We demonstrate that Franz-Keldysh oscillations (FKOs) observed by photoreflectance (PR) spectroscopy are highly sensitive to the surface morphology of Alx Ga1−x N layers in Alx Ga1−x N heterostructures. Three Al0.2Ga0.8N/GaN heterostructures with different surface-morphology profiles, which are confirmed with atomic force microscopy, have been investigated. The X-ray-diffraction patterns are hardly affected by the Al0.2Ga0.8N/GaN-layer morphology. In contrast, it is revealed that cracks and pits dominating the morphology remarkably reduce the amplitude of the FKOs from the Al0.2Ga0.8N/GaN layer, which is attributed to the following two mechanisms related to the cracks and pits. One is lifetime broadening due to carrier scattering, and the other is the suppression of the modulation magnitude for the built-in electric field, which is caused by the trapping and recombination of photogenerated carriers at the surface.

Temperature and frequency dependence of a.c. conductivity have been studied in glassy Se70Te30−x Sbx (2 ≤ x ≤ 10). An agreement between experimental and theoretical results suggests that the a.c. conductivity behaviour of Se-Te-Sb system can be successfully explained by correlated barrier hopping (CBH) model. The observation of Meyer-Neldel rule in case of a.c. conductivity at high temperatures is also reported for glassy Se70Te30−x Sbx (2 ≤ x ≤ 10) alloys.

This paper presents a commercial high voltage thyristor used as a switch allowing a tank capacitor to discharge in a load. In classical high power pulse technology applications the output voltage pulse has to be characterized mainly by its crest value, its rise-time, the period the thyristor is held in the on-state and the fall-time. These parameters are studied as a function of the power circuit and of the trigger circuit. The thyristor presents two behaviours: the main current is either higher or lower than the latching current. The “low current” behaviour is extensively investigated as it allows repetitive operation of the device. Two pulse power applications triggering electrical discharges are presented. Each one necessitates a specific pulsed power supply using series thyristor stacks or Marx structures. The first pulsed source delivers negative pulses with a crest voltage $V_{oM}=-35$  kV, a turn on capability of $T_{r}=90$  ns and a repetition rate F = 900 Hz. The second is built using Marx structure and is characterized by $V_{oM}=60$  kV, $T_{r}=250$  ns, F = 900 Hz.

Electrical and photoelectrical measurements are made at different temperatures on heterojunction photovoltaic cells fabricated by vacuum deposition of n-(Ga2Se3-Ga2S3) thin films onto p-Si single crystals. A complete study of the current density as a function of voltage and temperature is carried out in order to gain fundamental information on trap depth, trap distribution and position of the Fermi level. At high forward bias, the results are consistent with space-charge-limited conduction due to an exponentially decreasing distribution of traps. At low voltages, the dark current density in the forward direction varies exponentially with voltage. The devices exhibit strong photovoltaic characteristics with an open-circuit voltage of 0.70 V, a short-circuit current density of 9.42 mA cm−2 and a power conversion efficiency of 5.20%. These parameters have been estimated at room temperature and under light illumination provided by a halogen lamp with an input power density of 50 mW cm−2.

We have demonstrated debundling of molydenym-sulphur-iodine nanowires simply by diluting nanowire dispersions in isopropanol. Using atomic-force-microscopy we observe the bundle diameter distribution to decrease dramatically with concentration. Detailed analysis of the data suggests the presence of an equilibrium bundle number density. The population of individual nanowires increases with decreasing concentration until almost half of all dispersed objects are individual nanowires at a concentration of 4 × 10−3 mg/ml. The partial concentration of individual nanowires peaks at a concentration of ~7 × 10−3 mg/ml. This debundling also occurs spontaneously without the input of sonic energy, suggesting thermodynamic solubility. The absorbance of the nanowire dispersions, measured in the visible region increases linearly with concentration indicating a concentration independent absorption coefficient. However, for the infra-red feature that has been associated with band edge transitions, the absorption coefficient increases with increasing concentration for both stoichiometries. This suggests that this transition may be quenched by the inter-nanowire interactions associated with bundling. Finally, nanowire re-aggregation can be induced by the addition of small quantities of non-solvents.

