Peculiarities of thermo-optic coefficient under different temperature regimes in optical fibers containing fiber Bragg gratings

Direct experimental measurements of the thermo-optic for fixed temperature intervals (20–200 °C, 200–500 °C, 500–660 °C, 660–780 °C) in fused silica fiber containing fiber Bragg gratings (FBGs) were conducted. The diffraction efficiency of a FBG fluctuated with temperature between 2.01 × 10^? 4 and 0.17 × 10^? 4 while the temperature shift of the Bragg's peak was monitored between 1300 and 1311 nm with sub-Angstrom precision. Numerical simulations were focused on FBG's diffraction efficiency calculations accounting for the temperature drift of the gratings, and found to be in excellent agreement with obtained experimental data.It was found that the first-order thermo-optic coefficient changes between 1.29 and 1.85 × 10^? 5 K^? 1 for the linear fit and at T = 0 °C its value was found to be close to 2.37 × 10^? 5 K^? 1 for the polynomial fit of experimental data. The average thermo-optic coefficient undergoes a minimum in the vicinity of 440 °C. Additional observation indicates a negative sign of the second-order thermo-optic coefficient. The value of thermal expansion coefficient was much less (0.5 × 10^? 6 K^? 1) than that for the average thermo-optic coefficient. Based on the energy dispersive spectroscopy it was determined that thermal erasing of the FBGs at a temperature around 780 °C corresponds to germanium monoxide diffusion out of core in silica-based fibers.     

Bootstrap Approximation to Distributions of Finite Population U-statistics

We show that the without replacement bootstrap of Booth, Butler and Hall (J. Am. Stat. Assoc. 89, 1282–1289, 1994) provides second order correct approximation to the distribution function of a Studentized U-statistic based on simple random sample drawn without replacement. In order to achieve similar approximation accuracy for the bootstrap procedure due to Bickel and Freedman (Ann. Stat. 12, 470–482, 1984) and Chao and Lo (Sankhya Ser. A 47, 399–405, 1985) we introduce randomized adjustments to the resampling fraction.      

Fifty years in the field of probability: A conversation with professor Vygantas Paulauskas

Lithuanian Mathematical Journal - This year professor Vygantas Paulauskas is celebrating his 75 birthday. An outstanding probabilist he is well known for his research in the field of probability...     

Investigation of Active and Inactivated Yeast Cells by Scanning Electrochemical Impedance Microscopy

Scanning electrochemical microscopy (SECM), electrochemical impedance spectroscopy (EIS) and scanning electrochemical impedance microscopy (SEIM) were used to investigate electrochemical activity of active and inactivated yeast Saccharomyces cerevisiae cells. SEIM experiment was performed using a unique electrochemical impedance spectrometer with a fast Fourier transform (FFT‐EIS) function, which enabled simultaneously perturb/evaluate electrochemical system at 50 frequencies. This allowed very quick observing the differences between impedance spectra, which were taken every few seconds. Therefore, we were able to apply SEIM for relatively fast determination of electrochemical impedance dependence on the distance between ultramicroelectrode (UME) and surface modified by immobilized yeast cells. It was determined that electrochemical activity and ‘breathing’ (a consumption of dissolved oxygen) of yeast can be electrochemically observed when the distance between UME and surface of yeast cells is in the range from 0 μm to 25 μm. Therefore, 25 μm is the maximum distance suitable for efficient investigation of yeast cell activity when experiments are performed in FFT‐SEIM mode. Charge transfer resistance of active and inactivated yeast cells was determined using EIS. It was calculated that charge transfer resistance of active yeast cells is 1.5 times lower than that of inactivated yeast cells. Lipophilic vitamin K₃ (Vit‐K₃) and hydrophilic vitamin K₁ (Vit‐K₁) were mixtured and used as redox mediators for charge transfer from yeast cells.     

Factors controlling charge recombination under dark and light conditions in dye sensitised solar cells

