Showing papers by "Defence Research Agency published in 1992"
TL;DR: Comparison with effective-mass theory shows that the latter is valid for wires wider than 23 A, and first-principles pseudopotential calculations for H-terminated Si wires with thicknesses from 12 to 23 A are performed.
Abstract: We have performed first-principles pseudopotential calculations for H-terminated Si wires with thicknesses from 12 to 23 \AA{}, calculating the band gaps and optical matrix elements. Comparison with effective-mass theory shows that the latter is valid for wires wider than 23 \AA{}. We have used our data to analyze the luminescent properties of highly porous Si fabricated by electrochemical etching of Si wafers in HF-based solutions.
365 citations
TL;DR: The experimental setup consists of an external source of correlated photon pairs which propagate to two widely separated unbalanced Mach-Zehnder interferometers and can be fully modulated by phase plates in either interferometer.
Abstract: We propose an experimental realization of cryptographic-key-sharing scheme exploiting quantum correlations between pair photons. Our experimental setup consists of an external source of correlated photon pairs which propagate to two widely separated unbalanced Mach-Zehnder interferometers. The probability of detection of photon pairs in any two outputs of the interferometers can be fully modulated by phase plates in either interferometer.
288 citations
TL;DR: In this paper, the morphology of x = 0.20−0.26 Si1−xGex alloy layers during vapour phase epitaxial (VPE) growth at 610-750 °C using in situ laser light scattering (LLS) and ex situ atomic force microscopy (AFM).
186 citations
TL;DR: In this paper, the use of an amphiphilic anion as both counterion and spacer molecule gives rise to an ordered segregation of the hemicyanine chromophores, and greatly enhanced second-harmonic generation.
Abstract: THE phenomenon of second-harmonic generation (SHG), whereby a material under illumination generates light at twice the incident frequency, is finding increasing use in optical signal processing. The principal requirement for SHG is a non-centrosymmetric structure; in organic materials, this can be achieved through the use of Langmuir–Blodgett films, which offer control of structure at the molecular level. SHG has been demonstrated in films composed of hemicyanine dyes of the general formula D—C6H4— CH=CH—C5H4N+— RX-, where D is an electron donor, R is usually a hydrophobic alkyl chain and X- is a counterion such as Br- or I-. The frequency-doubling properties of these films are sensitive to the choice of donor group1,2 the extent of molecular aggregation3,4and the type of packing5–7 Mixed films, in which the dye molecules are interspersed with an inert phase, have been used to reduce aggregation and consequently enhance SHG3,8, but these films have the potential disadvantage of phase separation. Here we show that the use of an amphiphilic anion as both counterion and spacer molecule gives rise to an ordered segregation of the hemicyanine chromophores, and greatly enhanced SHG.
186 citations
TL;DR: It is shown that the biexciton density at a given optical injection rate is much enhanced by resonant generation at either the light-hole or the heavy-hole exciton.
Abstract: The conditions required for the generation of biexcitons in quantum wells are discussed, and a model line shape for biexciton recombination is fitted to observed biexciton photoluminescence spectra. It is shown that the biexciton density at a given optical injection rate is much enhanced by resonant generation at either the light-hole or the heavy-hole exciton. Unlike the case of silicon, the biexciton density is found not to vary as the square of the exciton density and this is attributed in part to the short lifetime of the excitons and biexcitons.
82 citations
TL;DR: Novel waveguide beam splitters and recombiners based on multimode propagation phenomena in hollow step-index waveguides are predicted and demonstrated and are considered to be more widely applicable.
Abstract: Novel waveguide beam splitters and recombiners based on multimode propagation phenomena in hollow step-index waveguides are predicted and demonstrated. The splitter designs are based on symmetrically feeding the fundamental-mode field from a square cross-section waveguide, 2a × 2a, into a multimode rectangular guide, 2a × 2b (b > a). As a result of multimode superposition phenomena, unique transverse-field patterns representing different-order multiway splitting of the input field occur at predictable positions along the rectangular-guide axis. The predictions are verified experimentally at 10.6 μm with hollow dielectric waveguides but are considered to be more widely applicable.