High resolution spectroscopy of quantum systems is a key point in the quantum information field. Rare earth ions in crystals are interesting candidates for this application because of the long coherence lifetime of some of their transitions. These ions can also be manipulated by optical excitation which has, however, to be produced by ultra-stable laser sources. This proves to be a very difficult task in the case of dye lasers which are necessary to excite Pr3+ doped crystals and especially Pr3+:Y2SiO5. This compound is by far the most used host in rare earth based quantum information studies. In this paper, we discuss the use of Pr3+,Yb3+ codoped materials to build an infrared pumped solid state laser suitable to excite Pr3+:Y2SiO5 around 606 nm. We show by the analysis of a rate equation model that avalanche upconversion is not very efficient to obtain a high power laser. This is more easily obtained if a second laser is set to pump resonantly Yb3+ ions. The spectroscopic properties of a new matrix Pr3+,Yb3+:PbF2 are also investigated. We found that this compound emits at 606.18 nm with a width of 5 nm and would be therefore suitable to excite Pr3+:Y2SiO5. Moreover, it can be excited at 857 nm and 975 nm, i.e. in the range of high power laser diodes.

A free electron laser (FEL) amplifier with a curved parallel plate waveguide and a planar wiggler is presented. A set of operating equations for this FEL amplifier is derived by using the three-dimensional nonlinear theory. The characteristics including the evolution of power, efficiency and bandwidth of the FEL operating at the frequency of 94 GHz are numerically analyzed. The effects of electron energy spread and wiggler taper on saturation efficiency are also studied.

Small form factor optical disk (SFFOD) with a diameter of 27.4 mm was prepared for a mobile application. Near-field recording (NFR) technology with a flying optical head is applied to SFFOD for high density and small volume. A problem of contamination was observed at the head-disk interface (HDI) for first-surface recording. As a solution of the problem, a cover-layer was laminated onto the disk to move the focal plane into the media, which could solve the contamination problem. An edge bead free cover-layer could be coated with UV curable resin by using an outer-ring technique during the spin coating process. A diamond-like carbon (DLC) film and a lubricant film were coated on the disk to reduce the HDI problems.

The determination of the optical trajectories on which the radiative energy is transported has been achieved by solving the anisotropic Fermat's principle in a sphere of uniaxial semi-transparent material when the dielectric perpendicular and parallel permittivities are space dependant; first established for the general case of an optical axis function of the location inside the medium, it has completely been solved for the particular case of a constant optical axis with a constant anisotropy factor when the perpendicular permittivity is a function of the radius; the calculation shows a lack of the spherical symmetry due to the presence of the optical axis which induces non planar trajectories contrarily to the isotropic case.

An algorithm for generating and connecting 3-D nonconforming mesh with Finite Volume Method FVM is developed. The accuracy and computational performance using the nonconforming mesh technique are compared with those using the conventional conforming mesh. The algorithm is applied to solve a 3-D magnetostatic workshop problem proposed by IEEJ. It is shown that the memory requirements and CPU time using the nonconforming mesh can be decreased to about 1/2 of those using the conforming one. Moreover, the accuracy of the magnetic flux density is significantly improved.

Nano-crystalline magnetic alloys of Fe-Ni and Fe-Co are fabricated by high-energy milling. The coexistence of bcc and fcc lattice structures is observed in some of Fe-Ni alloys, as opposed to Fe-Co ones which exhibit the bcc phase only. The grain sizes increase linearly with the lattice strains for Fe-Co, and exponentially so for Fe-Ni. The cut-off frequency decreases with increasing nickel content in Fe-Ni alloys, but increases with increasing cobalt content in Fe-Co alloys. The larger the cut-off frequency is, the smaller the initial permeability. Resonant behaviour is observed in the pure Fe and Fe-Ni samples, which is replaced by relaxant behaviour in Fe-Co, implying that the Co plays an important role in hindering the resonance. The initial permeability at 10 kHz varies inverse proportionally with the coercivity H c . It reaches a maximum while H c goes to a minimum at 7.69 at.% Ni or Co. It is interesting that average atomic moments of nanocrystalline alloys are inconsistent with the Slater-Pauling relation.