A simple and powerful approach for assessing the recombination losses in dye sensitised solar cells (DSSCs) across the current voltage curve (j-V) as a function of TiO(2) electron concentration (n) is demonstrated. The total flux of electrons recombining with iodine species in the electrolyte and oxidised dye molecules can be thought of as a recombination current density, defined as j(rec) = j(inj)-j where j(inj) is the current of electrons injected from optically excited dye states and j is the current density collected at cell voltage (V). The electron concentration at any given operating conditions is determined by charge extraction. This allows comparison of factors influencing electron recombination rates at matched n. We show that j(rec) is typically 2-3 times higher under 1 sun equivalent illumination (j(inj) > 0) relative to dark (j(inj) = 0) conditions. This difference was increased by increasing light intensity, electrolyte iodine concentration and electrolyte solvent viscosity. The difference was reduced by increasing the electrolyte iodide concentration and increasing the temperature. These results allowed us to verify a numerical model of complete operational cells (Barnes et al., Phys. Chem. Chem. Phys., DOI: 10.1039/c0cp01554g) and to relate the differences in j(rec) to physical processes in the devices. The difference between j(rec) in the light and dark can be explained by two factors: (1) an increase in the concentration of electron acceptor species (I(3)(-) and/or I(2)) when current is flowing under illumination relative to dark conditions where the current is flowing in the opposite direction, and (2) a non-trivial contribution from electron recombination to oxidised dye molecules under light conditions. More generally, the technique helps to assign the observed relationship between the components, processing and performance of DSSCs to more fundamental physical processes.     

Instability of nanostructures patterned in polystyrene under high electric field gradients

A nanolithography technique based on the lateral displacement of electrically biased AFM tip was developed for nanostructures formation of 30-100 nm in width and 1-10 nm in height in the polystyrene (PS) films. It was demonstrated that the nanostructures patterned in annealed PS films (90K Mw) show slow exponential relaxation between 55 and 265 h depending on their size. Relaxation of the nanostructures in non-annealed films usually occurred in minutes. It was observed that in the annealed samples a negative electric charge accumulated in the areas where the nanostructures formed while in the non-annealed samples only the positive charge in exposed areas was detected using the electric force microscopy. After 320 h of monitoring under the humidity maintained between 25 and 27% it was suggested that slow dynamical changes of the nanostructures can be attributed to the negative electric charge dissipation in the annealed samples.     

Perfect matchings in random intersection graphs

Let W ₁,??,W _n be independent random subsets of [m]={1,??,m}. Assuming that each W _i is uniformly distributed in the class of d-subsets of?[m] we study the uniform random intersection graph G _s (n,m,d) on the vertex set {W ₁,??W _n }, defined by the adjacency relation: W _i ??W _j whenever |W _i ??W _j |?Rs. For even?n we show that as n,m???? the edge density threshold for the property that G _s (n,m,d) contains a perfect matching is asymptotically the same as that for G _s (n,m,d) being connected.     

Patterning of indium-tin oxide on glass with picosecond lasers

The results of patterning of the indium-tin oxide (ITO) film on the glass substrate with high repetition rate picosecond lasers at various wavelengths are presented. Laser radiation initiated the ablation of the material, forming grooves in ITO. Profile of the grooves was analyzed with a phase contrast optical microscope, a stylus type profiler, scanning electron microscope (SEM) and atomic force microscope (AFM). Clean removal of the ITO film was achieved with the 266 nm radiation when laser fluence was above the threshold at 0.20 J/cm², while for the 355 nm radiation, the threshold was higher, above 0.46 J/cm². The glass substrate was damaged in the area where the fluence was higher than 1.55 J/cm². The 532 nm radiation allowed getting well defined grooves, but a lot of residues in the form of dust were generated on the surface. UV radiation with the 266 nm wavelength provided the widest working window for ITO ablation without damage of the substrate. Use of UV laser radiation with fluences close to the ablation threshold made it possible to minimize surface contamination and the recast ridge formation during the process.     

Biopharmaceutical Evaluation of Capsules with Lyophilized Apple Powder

Apples are an important source of biologically active compounds. Consequently, we decided to model hard gelatin capsules with lyophilized apple powder by using different excipients and to evaluate the release kinetics of phenolic compounds. The apple slices of “Ligol” cultivar were immediately frozen in a freezer (at −35°C) with air circulation and were lyophilized with a sublimator at the pressure of 0.01 mbar (condenser temperature, −85°C). Lyophilized apple powder was used as an active substance filled into hard gelatin capsules. We conducted capsule disintegration and dissolution tests to evaluate the quality of apple lyophilizate-containing capsules of different encapsulating content. Individual phenolic compounds can be arranged in the following descending order according to the amount released from the capsules of different compositions: chlorogenic acid > rutin > avicularin > hyperoside > phloridzin > quercitrin > (−)-epicatechin > isoquercitrin. Chlorogenic acid was the compound that was released in the highest amounts from capsules of different encapsulating content: its released amounts ranged from 68.4 to 640.3 μg/mL. According to the obtained data, when hypromellose content ranged from 29% to 41% of the capsule mass, the capsules disintegrated within less than 30 min, and such amounts of hypromellose did not prolong the release of phenolic compounds. Based on the results of the dissolution test, the capsules can be classified as fast-dissolving preparations, as more than 85% of the active substances were released within 30 min.