78 citations
01 Jan 1992
TL;DR: In this paper, the authors present data for mild steel at -50°C where failure is by J c (cleavage with no prior stable crack growth) and show that these trends are predictable by elastic T-stress analysis, and may be quantified very precisely by more exact elastic-plastic constraint theories.
Abstract: Recent years have seen important advances in constraint based fracture prediction methods. These methods remove the conservatism of current procedures based on lower bound (high constraint) fracture toughness. This report presents data for mild steel at -50°C where failure is by J c (cleavage with no prior stable crack growth). J c shows clear geometry dependence, with higher values in shallow cracked three point bend (3PB) specimens, and in centre cracked tension (CCT) specimens, than in conventional, deeply cracked 3PB specimens. It is shown that these trends are predictable by elastic T-stress analysis, and may be quantified very precisely by more exact elastic-plastic constraint theories. It is concluded that these advanced methods will soon be useable in structural fracture analyses.
74 citations
TL;DR: In this paper, it was shown that both the intrasubband and intersubband electron-phonon scattering rates are independent of the basis set used to describe the phonon modes, as long as this set is orthogonal and complete.
Abstract: Slab vibrations, reformulated slab vibrations, and guided vibrations have been proposed by various authors as the confined polar-optic-phonon modes in heterostructures. It is shown, with reference to these three cases, that both the intrasubband and intersubband electron-phonon scattering rates are independent of the basis set used to describe the phonon modes, as long as this set is orthogonal and complete. Earlier conflicting results are attributed to the use of basis sets that do not satisfy these criteria, and to scattering potentials that do not obey the correct boundary conditions. Nearly all the scattering is by modes that are well described by continuum models. A dispersive continuum theory of the lattice dynamics is developed. Without recourse to microscopic theory, we derive the connection rules that relate fields on either side of an interface, and identify the circumstances in which each above-mentioned set of vibrations corresponds to the normal modes. The controversy over whether the normal modes satisfy electromagnetic or mechanical boundary conditions is resolved. The design of lattice properties to minimize electron-phonon scattering rates is discussed.
58 citations
TL;DR: In this article, the synthesis of several functionalised aryl cyanate esters bearing reactive allyl groups was presented, and the monomers were purified using preparative HPLC and a range of techniques, including heteronuclear NMR spectroscopy, was employed in their characterisation.
Abstract: The syntheses of several functionalised aryl cyanate esters bearing reactive allyl groups are presented The monomers were purified (to c 98%) using preparative HPLC and a range of techniques, including heteronuclear nuclear magnetic resonance (NMR) spectroscopy, was employed in their characterisation A model functionalised aryl cyanate ester was doped with a variety of additives and the resulting thermal behaviour examined using dynamic differential scanning calorimetry (DSC) Furthermore, the postulated co-reaction of the model functionalised cyanate ester with bis-maleimides was investigated using heteronuclear NMR and dynamic mechanical techniques
52 citations
01 Jun 1992
TL;DR: The concept of algorithmic engineering is introduced and discussed in the context of parallel digital signal processing, mainly relating to the use of QR decomposition by square-root-free Givens rotations as applied to adaptive filtering and beamforming.
Abstract: Algorithmic engineering provides a rigorous framework for describing and manipulating the type of building blocks commonly used to define parallel algorithms and architectures for digital signal processing. The concept is first illustrated by means of some fairly simple worked examples. These relate to the use of QR decomposition by Givens rotations for the purposes of adaptive filtering and beamforming. It is then shown how a novel modular architecture for linearly constrained adaptive beamforming has been derived by transforming an established least squares processor design using some simple algorithmic engineering techniques. This novel architecture constitutes a stable and efficient recursive realisation of the modular adaptive beamformer proposed by Liu and Van Veen.
38 citations
TL;DR: The concept of integrated circuits (ICs), conceived in the 1950s, overcame what had been called a "tyranny of numbers" problem as discussed by the authors, that of connecting and soldering together prohibitive numbers of small, discrete electronic components.
