Levelling the focal spot intensity of the focused gaussian beam

Abstract

It is experimentally and numerically shown that a simple binaryphase optical element can be used for levelling the light energy in the focal plane of the focused Gaussian beam, generating a square-shaped focal beam, and transforming the Gaussian beam into a uniform beam which preserves its radius at a definite length of its path.     

Task assignment in loosely‐coupled multiprocessor systems

Abstract

In loosely‐coupled multiprocessor systems, a parallel program has its modules distributedly assigned among the processors. The assignment policy is to minimize interprocessor communication cost and to balance the workload of processors. However, there exists conflict between these two criteria and a compromise must be made to obtain an optimal solution. In this paper, we propose a new task assignment model for distributed computing systems, and for solving the assignment problem based on partitioning graphs. The problem of finding an optimal solution had been shown to be in the class of NP‐complete. For the sake of computation efficiency, we propose some heuristics for obtaining suboptimal solutions.     

Characterization of Bi and Fe co-doped PZT capacitors for FeRAM

Abstract

Ferroelectric random access memory (FeRAM) has been in mass production for over 15 years. Higher polarization ferroelectric materials are needed for future devices which can operate above about 100 °C. With this goal in mind, co-doping of thin Pb(Zr₄₀,Ti₆₀)O₃ (PZT) films with 1 at.% Bi and 1 at.% Fe was examined in order to enhance the ferroelectric properties as well as characterize the doped material. The XRD patterns of PZT-5% BiFeO₃ (BF) and PZT 140-nm thick films showed (111) orientation on (111) platinized Si wafers and a 30 °C increase in the tetragonal to cubic phase transition temperature, often called the Curie temperature, from 350 to 380 °C with co-doping, indicating that Bi and Fe are substituting into the PZT lattice. Raman spectra revealed decreased band intensity with Bi and Fe co-doping of PZT compared to PZT. Polarization hysteresis loops show similar values of remanent polarization, but square-shaped voltage pulse-measured net polarization values of PZT-BF were higher and showed higher endurance to repeated cycling up to 10¹⁰ cycles. It is proposed that Bi and Fe are both in the +3 oxidation state and substituting into the perovskite A and B sites, respectively. Substitution of Bi and Fe into the PZT lattice likely creates defect dipoles, which increase the net polarization when measured by the short voltage pulse positive-up-negative-down (PUND) method.     

Using the Finite Element Method to Solve Coupled Wave Equations in Volume Holograms

Abstract

Galerkin's finite element method has been used to solve the coupled wave equations in a volume hologram. The expressions which transform the coupled wave equations into a matrix equation are given. Numerical examples show that the method is very accurate and efficient.     

Spontaneous emission from a double V-type four-level atom in a double-band photonic crystal

Abstract

Quantum interferences in spontaneous emission from a double V-type four-level atom in a double-band photonic crystal have been investigated. The double V-type transitions from the two upper levels to the two lower levels can interact not only with modes near the edges of a double-band photonic band gap, but also with free vacuum modes. The resulting two types of quantum interferences give rise to not only dark lines, but also a narrow spontaneous line in the spectrum from the transition coupled to the free vacuum modes. The dark lines and narrow spontaneous line are shown to be dependent on the width of the forbidden gap, relative position of the upper levels from the band edges, coupling constants, and initial coherent superposition state of the system.     

Tunneling Photocurrent Assisted by Interlayer Excitons in Staggered van der Waals Hetero‐Bilayers

Vertically stacked van der Waals (vdW) heterostructures have been suggested as a robust platform for studying interfacial phenomena and related electric/optoelectronic devices. While the interlayer Coulomb interaction mediated by the vdW coupling has been extensively studied for carrier recombination processes in a diode transport, its correlation with the interlayer tunneling transport has not been elucidated. Here, a contrast is reported between tunneling and drift photocurrents tailored by the interlayer coupling strength in MoSe₂/MoS₂ hetero‐bilayers (HBs). The interfacial coupling modulated by thermal annealing is identified by the interlayer phonon coupling in Raman spectra and the emerging interlayer exciton peak in photoluminescence spectra. In strongly coupled HBs, positive photocurrents are observed owing to the inelastic band‐to‐band tunneling assisted by interlayer excitons that prevail over exciton recombinations. By contrast, weakly coupled HBs exhibit a negative photovoltaic diode behavior, manifested as a drift current without interlayer excitonic emissions. This study sheds light on tailoring the tunneling transport for numerous optoelectronic HB devices.     

Photonic Band Calculations for Woodpile Structures

Abstract

Photonic band structure has been computed for ‘woodpile’ structures having the periodicity of the simple tetragonal lattice. Bandgaps have been found. Further research directions are explored.     

