4H-SiC-based photodetector of Carcinogenic UV radiation

Carcinogenic (bactericidal) radiation (200–300 nm with a peak at 254 nm) is present in natural (Sun) and artificial (lamps) source of UV radiation. Its intensity is very low as compared to other types of radiation, but it strongly affects the health of human beings. To prevent oncological diseases, it is important to monitor the carcinogenic radiation level; i.e., selective photodetectors are required. A UV photodetector based on n-4H-SiC Schottly barriers and the p ⁺-n junction is proposed. The quantum efficiency spectrum of such detectors is very close to the spectrum of relative action of carcinogenic radiation on human beings. The quantum efficiency (at the spectral peak (254 nm) amounts to about 0.3 electrons/photon for virtually zero sensitivity in other spectral regions. Quantum efficiency in the wavelength range 247–254 nm is practically independent of temperature in the range from ?100 to +100°C.     

New fluorescent labels for time-resolved detection of biomolecules

New dyes with characteristic fluorescence lifetimes have been developed for bioanalytical applications. Based upon the concept of multiplex dyes, we have designed rhodamine dyes with nearly identical absorption and emission spectral characteristics but different fluorescence lifetimes. Extending this principle to applications with laser diodes, new rhodamines with functional groups for covalent coupling of analytes have been developed. The new labels exhibit absortion and fluorescence beyond 600 nm and have a high quantum efficiency, even in aqueous buffer systems.     

Processes of Electron Excitation Energy Conversion in Europium Tris‐Naphthoyltrifluoroacetonate

The absolute quantum yield and duration of ⁵ D ⁷ F _j luminescence of the Eu³⁺ ion in europium trisnaphthoyltrifluoroacetonate has been measured on excitation of luminescence in the UV S ₀ S ₁ absorption band of the ligand in the temperature range 77–365 K. Based on the analysis of the experimental dependences obtained and solution of the balance equations for stationary and nonstationary cases, we have developed a new approach to the determination of the scheme and rate constants of actual transitions. The rate constants of all processes of electron excitation energy conversion have been determined. The role of the state with charge transfer in these processes has been revealed.     

On the origin of the acoustic signals generated in KDP by pulsed UV laser excitation

The origin of the acoustic signals that are generated in the bulk of a KDP crystal during irradiation with short UV laser pulses is determined. The generation of these signals by excitation with moderate or high optical fluences is linked to the evolution of the population of point defects that is generated in the crystal by absorption of two UV photons. These defects are bleached due to their efficient linear absorption of UV radiation, and their non-radiative relaxation is shown to be the origin of the acoustic signals. The rate constants for the different processes involved in both the linear and the non-linear interactions were determined from the experiments presented here. Characteristic values for the quantum efficiency for the generation of defects, _F = 0.95 ± 0.05, and for the quantum efficiency for bleaching of defects, _B = 0.065 ± 0.005, were obtained for 266-nm laser radiation. The model developed for the intensity of the acoustic signals reproduces the experimental facts with very good accuracy.     

Pulsed mercury capillary lamps for dye laser pumping: Spectral measurements

Calibrated spectral measurements of the light emitted by a pulsed high-pressure mercury capillary lamp are presented. These pulsed lamps have been used in a previous work to pump a Rhodamine 6G laser. The measurements presented here show a very high efficiency in the blue and near UV part of the spectrum, suggesting that these lamps represent a very attractive pumping source for dye lasers emitting in the blue. In particular, as an example, a computation has been made for a basic solution of 4-MU yielding a spectral efficiency of 24%.     

Photophysical properties and laser performance of photostable UV laser dyes

The photophysical properties such as singlet absorption and fluorescence spectra, the fluorescence quantum yield and fluorescence decay time as well as the laser performance data such as the tuning range, the conversion efficiency, and the photochemical stability of 12 sterically hinderedp-quaterphenyls have been measured in ethanol and/or dioxane at room temperature. The sterically hinderedp-quaterphenyls exhibit shorter laser dye emissions in the 330–380 nm range than the parent compoundp-quaterphenyl. The conversion efficiencies of the sterically hinderedp-quaterphenyls range between 1 and 21%. The photochemical stability of the sterically hinderedp-quaterphenyls is by a factor of 10 to 20 better in dioxane than in ethanol. 2-Methyl-5-tert. butyl-p-quaterphenyl and 2,5,2,5-tetramethyl-p-quaterphenyl are among the most stable UV laser dyes known today.     

Spectral dependence of light extraction efficiency of high‐power III‐nitride light‐emitting diodes

Using the recently suggested method of processing the data on external quantum efficiency as a function of output optical power, we have estimated the dependence of light extraction efficiency of high‐power light‐emitting diodes (LEDs) on their emission wavelength varied between 425 nm and 540 nm. The extraction efficiency is found to increase with the wavelength from ～80% to ～85% in this spectral range and to correlate with the wavelength dependence of reflectivity of the large‐area p‐electrode being the essential unit of the LED chip design. The correlation found identifies the incomplete reflection of emitted light from the electrode as the major mechanism eventually controlling the spectral dependence of the efficiency of light extraction from the LEDs.

