V. Ya. Panchenko

Optical and structural characteristics of Ga-doped ZnO films

The n-ZnO films doped with Ga to the content 2.5 at % are produced by pulse laser deposition onto the (0001) oriented single crystal sapphire substrates. The transmittance spectra of the ZnO films in the range from 200 to 3200 nm are studied in relation to the Ga dopant content. It is established that an increase in the Ga content shifts the fundamental absorption edge to the blue region, but reduces the transparency of the ZnO films in the infrared spectral region. The dependence of the band gap on the level of doping with Ga is determined. The photoluminescence spectra of the ZnO films doped to different levels are recorded. It is established that the PL intensity and peak position vary unsteadily with the level of doping. X-ray diffraction studies of the structure of the films are carried out. It is found that the crystallographic parameters (the lattice constant c) of the ZnO film depend on the Ga dopant content and the conditions of deposition of the films.

Fabrication of polymer scaffolds for tissue engineering using surface selective laser sintering

A new approach to the fabrication of individual implants and scaffolds for tissue engineering—surface selective laser sintering (SSLS)—is proposed and realized. In contrast to the conventional selective laser sintering, the SSLS method makes it possible to sinter polymer microparticles and melt the near-surface layer rather than the microparticle as a whole. The effect of the laser radiation parameters and the structure and composition of the raw products on the structure and properties of the biomaterials sintered by the laser radiation is analyzed. This approach makes possible both the application of thermally unstable polymers (e.g., polylactides or polylactoglycolides) and the fabrication of scaffolds with incorporated bioactive proteins. The results obtained yield a regular physical basis for a new technology of the fabrication of various polymer scaffolds that represent important materials and elements of modern tissue engineering. The flexibility of the SSLS method is especially important at the stage of investigation of the cell and tissue responses needed for the optimization of the material for a specific application in tissue engineering.

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.

High-temperature ferromagnetism in Si1 − xMnx (x ≈ 0.5) nonstoichiometric alloys

It has been found that the Curie temperature (TC ≈ 300 K) in nonstoichiometric Si1 − xMnx alloys slightly enriched in Mn (x ≈ 0.52–0.55) in comparison to the stoichiometric manganese monosilicide MnSi becomes about an order of magnitude higher than that in MnSi (TC ∼ 30 K). Deviations from stoichiometry lead to a drastic decrease in the density of charge carries (holes), whereas their mobility at about 100 K becomes an order of magnitude higher than the value characteristic of MnSi. The high-temperature ferromagnetism is ascribed to the formation of defects with the localized magnetic moments and by their indirect exchange interaction mediated by the paramagnetic fluctuations of the hole spin density. The existence of defects with the localized magnetic moments in Si1 − xMnx alloys with x ≈ 0.52–0.55 is supported by the results of numerical calculations performed within the framework of the local-density-functional approximation. The increase in the hole mobility in the nonstoichiometric material is attributed to the decay of the Kondo (or spin-polaron) resonances presumably existing in MnSi.

Characterization of ZnO:Ga and ZnO:N films prepared by PLD

The thin films of zinc oxide have been produced by the pulse laser deposition method at various levels of gallium and nitrogen doping. To obtain the n-type films we used gallium doping with concentration of gallium from zero up to 5 at %. The dependence of photoluminescence of the epitaxial ZnO:Ga films on the concentration of gallium doping has been studied. An optimum range of the n-type ZnO films doping with gallium has been determined to obtain highly effective films from the viewpoint of realizing p-n transitions. This range, on the one hand, defines the maximal PL amplitude and, on the other hand, specifies the minimal specific resistance that corresponds to an interval of 0.125–1.000 at % Ga. To produce the p-type ZnO:(Ga, N) films, the ZnO targets with the content of GaN from zero up to 2 at % were used. N2O was used as a buffer gas. A difference is observed in the positions of the peaks of the emission lines of the photoluminescence spectra for the ZnO films, doped with gallium (Ga) and co-doped with gallium and nitrogen (N).

Post-filament self-trapping of ultrashort laser pulses.