Abstract: Silicon is the second most abundant element in the Earth's crust. As well as being cheap, it possesses certain physical and chemical properties that have made it the dominant material in microelectronics for over three decades. The concept of integrated circuits (ICs), conceived in the 1950s, overcame what had been called a "tyranny of numbers" problem – that of connecting and soldering together prohibitive numbers of small, discrete electronic components. But it was the 1960s, when the practical fabrication of such devices became possible, that heralded the silicon chip revolution.
TL;DR: In this article, three glycidylether epoxy resins with functionalities of 2, 3 and 4 were purified and extensively characterized using rheometry and differential scanning calorimetry.
TL;DR: In this paper, a simple uniform switching model was used to investigate the behavior of voltage dependent switching time of surface stabilized ferroelectric liquid crystals as a function of the biaxial permittivity tensor and the layer tilt angle.
Abstract: Using a simple uniform switching model, we investigate the behaviour of the voltage dependent switching time of surface stabilized ferroelectric liquid crystals as a function of the biaxial permittivity tensor and the layer tilt angle. We show that the dielectric biaxiality can markedly effect the response time (τ) of the device and is the origin of the minimum in τ as a function of voltage (V) in tilted layer systems. The dielectric biaxiality should, therefore, be optimized for multiplexing schemes which use the τ—V minimum.
TL;DR: In this article, the authors discuss recent progress in the study of the nonclassical properties of light beams generated by non-degenerate parametric splitting in χ(2) nonlinear birefringent crystals, with special emphasis on their quantum correlation.
Abstract: We discuss recent progress in the study of the non-classical properties of light beams generated by non-degenerate parametric splitting in χ(2) nonlinear birefringent crystals, with special emphasis on their quantum correlation (“twin beams”). We describe experimental results using successively pure parametric fluorescence, parametric amplification of a weak signal beam pumped by a pulsed laser, and parametric oscillation in a cavity pumped by a cw laser. In this review, we compare the respective advantages and drawbacks of the different approaches.
TL;DR: In this article, the average local time variation of the velocity for Bz northward under various By conditions is presented, and the nightside drifts for BZ northward tend to be antisunward, and resemble those which occur for southward IMF, as pointed out by Heppner and Maynard.
Abstract: The Digisonde at Qanaq, Greenland (87°Λ) measures the ionospheric convection velocity in the central polar cap. The average local time variation of the velocity for Bz northward under various By conditions is presented. The nightside drifts for Bz northward tend to be antisunward, and resemble those which occur for southward IMF, as pointed out by Heppner and Maynard (1987). However, it is difficult to reconcile the observed diurnal drift variation with the Heppner-Maynard distorted two-cell model. Modified forms of the Potemra et al. (1984) four cell model offer a more plausible framework in which to understand the Qanaq data. The polar cap cells are relatively small, and are displaced towards the dayside.
TL;DR: Six fibers of length 1.5 m are cut with a mean difference in length of 10.8 μm, and the pigtailed ends of the couplers are also cut to this accuracy.
Abstract: Six fibers of length 1.5 m are cut with a mean difference in length of 10.8 μm. The pigtailed ends of the couplers are also cut to this accuracy.
01 Jan 1992
TL;DR: A notation for presenting a claim that a program written in Ada satisfies a specification written in Z, which has both formal and informal elements: the formal elements follow the style of the refinement calculus, while the informal elements follow Knuth’s literate programming style.
Abstract: This paper describes a notation for presenting a claim that a program written in Ada satisfies a specification written in Z The notation has both formal and informal elements: the formal elements follow the style of the refinement calculus, while the informal elements follow Knuth’s literate programming style The combination of formal and informal elements allows for selective verification, an important requirement for practical use of formal refinement
23 Feb 1992
TL;DR: SRU's first experiences collecting unscripted speech data using the 'Wizard of Oz' technique to provide a genuine telephone-based route planning service are presented and valuable insights into the nature of future speech-based human-machine interaction are obtained.
Abstract: This paper presents SRU's first experiences collecting unscripted speech data using the 'Wizard of Oz' technique to provide a genuine telephone-based route planning service. Although only a limited quantity of data has been collected so far, several valuable insights into the nature of future speech-based human-machine interaction have been obtained.