     

Laser Tunable Source for the Red and Near UV Regions of the Spectrum

The spectral and lasing characteristics of a number of dyes for the red and near IR portions of the lasing spectrum in laser and lamp pumpings are investigated. Smooth tuning of a pulse-periodic lamp-pumped laser in the 660–720 nm range and generation of its second harmonic in the 330–360 nm region were obtained. As a result, the radiation range of the earlier developed tunable laser source has been extended in the red and near UV regions of the spectrum. The use of mixtures of dyes and proton-donating additives in working solutions has substantially improved the energy characteristics of this source.     

Quantum Orbifolds

This is a study of orbifold-quotients of quantum groups (quantum orbifolds \({\Theta } \rightrightarrows G_{q}\)). These structures have been studied extensively in the case of the quantum S U ₂ group. A generalized theory of quantum orbifolds over compact simple and simply connected quantum groups is developed. Associated with a quantum orbifold there is an invariant subalgebra and a crossed product algebra. For each spin quantum orbifold, there is a unitary equivalence class of Dirac spectral triples over the invariant subalgebra, and for each effective spin quantum orbifold associated with a finite group action, there is a unitary equivalence class of Dirac spectral triples over the crossed product algebra. A Hopf-equivariant Fredholm index problem is studied as an application.     

Modeling,design, growth and characterization of red wavelength range microcavity emitters for plastic optic fiber applications

Resonant cavity light emitting diodes (RC-LEDs) and vertical cavity surface emitting lasers (VCSELs) for the red wavelength range are presented in the paper. A wide variety of simulation tools were employed in the device design and optimization and the good agreement between simulations and measurements enabled effective device development. Our red wavelength range RC-LEDs were mainly intended for short-haul communication systems based on polymethyl methacrylate plastic optical fiber and were optimized accordingly. They are achieving, under different structure and working regime variants, high output power (15 mW), high external efficiency (9.5%), record small-signal modulation bandwidth (f _-3dB up to 350 MHz), error-free back-to-back transmission rates beyond 622 Mbits/s, adjustable far-field pattern and good POF coupling efficiency with reasonably wide tolerances and without using auxiliary optics. In view of the possible use of graded-index POFs, free-space transmission and other high bandwidth or high spectral purity applications, VCSELs in the red wavelength range were also realized. They have achieved sub-milliamp room-temperature (RT) continuous-wave (CW) lasing for an 8 m diameter emission window and exhibited an external quantum efficiency of 6.65% for 10 m devices in RT CW operation. The VCSEL structures were far from optimal – not even incorporating all the RC-LED structure refinements – and significant improvement in performance characteristics are predicted for the optimized layer structure.     

Small-signal modulation characteristics for 1.5 μm lattice-matched InGaNAs/GaAs and InGaAs/InP quantum well lasers

The small-signal modulation characteristics of 1.5 m lattice-matched InGaNAs/GaAs and InGaAs/InP quantum well lasers and their temperature dependence have been calculated. It is found that the maximum bandwidth of the InGaNAs/GaAs quantum well lasers is about 2.3 times larger than that of the InGaAs/InP quantum well lasers due to the high differential gain which results from the large electron effective mass in the dilute nitride system. The slope efficiency for the 3 dB bandwidth as a function of optical density is twice as large for InGaNAs/GaAs as for InGaAs/InP quantum well lasers.     

Visible emission from ZnCdO/ZnO multiple quantum wells

Polar c‐axis oriented Zn_0.75Cd_0.25O/ZnO multiple quantum wells (MQWs), grown by pulsed‐laser deposition (PLD), emitting in the visible spectral range are reported. By applying a low growth temperature of ≈300 °C a large Cd content of 0.25 and abrupt interfaces could be achieved using PLD. The emission energy was tuned from the green to the violet spectral range (2.5 eV to 3.1 eV) by tuning the quantum well thickness. It is determined by the quantum confinement effect and the quantum‐confined Stark effect. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

     

Optical and electrical characterizations of 4H-SiC-oxide interfaces by spectroscopic ellipsometry and capacitance-voltage measurements

4H-SiC-oxide interfaces formed by various oxidation methods on SiC (0 0 0 1) Si- and () C-face substrates have been characterized by performing spectroscopic ellipsometry in wide spectral region including deep UV spectral range and capacitance-voltage measurements. The results exhibit that the refractive indices of the interface layers well correlate with interface state density in all the cases of oxidation processes. To investigate the difference in interface characteristics between wet and dry oxidation, we compared to the sample fabricated by wet oxidation followed by heating in Ar or O₂ atmosphere, aiming to remove hydrogen related species at the interface. We also tried to make clear the difference in the interface characteristics between Si- and C-faces by lowering the oxidation rate of C-face down to those for Si-face. Putting together with all of the results obtained, we discuss the origins that determine the interface characteristics in terms of both the optical and electrical characterizations.     