Laser filamentation is understood to be self-channeling of intense ultrashort laser pulses achieved when the self-focusing because of the Kerr nonlinearity is balanced by ionization-induced defocusing. Here, we show that, right behind the ionized region of a laser filament, ultrashort laser pulses can couple into a much longer light channel, where a stable self-guiding spatial mode is sustained by the saturable self-focusing nonlinearity. In the limiting regime of negligibly low ionization, this post-filamentation beam dynamics converges to a large-scale beam self-trapping scenario known since the pioneering work on saturable self-focusing nonlinearities.

Enhanced spatial resolution in optical imaging of biotissues labelled with upconversion nanoparticles using a fibre-optic probe scanning technique

The new generation of synthetic nanomaterials, upconversion nanoparticles (UCNPs), have the potential for high-contrast optical imaging of biological tissue by virtue of their unique luminescent properties which enable the autofluorescence and excitation signals to be completely suppressed and avoid biotissue absorption. The potential for deep tissue imaging, such as whole animal imaging, is demonstrated in this report on a comparative study of two epiluminescent imaging methods suitable for the localization of a UCNP-labelled pathology site buried in highly scattering biological tissue modelled by an optical tissue phantom. The lateral resolution exhibited in scanning imaging by an illumination-collection fibre-optic probe appeared to be almost 1.73 times better than that shown by the wide-field CCD commonly used in diffuse optical tomography systems. We attribute this improved lateral resolution to the enhanced angular selectivity of the illumination-collection regime and to the nonlinear dependence of the UCNP luminescence on the excitation intensity.

Laser technologies for fabricating individual implants and matrices for tissue engineering

This paper presents the results of work on the development and modification of laser methods of rapid prototyping and fabrication of individual implants and matrices for tissue engineering. A technology has been developed for shaping mineral-polymer bioactive structures, based on laser photopolymerization of a liquid mixture of polyfunctional acrylic oligomers with microparticles of hydroxyapatite. A method is proposed and developed for surface-selective laser sintering, making it possible to sinter polymeric microparticles without melting them completely, but only submelting their surface. This made it possible to obtain bioactive polymeric matrices based on bioresorbable aliphatic polyesters.

Defect-induced high-temperature ferromagnetism in Si1-xMnx(x???0.52?0.55) alloys

We present a comparative study of the anomalous Hall effect (AHE) and of the transverse Kerr effect (TKE) in the nonstoichiometric alloys. The data on AHE and TKE are consistent with each other and clearly indicate the intrinsic above room-temperature ferromagnetic order in the studied samples. We argue that this order is not produced by the phase segregation effects, but rather has a global character, while even a small level of the nonstoichiometry in alloys drastically changes their magnetic, electrical, optical and magneto-optical properties as compared to those of the stoichiometric manganese monosilicide MnSi. We propose a qualitative explanation of the obtained experimental results in the frame of the model of defect-induced ferromagnetic order using the first-principles calculations of the electronic and magnetic structure of the system.

Determination of the refractive index of β-NaYF4/Yb3+/Er3+/Tm3+ nanocrystals using spectroscopic refractometry

A method for measuring refractive index n of nanosize particles in the visible and near-IR spectral ranges is proposed. The method is based on comparing refractive index ncolloid of a colloid solution of nanoparticles in several solvents with refractive indices nsolvent of corresponding pure solvents and has an accuracy of ±2 × 10−4. Upconversion nanosize phosphors (UCNPs) are synthesized in the form of a β-NaYF4 crystalline matrix doped with Yb3+, Er3+, and Tm3+ rare earth ions. UCNPs have a doped core with a diameter of 40 ± 5 nm and undoped shell with a thickness of 3–5 nm. Synthesized nanocrystals possess intense photoluminescence in the blue, green, and red spectral ranges upon excitation by IR radiation with a wavelength of 977 nm. Using a spectroscopic refractometer, the dispersion of the refractive index of β-NaYF4/Yb3+/Er3+/Tm3+ nanocrystals was measured for the first time in the spectral range of 450–1000 nm with an accuracy of ±2 × 10−4.