A structural analysis of ultrathin barrier (In)AlN/GaN heterostructures for GaN-based high-frequency power electronics

Metal–organic chemical vapor deposition (MOCVD) is one of the best growth methods for GaN-based materials as well-known. GaN-based materials with very quality are grown the MOCVD, so we used this growth technique to grow InAlN/GaN and AlN/GaN heterostructures in this study. The structural and surface properties of ultrathin barrier AlN/GaN and InAlN/GaN heterostructures are studied by X-ray diffraction (XRD) and atomic force microscopy (AFM) measurements. Screw, edge, and total dislocation densities for the grown samples have been calculated by using XRD results. The lowest dislocation density is found to be 1.69 × 10⁸ cm⁻² for Sample B with a lattice-matched In_0.17Al_0.83N barrier. The crystal quality of the studied samples is determined using (002) symmetric and (102) asymmetric diffractions of the GaN material. In terms of the surface roughness, although reference sample has a lower value as 0.27 nm of root mean square values (RMS), Sample A with 4-nm AlN barrier layer exhibits the highest rough surface as 1.52 nm of RMS. The structural quality of the studied samples is significantly affected by the barrier layer thickness. The obtained structural properties of the samples are very important for potential applications like high-electron mobility transistors (HEMTs).     

Accurate Turbine Inertia Measurement

High fidelity power measurements in free wheel devices require accurate inertia measurements. To evaluate the turbine efficiency, a new experimental technique to measure the momentum of inertia has been developed at the von Karman Institute (VKI). This experimental methodology allows the determination of the inertia of complex shape bodies without being dismounted from their rotating housing.     

Aero-thermal investigation of a multi-splitter axial turbine

This paper reports the external convective heat transfer in an innovative low-pressure vane, designed with a multi-splitter configuration. Three aerodynamic airfoils are positioned in between larger structural vanes, replacing struts presently used in current aero-engines, which results in superior aerodynamic performance. Static pressure and heat flux measurements were carried out in the large compression tube facility of the von Karman Institute, using pneumatic taps and single-layered thin film gauges respectively. The steady and unsteady heat transfer distributions were obtained at representative conditions of modern aero-engines, with M_2,is close to unity and a Reynolds number of approximately 10⁶. This facility is specially suited to control the gas-to-wall temperature ratio that drives transition mechanisms. The heat transfer across the multi-splittered passages is confronted with correlations on ducts to further characterize the boundary layer status. The data will be used to guide code developers by verifying their boundary layer transition models, and designers by showing the areas of the vane where heat transfer is most sensitive to the off-design conditions.     

Highly efficient widely tunable subpicosecond double mode-locked laser

A high average power (≈150 mW) subpicosecond (0.5 ps) passively double mode-locked dye laser is described. Two synchronized cw pulse trains are generated which are independently tunable over broad wavelength ranges (≈600 Å).     

Theory of synchronously pumped dye lasers

An analysis of cw synchronously pumped dye lasers is presented. Under the assumption that the cavity (tuning element) bandwidth is much wider than the bandwidth of the transform limited pulses generated, the pulse forming dynamics is rigorously treated. It is shown that for a finite mismatch between the lengths of the dye and the pump lasers, a steady-state pulse develops in the dye laser cavity with a conserved pulseshape. The characteristics (energy, shape, peak power, duration) of these pulses of ultimate width are quantitatively determined as a function of cavity mismatch. An analytical solution for the pulse envelope is determined, which yieldsI(t)∝Sech²(t/t _p ) to a good approximation.     

Comparing Singlet Oxygen Generation of Schiff Base Substituted Novel Silicon Phthalocyanines by Sonophotochemical and Photochemical Applications

Although the sonophotodynamic method has an effective therapeutic outcome for anticancer treatment compared with the photodynamic method, there are not enough related studies in the literature and this study aims to contribute to the development of sonophotodynamic studies. For this purpose, the Schiff base substituted silicon phthalocyanines were designed and synthesized as effective sensitizer candidates and the photophysicochemical and sonophotochemical features of the phthalocyanines were examined to increase singlet oxygen efficiency. The calculated Φ_Δ values indicate that the contribution of substituent groups improved the production of singlet oxygen compared with silicon (IV) phthalocyanine dichloride (SiPcCI₂) and also the sonophotochemical applications increased the singlet oxygen yields. The Φ_Δ values (Φ_Δ = 0.76 for axially bis-{4-[(E)-(pyridin-3-ylimino)methyl]phenol} substituted silicon (IV) phthalocyanine ( 2a ), 0.68 for axially bis-4-[(E)-{[(pyridin-3-yl)methyl]imino}methyl]phenol substituted silicon (IV) phthalocyanine ( 2b ) in photochemical study) reached to Φ_Δ = 0.98 for 2a , 0.94 for 2b in sonophotochemical study. This article will enrich the literature on increasing singlet oxygen yield.     

A rapid-scanning autocorrelation scheme for continuous monitoring of picosecond laser pulses

We describe a scheme for rapidly introducing a periodic linear time delay to a train of picosecond laser pulses. By incorporating this scheme in one arm of the Michelson interferometer of a conventional autocorrelator, the second order intensity autocorrelation function of a cw train of picosecond pulses is continuously displayed on an oscilloscope.     

Picosecond pulses from passively mode-locked dye lasers

Successive stages of the development of ultrashort pulses in passively mode-locked dye lasers are studied. It is numerically shown that the initial fluctuations evolve into a picosecond pulse which retains its magnitude and particular shape and moves slower than the speed of light.     

Novel peripherally substituted zinc phthalocyanine: synthesis,characterization, investigation of photophysicochemical properties and theoretical study

The new thiohexanoic acid substituted zinc phthalocyanine was synthesized and characterized by FT-IR, ¹H–NMR, electronic spectroscopy, and mass spectrometry as well as DFT calculation studies. The photochemical properties (singlet-oxygen quantum yields and photodegradation quantum yields) and photophysical properties (fluorescence quantum yields and fluorescence behavior) of the compound were studied in dimethylsulfoxide (DMSO), dimethylformamide (DMF) and tetrahydrofuran (THF). Singlet-oxygen quantum yields ranged from 0.29 to 0.43. However, energy-minimized structure, vibrational frequency, electronic distribution and molecular orbitals were obtained by DFT calculations which were supported by experimental results.     

Determination of vigabatrin in human plasma and urine by high-performance liquid chromatography with UV-Vis detection

A simple and reliable high-performance liquid chromatographic (HPLC) method with UV-Vis detection has been developed and validated for the determination of vigabatrin (VG) in human plasma and urine. The samples were pre-column derivatizated with 1,2-naphthoquinone-4-sulphonic acid sodium salt (NQS). A good chromatographic separation was achieved on a C18 column with a mobile phase consisting of acetonitrile and 10 mM orthophosphoric acid (pH 2.5) gradient elution. Tranexamic acid was used as an internal standard (I.S.). The method was linear over the concentration range of 0.8-30.0 microg/ml for both samples. The method is precise (relative standard deviation (R.S.D.) <9.13%) and accurate (relative mean error (RME) <-8.75%); analytical recoveries were 81.07% for plasma and 83.05% for urine. The assay was applied to pharmacokinetic study in a healthy volunteer after a single oral administration of 1 g of vigabatrin.