Topological birefringence and combined Rytov-Magnus effect

It is shown that the symmetry properties of the locally isotropic inhomogeneous medium of an optical fiber cause circular and linear topological birefringence. The circular birefringence δn _C in graded-index fibers is ∼(λ/ρ)² (where λ is the wavelength and ρ is the core radius), while the linear birefringence is δn _L ∼(λ/ρ)³. This topological birefringence is characterized not only by the polarization basis (as in crystals for example) but also by the magnitude and sign of the topological charge of the guided vortex. This topological birefringence forms the basis of the instability of the fiber IV vortex and is observed experimentally as the combined Rytov-Magnus effect. Pis’ma Zh. Tekh. Fiz. 25, 41–46 (March 12, 1999)     

Sealed efficient excilamps excited by a capacitive discharge

The development of sealed XeCl (λ∼308 nm), KrCl (λ∼222 nm), and XeI (λ∼253 nm) excilamps excited by a capacitive rf discharge is reported. It is shown that highly efficient emission of exciplex molecules is achieved under capacitive discharge excitation and the emitter has a simple design. An average emission power of 3W was obtained with a ∼12% efficiency and the lifetime of the sealed excilamps was longer than 1000 h. Pis’ma Zh. Tekh. Fiz. 25, 27–32 (November 12, 1999)     

Instability induced in a relativistic plasma flux by the excitation of surface waves

An analysis is made of the instability of a plasma flux caused by the excitation of a new type of cylindrical surface electromagnetic waves at the interface between the flux and a stationary plasma. It is shown that, unlike the conventional case ε₁>0 and ε₂,<0, at the interface of a relativistic plasma beam there exist growing surface waves at frequencies corresponding to positive values of the permittivities on both sides of the discontinuity. For a given geometry and plasma density the critical parameter for the excitation of these waves is the wave radius of the flux. Pis’ma Zh. Tekh. Fiz. 23, 76–79 (March 12, 1997)     

Spinodal decomposition in fine grained materials

We have used a phase field model to study spinodal decomposition in polycrystalline materials in which the grain size is of the same order of magnitude as the characteristic decomposition wavelength (Xsu). In the spirit of phase field models, each grain (i) in our model has an order parameter (η _i) associated with it;η _i has a value of unity inside the ith grain, decreases smoothly through the grain boundary region to zero outside the grain. For a symmetric alloy of composition,c = 0–5, our results show that microstructural evolution depends largely on the difference in the grain boundary energies, y_gb, of A-rich (a) and B-rich (β) phases. If Y _gb ^α is lower, we find that the decomposition process is initiated with an a layer being formed at the grain boundary. If the grain size is sufficiently small (about the same as λ_sd), the interior of the grain is filled with the β phase. If the grain size is large (say, about 10λ_SD or greater), the early stage microstructure exhibits an A-rich grain boundary layer followed by a B-rich layer; the grain interior exhibits a spinodally decomposed microstructure, evolving slowly. Further, grain growth is suppressed completely during the decomposition process.     

Changes in emission characteristics of light-emitting diodes under variation of internal factors and pulse-excitation parameters

A relationship is established between the wavelength λ at the maximum of the near-edge emission band of a light-emitting diode (LED) and the external parameters of the pulse current t_i and Q and the thermal time constant τ_i, which is determined by the product of the thermal resistance and capacitance. A dependence of the shift in λ on the relationship between the LED’s internal and external parameters is discovered. It is shown that for LED operation in the pulsed mode, the shift in λ with variation of t_i. Q, and especially τ_i can reach values on the order of 10 nm (or more), which is a considerable source of errors. Ukraine. Translated from Izmeritel’naya Tekhnika, No. 12, pp. 37–38, December, 1999.     