Optical properties of samarium doped zinc-tellurite glasses

Glasses with the composition, (Sm₂ O ₃) _x (ZnO)_(40-x)(TeO ₂)₍₆₀₎, were prepared by conventional melt quenching method. The density, molar volume, and optical energy band gap of these glasses have been measured. The refractive index, molar refraction and polarizability of oxide ion have been calculated by using Lorentz-Lorentz relations. Optical absorption spectra of these glasses were recorded in the range 300–700 nm at room temperature. The oxide ion polarizabilities deduced from two different quantities, viz. refractive index and optical energy band gap, agree well compared with other glasses. The nonlinear variation of the above optical parameters with respect to samarium dopant has been explained.     

Mixed Valence Perovskite Cs2Au2I6: A Potential Material for Thin‐Film Pb‐Free Photovoltaic Cells with Ultrahigh Efficiency

New light is shed on the previously known perovskite material, Cs₂Au₂I₆, as a potential active material for high‐efficiency thin‐film Pb‐free photovoltaic cells. First‐principles calculations demonstrate that Cs₂Au₂I₆ has an optimal band gap that is close to the Shockley–Queisser value. The band gap size is governed by intermediate band formation. Charge disproportionation on Au makes Cs₂Au₂I₆ a double‐perovskite material, although it is stoichiometrically a single perovskite. In contrast to most previously discussed double perovskites, Cs₂Au₂I₆ has a direct‐band‐gap feature, and optical simulation predicts that a very thin layer of active material is sufficient to achieve a high photoconversion efficiency using a polycrystalline film layer. The already confirmed synthesizability of this material, coupled with the state‐of‐the‐art multiscale simulations connecting from the material to the device, strongly suggests that Cs₂Au₂I₆ will serve as the active material in highly efficient, nontoxic, and thin‐film perovskite solar cells in the very near future.     

On the optical and electrical properties of rf and a.c. plasma polymerized aniline thin films

Polyaniline is a widely studied conducting polymer and is a useful material in its bulk and thin film form for many applications, because of its excellent optical and electrical properties. Pristine and iodine doped polyaniline thin films were prepared by a.c. and rf plasma polymerization techniques separately for the comparison of their optical and electrical properties. Doping of iodine was effectedin situ. The structural properties of these films were evaluated by FTIR spectroscopy and the optical band gap was estimated from UV-vis-NIR measurements. Comparative studies on the structural, optical and electrical properties of a.c. and rf polymerization are presented here. It has been found that the optical band gap of the polyaniline thin films prepared by rf and a.c. plasma polymerization techniques differ considerably and the band gap is further reduced byin situ doping of iodine. The electrical conductivity measurements on these films show a higher value of electrical conductivity in the case of rf plasma polymerized thin films when compared to the a.c. plasma polymerized films. Also, it is found that the iodine doping enhanced conductivity of the polymer thin films considerably. The results are compared and correlated and have been explained with respect to the different structures adopted under these two preparation techniques.     

Characteristics of shear banding in dual phase steel

Abstract

The evolution process of shear banding in a ferrite–martensite dual phase steel has been investigated via in situ tensile testing in a scanning electron microscope. Shear band type deformation localisation occurs at the maximum loading point of uniaxial tension. Necking occurs simultaneously and locally. Voids nucleate in ferrite domains and on the interfaces between the two component phases or grain boundaries. The void volume fraction is greater within the shear band than away from the band and is greater in the interior than at the specimen surface. The results also show that void damage promotes the initiation of shear bands and the development of shear banding stimulates further void damage.

MST/1819     

Photonic Bands

Abstract

Photonic band structure has been computed using ellipsoidal grains in f.c.c. lattice. Bandgaps have been found and the conditions for the appearance of such gaps are discussed. The effective long-wavelength dielectric constants for the ordinary and the extraordinary rays are calculated and compared with the predictions of effective medium and Maxwell-Garnett theories.     

Superluminal tunnelling times as weak values

Abstract

We consider the tunnelling particle as a pre- and post-selected system and prove that the tunnelling time is the expectation value of the position of a ‘clock’ degree of freedom weakly coupled to it. Such a value, called a ‘weak value’, typically falls outside the eigenvalue spectrum of the operator. The appearance of unusual weak values has been associated with a unique interference structure called ‘superoscillations’ (band-limited functions which on a finite interval, approximate functions with spectra well outside their band). It is proposed that superoscillations play an important role in the interferences which give rise to superluminal effects. To demonstrate that, we consider a certain simple tunnelling barrier which allows a wave packet to travel in zero time and negligible distortion, a distance arbitrarily longer than the width of the wave packet. The peak is shown to result from a superoscillatory superposition at the tail. Similar reasoning applies to the dwell time. For this system, both the Wigner time (related to the group velocity) and a clock time correspond to superluminal velocities.     

On the Action of Ultraviolet Light on C70 Fullerene

Abstract

C₇₀ was photopolymerized in solution of CCl₄ under nitrogen flow by an high pressure mercury lamp. The resulting photoproduct was still soluble in common solvents and has been studied by electronic and FT-IR spectroscopy. FT-IR spectroscopy reveals a very peculiar band pattern never reported to date for C₇₀ photoproduct. Since this band pattern is very similar to that of C₆₀ photopolymer, it suggests that C₇₀ photoproduct should have a chemical structure very close to that assigned to C₆₀ photopolymer and piezopolymer.