Longitudinal rf discharge in Xe/Cl<Subscript>2</Subscript> mixtures

Results are presented from the studies of the electrical and emission characteristics of the low-temperature plasma of a longitudinal rf (f₀=1.76 MHz) discharge in Xe/Cl₂ mixtures at pressures of 100–800 Pa. The discharge was ignited in a cylindrical quartz tube with an inner diameter of 1.4 cm and interelectrode distance of 3.0 cm. The discharge emission within the spectral range of 190–670 nm is studied. The dynamics of the discharge current and discharge emission at different pressures and compositions of a Xe/Cl₂ mixture are investigated. It is shown that a discharge in a Xe/Cl₂ mixture acts as a wideband excimer-halogen lamp with a cylindrical output aperture emitting in the spectral range of 220–320 nm. The broad plasma emission spectrum is formed due to the overlap of the XeCl(D, B-X; B, C-A) bands that are broadened at low working-gas pressures. The composition of the working mixture is optimized to achieve the maximum power of the wideband UV plasma emission. Longitudinal rf discharges in low-pressure Xe/Cl₂ mixtures are of interest for developing small-size wideband (Δλ=220–450 nm) cylindrical-aperture lamps, whose efficiency can, on average, exceed the efficiency of conventional hydrogen lamps by more than one order of magnitude.     

Highly efficient and bright doped organic electroluminescent diodes using an aluminum electrode

Remarkable improvement in efficiency and electroluminescence (EL) has been observed in an organic EL device, which consists of a hole-transport layer and a luminescent layer. The hole-transport layer is an N,N-bis(3-methyphenyl)-N,N-diphenylbenzidine film. The doped emitting layer consists of 8-(quinolinolate)-aluminum as the host and rubrene as the emission dopant. The doped cell with aluminum cathode demonstrated a luminance in excess of 20,000 cd/m² and an external quantum efficiency of 2.7%, which is about four times and three times, respectively, greater than those of the undoped cell. The EL emission from the device shows spectral narrowing and a shift to higher energy.     

Direct shaping of tunable UV ultra-short pulses

Time shaping of ultra-short UV pulses has been performed using a specially designed acousto-optic programmable dispersive filter in KDP material. A time shaping window up to 5 ps has been obtained in the 260–410 nm range. A temporal/spectral characterization of shaped pulses on this broad UV spectral range is presented. The efficiency at shorter wavelength is limited by two-photon absorption. However several µJ is obtained at the output of the device allowing chemistry coherent control experiment.     

Determination of the Thermodynamic Characteristics of Complexes of Chlorosubstituted Anilines with Proton Acceptors

The influence of temperature on the spectral parameters of the absorption bands (NH) of free and hydrogen-bonded molecules of chlorosubstituted anilines with various proton acceptors in a CCl₄ medium has been studied in a temperature range of 290–330 K. The temperature dependence of the monomer–complex equilibrium constants has been investigated and the thermodynamic characteristics of the complexes have been calculated.     

New Series of Dyes for Flashlamp-Excited Lasers

A new series of laser compounds which are derivatives of 7H-benzo[4, 5]imidazo[1, 2-c]chromeno[3, 2–e]pyrimidine have been developed and their working characteristics are presented. It is shown that the new compounds fluoresce with a high quantum yield (0.83–0.97) in the green spectral region; their methanol solutions gave effective lasing upon flashlamp excitation with a pulse duration of about 2.0 sec at the level 0.5. In a nonselective cavity they emit laser radiation in the range 540–570 nm. The influence of the molecular structure of the investigated dyes on their fluorescence and laser characteristics is discussed.     

An even magneto-optic effect: Linear polarization of the spectral thermal emission from ferromagnetic iron

The thermal spectral emission from ferromagnetic iron observed at normal incidence and with the magnetization parallel to its surface is partially linearly polarized. Measurements of the new even magneto-optic effect have been performed in the spectral range from 1.8 m to 5 m wave-length.     

Explicit inertial range renormalization theory in a model for turbulent diffusion

The inertial range for a statistical turbulent velocity field consists of those scales that are larger than the dissipation scale but smaller than the integral scale. Here the complete scale-invariant explicit inertial range renormalization theory for all the higher-order statistics of a diffusing passive scalar is developed in a model which, despite its simplicity, involves turbulent diffusion by statistical velocity fields with arbitrarily many scales, infrared divergence, long-range spatial correlations, and rapid fluctuations in time-such velocity fields retain several characteristic features of those in fully developed turbulence. The main tool in the development of this explicit renormalization theory for the model is an exact quantum mechanical analogy which relates higher-order statistics of the diffusing scalar to the properties of solutions of a family ofN- body parabolic quantum problems. The canonical inertial range renormalized statistical fixed point is developed explicitly here as a function of the velocity spectral parameter, which measures the strength of the infrared divergence: for<2, mean-field behavior in the inertial range occurs with Gaussian statistical behavior for the scalar and standard diffusive scaling laws; for>2 a phase transition occurs to a fixed point with anomalous inertial range scaling laws and a non-Gaussian renormalized statistical fixed point. Several explicit connections between the renormalization theory in the model and intermediate asymptotics are developed explicitly as well as links between anomalous turbulent decay and explicit spectral properties of Schrödinger operators. The differences between this inertial range renormalization theory and the earlier theories for large-scale eddy diffusivity developed by Avellaneda and the author in such models are also discussed here.