Characteristics of stimulated emission from an optically pumped GaN/AlGaN double heterostructure

The luminescence properties of a GaN/Al_0.1Ga_0.9N double heterostructure grown by vapor-phase deposition from organometallic compounds are studied. When luminescence is observed from the end, the radiation intensity shows a sharply defined threshold dependence on the pump density. The threshold excitation density at T=77 K was ∼40 kW/cm² and the wavelength of the stimulated emission was λ=357 nm. The long-wavelength shift of the emission line at high pump densities may be attributed to renormalization of the band gap caused by many-particle interactions in the electron-hole plasma. Pis’ma Zh. Tekh. Fiz. 23, 53–59 (August 12, 1997)     

Eliminating nuisance parameters: two characterizations

Given a statistical modelP = {P_θ : θ ∈ x} and a surjective functiong: ϑ→Λ the problem of transformingP into a new modelQ= {_λ : λ ∈ Λ} indexed by Λ is investigated. Two characterizations are given for those modelsQ of the form Q_λ = ∫ P_θ π_λ(dθ), where π_λ is some probability such that π_λ(g=λ)=1. The first is related to a geometric property ofQ, while the second rests on the inferential implications of adoptingQ. Also, in the first π_λ is allowed to be finitely additive, while in the second π_λ is σ-additive. Finally, integrated likelihoods are revisited in light of the second characterization.     

LEDs based on InAsSbP/InAsSb heterostructures (λ = 4.7 μm) for carbon monoxide detection

Light-emitting diodes (LEDs) operating in the 4.4–4.8-μm wavelength range have been developed for detecting the presence of carbon monoxide (CO) in air. The proposed LEDs are based on InAsSbP/InAsSb heterostructures with InAs_0.85Sb_0.15 active region, which were grown by metalorganic vapor-phase epitaxy. The electrolumuinescent properties of LEDs have been studied. The output power of LED operating in a pulsed mode reaches 50 μW at a current pulse amplitude of 2 A, and that in a quasi-continuous wave mode is 1 μW at a current of 200 mA. The absorption of LED radiation in a gaseous medium containing 10% CO has been studied. The proposed LEDs can be used to simultaneously detect both CO₂ (absorbing at λ = 4.3 μm) and CO (λ = 4.67 μm).     

Pattern scaling in axial segregation

Granular tumbled frequently segregate by grain size along the axis of partially-filled, horizontal, rotating tubes. When segregation approaches saturation at the surface, a well-defined pattern of bands with wavelength λ emerges. The long-term dynamics of the pattern involves a much slower coarsening process. We characterized the initial saturated wavelength λ as a function of the diameter of the tube D for a filling fraction of 30 %, for 1.9 cm ≤D≤11.5 cm. We also studied the initial growth-rate of the bands as D varies. We find that λ/ D is not constant, but rather increases rapidly for small D. The growth-rate of bands decreases with smaller D and segregation is suppressed completely for sufficiently small D. These relatively simple features are not captured by any of the existing models of axial segregation.     

λCalculi with Explicit Substitutions Preserving Strong Normalization

This paper studies preservation of β-strong normalization by two different confluent λ-calculi with explicit substitutions defined in [96]; the particularity of these calculi, called λ _d and λ _dn respectively, is that both have a (weak) composition operator for substitutions. We apply an abstract simulation technique to reduce preservation of β-strong normalization of λ _d and λ _dn to that of another calculus, called λ _f having no composition operator. Then, preservation of β-strong normalization of λ _f is shown using the same technique as in [2]. As a consequence, λ _d and λ _dn become the first λ-calculi with explicit substitutions having (weak) composition and preserving β-strong normalization. As an aside, we also show how to apply our technique to reduce preservation of β-strong normalization of the calculus λ _v in [20] to that of λ _f . Received: August 19, 1997; revised version: November 13, 1998     

A numerical study of the similarity of fully developed laminar flows in orthogonally rotating rectangular ducts and stationary curved rectangular ducts of arbitrary aspect ratio

