, a single photon pathway and a two photon pathway. The process is discussed.

Trapping of nanoparticles in a liquid by laser-induced microbubbles

Nanoparticles a few nanometers in diameter contained in a colloidal solution can be captured by the surface of a microbubble produced by a well-focused beam of a continuous wave visible laser. The concentration of the nanoparticles on the surface of the microbubble is found to gradually increase during the course of irradiation. The mechanism of such a trapping is associated with the laser-induced Marangoni convection developing in the vicinity of the microbubble as a result of the temperature gradient obtained at its surface.