The role played by CCl₄ in C₇₀ photopolymerization seems to be comparable to that already reported for C₆₀, i.e. it acts as a polymerization promoter.     

CONTINUOUS COOLING OF WET FOUNDRY SAND USING A FLUIDIZED BED

Abstract

In order to establish the optimum cooling system for hot returned sand in metal casting processes, a mathematical model including simultaneous heat and mass transfer has been constructed. The present study is especially directed towards clarifying the dynamical behavior of sand cooling in a one-pass fluidized bed. A series of continuous cooling experiments has been made for various conditions. Characteristics of both transient and steady-state bed temperature are explained reasonably by the present model. Furthermore, a possible proposal has been given on the optimal operation, which raises heat exchange efficiency in fluidization cooling of hot molding sand.     

Spectroscopic investigations of CdS nanoparticles in sol-gel derived polymeric thin films and bulk silica matrices

Absorption, photoluminescence and Raman scattering spectroscopies have been performed on two kinds of SiO₂ bulk matrices and sol-gel polymeric thin films, containing CdS nanoparticles. Waveguiding in the polymeric CdS films has been demonstrated for wavelengths in the visible region. A simple surface energy diagram, including defect levels, has been used to explain the size-selective luminescence of the nanocrystals. The luminescence spectrum is composed of an excitonic recombination band and a broader red-shifted band, imputed to the transition between the surface trap levels. The particle interaction with the silica xerogels appears to enhance these surface states. Resonance Raman scattering shows that the excitons are coupled to LO-phonon modes, but the spectra exhibit no matrix effect on the frequency vibration of the fundamental LO phonon.     

Post-deposition treatment behaviour of zinc oxide thin films in hydrogen peroxide solution

Abstract

Zinc oxide thin films with c axis orientation were grown on Si (100) by radio frequency magnetron sputtering in mixture of argon and oxygen environment using Zn target. These films are treated in hydrogen peroxide (H₂O₂) solution at room temperature for one hour and two hours. During this study it has been found that post-deposition treatment of ZnO films improves the film quality. Structural, electrical and optical properties have been compared before and after H₂O₂ treatment by X-ray diffraction analysis, ac conductivity, band gap and refractive index. The full width at half maximum decreases after post-deposition process, which improves the crystal quality. The relief in stress has been observed but films are not fully stress free. The film after treatment becomes highly insulating having resistivity of the order of 10¹⁴ Ω cm^?1 and can be used for piezoelectric applications. The increase in the band gap and refractive index, near to bulk value, has been observed after post-deposition treatment indicating the increase in grain size and crystal quality.     

A simulation study on the effect of spring-shaped fillers on the viscoelasticity of rubber nanocomposite

Background/PurposeRubber nanocomposites have been widely used in many engineering fields due to their unique properties such as high elasticity and viscoelasticity. Much attention has been paid to the viscoelasticity of rubbers because it directly relates to the performance of the rubber products.MethodsBased on the micromechanical theory, the finite element method is used to analyze the effect of elastic modulus and volume content of spring-shape nanofillers on the dynamic viscosity of composites.ResultsThe simulation results show that there is an optimal elastic modulus of spring-shape nanofillers to make the loss factor a minimum. There is a threshold value of spring-shape nanofiller content for the dissipation energy density of composite.ConclusionThe elastic modulus of spring-shape nanofillers has a large effect on the loss factor of composites. The selection of elastic modulus of spring-shape nanofillers is critical for applications of composites. The efficiency of spring-shape nanofillers in reducing the dynamic viscosity of composites is so high that volume content of spring-shape nanofillers as low as 0.1% can greatly reduce the loss factor of composites with bonding interface.     

Copper Chloride Nonlinear Optical Absorption under Quantum Confinement

Abstract

With the aid of two independently tunable lasers the influence has been studied of optical excitation on light absorption in the Z₃-band of CuCl crystals with size 25–75 Å in a glass matrix. It has been found that resonance pumping leads to a significant blue shift of the band caused by the interaction of excitations under quantum confinement. Interband excitation with intensity up to 0·3 GW cm^?2 does not affect the exciton absorption.     

Determination of transition temperatures during freezing and melting of interdendritic phases in Ni based superalloys

Abstract

Differential scanning calorimetry (DSC) coupled with microstructural analysis has been used to characterise the freezing and melting response in a typical Ni based superalloy, CMSX10K. Transition temperatures and evolution of fraction solid (or liquid) during freezing and melting are determined from enthalpy considerations. Significant differences in heat capacity (C_P) curves exist between the heating and cooling. It is shown that differences between freezing and melting kinetics occur because of undercooling, which is most significant beyond the solidus temperature up to the onset of eutectic freezing. The correlation of DSC heating and cooling curves to quantitatively characterise directional solidification has been demonstrated.