 The present study showed that a quantitative analogy of fully developed laminar flow in orthogonally rotating rectangular ducts and stationary curved rectangular ducts of arbitrary aspect ratio could be established. In order to clarify the similarity of the two flows, the dimensionless parameters K _LR=Re/(Ro)^1/2 and the Rossby number, Ro=w _m /Ωd _h , in a rotating straight duct were used as a set corresponding to the Dean number, K _LC=Re/λ^1/2, and curvature ratio, λ=R/d _h , in a stationary curved duct. Under the condition that the value of the Rossby number and the curvature ratio was large enough, the flow field satisfied the `asymptotic invariance property'; there were strong quantitative similarities between the two flows such as in the friction factors, flow patterns, and maximum axial velocity magnitudes for the same values of K _LR and K _LC. Based on these similarities, it is possible to predict the flow characteristics in rotating ducts by considering the flow in stationary curved ducts, and vice versa. Received: 10 September 2001 / Accepted: 13 May 2002     

Injection photodiode based on <Emphasis Type="Italic">p</Emphasis>-CdTe film

Photosensitive large-block p-type cadmium telluride (p-CdTe) film with a resistivity of ρ ≈ 10⁶−10⁷ Ω cm has been used to create an Al-p-CdTe-Mo structure with a Schottky barrier, which exhibits the properties of an injection photodiode. Being switched in the forward current passage direction (with “+” on the Mo contact) at high illumination levels, the proposed structure exhibits a responsivity of S _λ ≈ 2.6 A/W at a wavelength of λ = 0.625 μm, which is about five times the spectral response of the ideal photodetector for this wavelength range.     

Photoinduced non-linear optical effects in lanthanum calcium borate single crystals

We have revealed that lanthanum calcium borate (La₂CaB₁₀O₁₉) crystals show two-photon absorption (TPA) favored by UV laser field. UV-induced TPA measurements were performed under illumination of a Xe-F laser (λ = 217nm) as a photoinducing (pumping) beam. This pumping laser beam created a thin surface layer (about 85 nm) that was a source of the observed photoinduced TPA. Use of the longer photoinducing wavelength leads to a substantial decrease of the photoinduced TPA. The performed DFT calculations of the band energy dispersion clearly show that the effect observed is due to specific band energy dispersion between the sub-bands. The Raman shifted Nd-YAG laser radiation (λ = 1.9 μm) as well as its second and fourth harmonics (λ = 950 and λ = 475 nm, respectively) were used as fundamental (probing) beams of the TPA. The highest values of the TPA β coefficient were achieved for polarization of the pumping light directed along the second-order crystallographic axis. The obtained values of TPA coefficients are higher than for BiBO crystals that were also investigated by us.     

Temperature dependence of the intensity and phase of reflected light in a liquid-crystal structure with surface plasmons

The results of a theoretical calculation of the temperature dependence of the amplitude coefficient and phase of reflected radiation with wavelength λ=6328 Å in a liquid-crystal structure with surface plasmons are reported for the first time. The computational results could be helpful for developing temperature sensors and optically coupled thermal imagers based on liquid-crystal structures with surface plasmons. Pis’ma Zh. Tekh. Fiz. 23, 11–15 (September 12, 1997)     

The use of crystal phosphors based on rare-earth elements to determine the spectral resolution and to calibrate the wavelength scale of luminescence spectrometers

The results of an investigation of the crystal phosphor Y₂O₂S:Er,Yb to determine the possibility of using it to measure the spectral resolution and to calibrate the wavelength scale of luminescence spectrometers are presented. The luminescence spectrum of the crystal phosphor was measured on three spectrofluorescence instruments when excited by a laser with λ_max = 960 nm and with ultraviolet excitation with λ_max = 383 nm, the optical arrangements of which differ considerably in the radiation recording channel. The difference in the wavelength positions of the maxima of the spectra of the neon lamp used for calibration and of the crystal phosphor investigated is estimated.     