PbO-Sb2O3-B2O3 glasses containing different concentrations of MoO3 ranging from 0 to 5 mol % were prepared. A number of studies have been carried out on these glasses comprising differential thermal analysis, infrared and optical absorption, Raman and ESR spectra, magnetic susceptibility and measurements of dielectric properties (constant ε′, loss tan δ and a.c. conductivity $\sigma_{ac}$ over a range of frequency and temperature) of these glasses. The results have been analyzed in terms of different oxidation states of molybdenum ions. The analysis indicates that when the concentration of MoO3 is ≤0.6 mol %, molybdenum ions exist mostly in the Mo6+ state, occupy network forming positions with MoO $_{4}^{2-}$ structural units and increase the rigidity of the glass network. When MoO3 is present in higher concentrations, molybdenum ions seem to exist mostly in the Mo5+ state and occupy modifying positions.

Positron annihilation was used to characterize vacancy-type defects in two types of polycrystalline Si grown at temperatures above ~800 °C by chemical vapour deposition. The majority of vacancies (80%) consisted of monovacancies, and their thermal stability indicated them to be trapped at grain boundaries or at dislocations. Annealing above 500 °C caused a significant reduction in the monovacancy concentration, and an increase in divacancy concentration. Divacancies started to anneal above 1200 °C. Measurements between 8 and 293 K indicated that vacancies were neutral before as well as after annealing at 1380 °C. Fz-grown Si from one of these materials contained vacancy clusters with an average size of six to ten vacancies which persisted to 1380 °C. The cluster concentration corresponded to a monovacancy concentration of 1015 to 1016 cm−3, which is at least one order of magnitude larger than estimates based on voids [R. Falster, V.V. Voronko, F. Quast, Phys. Status Solidi B222, 219 (2000)].

Thermal stability of amorphous system xFe2O $_{3}(100-x)$ [ 20Na2O $\cdot $ 30CaO $\cdot $ 50P2O5] , with $x \le20$  mol%, was investigated by differential thermal analysis (DTA). Critical crystal radii are evaluated using the molar volumes and the thermal glass stabilities. Partial crystallisation of samples was obtained by heat treatment carried out for 24 hours at two temperatures. The sites occupied by iron in glass and partially crystallised samples are characterised according to electron paramagnetic resonance (EPR) results.

This paper deals with the guided ultrasonic propagation in the tri-layer structures. The propagation model and the procedure used to determine the dispersion curves and the displacement field in the structure are those presented earlier in B. Bougaze et al. [Eur. Phys. J. Appl. Phys. 32, 207 (2005)] and tested for the case of (aluminium/adhesive/aluminium) structure. Here, we consider the case of a tri-layer structure presenting two different adherents: copper and steel. We adjusted our programs for this case and determined the dispersion curves and the displacement field in the structure. The obtained dispersion curves are then used to reconstruct the material properties of the tri-layer structure via a proposed numerical inverse method based on an optimization process. These material properties include the longitudinal and shear wave velocities as well as the density of each layer of the structure. The agreement between the reconstructed and the exact material properties is very well demonstrating the efficiency of the proposed numerical inverse method.

This paper presents a numerical study of the Reynoldsnumber and scaling effects in microchannel flows. The configurationincludes a rectangular, high-aspect ratio microchannel with heatsinks, similar to an experimental setup. Water at ambienttemperature is used as a coolant fluid and the source of heating isintroduced via electronic cartridges in the solids. Two channelheights, measuring 0.3 mm and 1 mm are considered at first. TheReynolds number varies in a range of 500–2200, based on thehydraulic diameter. Simulations are focused on the Reynolds numberand channel height effects on the Nusselt number. It is found thatthe Reynolds number has noticeable influences on the local Nusseltnumber distributions, which are in agreement with other studies. Thenumerical predictions of the dimensionless temperature of the fluidagree fairly well with experimental measurements; however thedimensionless temperature of the solid does exhibit a significantdiscrepancy near the channel exit, similar to those reported byother researchers. The present study demonstrates that there is asignificant scaling effect at small channel height, typically ≤0.3 mm, in agreement with experimental observations. This scalingeffect has been confirmed by three additional simulations beingcarried out at channel heights of 0.24 mm, 0.14 mm and 0.1 mm,respectively. A correlation between the channel height and thenormalized Nusselt number is thus proposed, which agrees well withresults presented.