Thermal Conductivity of Standard Sands. Part I. Dry-State Conditions

A comprehensive thermal conductivity (λ) database of three dry standard sands (Ottawa C-109, Ottawa C-190, and Toyoura) was developed using a transient line heat source technique. The database contains λ data representing a variety of soil compactions and temperatures (T) ranging from 25 °C to 70 °C. The tested standard sands, due to their repeatable physical characteristics, can be used as reference materials for validation of thermal probes applied to similar dry granular materials. The measured data show an increasing trend of thermal conductivity at dryness (λ_dry) against T in spite of declining quartz λ with T. The air content (porosity) controls the λ of dry sands by acting as a very effective thermal insulator around solid soil particles. As a result, a diminutive increase of λ_dry with T is driven by increasing λ of air. The experimental λ data of dry sands were exceptionally well predicted by de Vries and Woodside–Messmer models, and also by a thermal conductance model, a product of λ of solids and the thermal conductance factor.     

A numerical method of determining the spectral-ratio temperature

A numerical method of calculating the spectral-ratio temperature for objects whose spectral emissive power ε_λ depends on the wavelength λ is described. A calculation is carried out for two types of broadband radiation receivers based on Si/Si and Si/InGaAs structures. The differences between the actual temperature of the objects and their spectral-ratio temperature are given for Fe, Ni, Cu, Ag, and Co as examples. It is shown that there are no disagreements between the results of numerical and analytical calculations of the spectral-ratio temperature.     

Synthesis and characterization of novel poly(aryleneethynylene)s derived from squaraines for photovoltaic applications

Two novel π-conjugated polymers composed of alkyl carbazole/dialkoxyphenylene and squaraine units were synthesized by Sonogashira cross-coupling reactions. The structures and properties of these copolymers were characterized using FT-IR, NMR, UV–Vis, gel permeation chromatography and cyclic voltammetry (CV). Both polymers possess adequate thermal stability and exhibit good solubility in common organic solvents such as chloroform, THF, and toluene. The polymer films show intense and broad visible and near IR absorption with the long wavelength absorption maximum peaks of 796 and 851 nm for P ₁ and P ₂ , respectively, which are apparently red-shifted compared with poly(phenyleneethynylene). CV studies reveal that the band gaps of these copolymers are about 1.45 eV, implying that the resulted polymers may be promising candidates for solar cells.     

Influence of acoustic treatment on the photoconductivity of zinc selenide crystals

It is shown that acoustic treatment of piezoelectric zinc selenide crystals by exciting strong natural elastic vibrations can specifically alter their photoelectric and other properties. Irreversible changes in the stressed-strained state of the crystals under this treatment are responsible for characteristic features in the spectral dependences of the permittivity ε′(λ) and the dielectric loss coefficient ε′’(λ). These dependences plotted in the form of ε*(λ) diagrams on the complex plane and also the spectrum of natural elastic vibrations can be used to monitor the treatment process. Pis’ma Zh. Tekh. Fiz. 25, 24–29 (December 26, 1999)     

Formation,thermal, optical and physical properties of GeS<Subscript>2</Subscript>–Ga<Subscript>2</Subscript>S<Subscript>3</Subscript>–AgCl novel chalcohalide glasses

Novel GeS₂–Ga₂S₃–AgCl chalcohalide glasses had been prepared by melt-quenching technique, and the glass-forming region was determined by XRD, which indicated that the maximum of dissolvable AgCl was up to 65 mol%. Thermal and optical properties of the glasses were studied by differential scanning calorimetry (DSC) and Visible-IR transmission, which showed that most of GeS₂–Ga₂S₃–AgCl glasses had strong glass-forming ability and broad region of transmission (about 0.45–12.5 μm). With the addition of AgCl, the glass transition temperature, Tg decreases distinctly, and the short-wavelength cut-off edge (λ_vis) of the glasses also shifts to the long wavelength gradually. However, the glass-forming ability of the glass has a complicated evolutional trend depended on the compositional change. In addition, the values of the Vickers microhardness, H _v , which decrease with the addition of AgCl, are high enough for the practical applications. These excellent properties of GeS₂–Ga₂S₃–AgCl glasses make them potentially applied in the optoelectronic field, such as all-optical switch, etc.