TL;DR: In this paper, the results from the theory of low-frequency scattering by a single sphere are used to obtain three coupled equations for the effective bulk modulus K ∗, shear modulusμ ∗ and density p ∗ of an elastic matrix containing spherical inclusions.
TL;DR: In this article, two principal nitration system s have been developed: (i) N 2 O 5 in pure nitric acid, and (ii) n 2 o 5 in organic solvents, mainly chlorinated hydrocarbons.
Abstract: The harsh and unselective nature of conventional nitrating agents, based on nitricsulphuric acid mixtures or pure nitric acid, has led to the development of new nitration methodologies based on dinitrogen pentoxide (N 2 O 5 ) that overcome many of the draw backs of conventional media, especially when dealing with sensitive substrates. Two principal nitration system s have been developed : (i) N 2 O 5 in pure nitric acid, and (ii) N 2 O 5 in organic solvents, mainly chlorinated hydrocarbons. The former system possesses strength similar to mixed acid (HNO 3 - H 2 SO 4 ) system s and is therefore potent, but displays advantages such as ease of generation of the nitration reagent by electrolysis and com patibility with nitram ine products, facilitating the synthesis of nitram ine explosives as well as certain aromatic nitro compounds. The second system (N 2 O 5 /organic solvent) encompasses a much wider range of nitrations, and with suitable choice of substrate permits the generation of polynitrated species directly without the formation of acidic by-products, an important environmental consideration. These conditions are also compatible with sensitive substrates, particularly those possessing strained-ring heterocyclic skeletons; thus the nitration of such compounds with N2O5 perm its precursors for energetic polymers (e.g. polyNIMMO) to be prepared by selective nitration. Likewise, nitrated derivatives of suitably functionalized elastomeric pre-polymers (e.g. NHTPB ) can also be manufactured; both of the latter category of compound find important application as energetic binders for novel explosive and propellant compositions, notably low-vulnerability (LOVA) propellant charges.
TL;DR: This paper describes the development, application and evaluation of a system dynamics based methodology for the assessment of computerised information systems carried out as part of a joint research project between the University of Bradford Management Centre and the Defence Research Agency (Military).
Abstract: This paper describes the development, application and evaluation of a system dynamics based methodology for the assessment of computerised information systems (CIS) carried out as part of a joint research project between the University of Bradford Management Centre and the Defence Research Agency (Military). It is suggested that assessment methods based solely on the internal attributes of the CIS are inadequate. There is a need for assessment in terms of the contribution which the CIS makes to the overall performance of the host system in which it will be implanted. A methodology based on system dynamics is described which embodies this principle.
01 Jul 1992
TL;DR: In this article, the authors demonstrate that the Reduced Navier-Stokes code RNS3D can be used very effectively to develop a vortex generator installation for the purpose of minimizing the engine face circumferential distortion by controlling the development of secondary flow.
Abstract: The present study demonstrates that the Reduced Navier-Stokes code RNS3D can be used very effectively to develop a vortex generator installation for the purpose of minimizing the engine face circumferential distortion by controlling the development of secondary flow. The computing times required are small enough that studies such as this are feasible within an analysis-design environment with all its constraints of time and costs. This research study also established the nature of the performance improvements that can be realized with vortex flow control, and suggests a set of aerodynamic properties (called observations) that can be used to arrive at a successful vortex generator installation design. The ultimate aim of this research is to manage inlet distortion by controlling secondary flow through an arrangements of vortex generators configurations tailored to the specific aerodynamic characteristics of the inlet duct. This study also indicated that scaling between flight and typical wind tunnel test conditions is possible only within a very narrow range of generator configurations close to an optimum installation. This paper also suggests a possible law that can be used to scale generator blade height for experimental testing, but further research in this area is needed before it can be effectively applied to practical problems. Lastly, this study indicated that vortex generator installation design for inlet ducts is more complex than simply satisfying the requirement of attached flow, it must satisfy the requirement of minimum engine face distortion.
TL;DR: Neutral cluster beam deposition (NCBD) is shown to be directly applicable to rugate filter fabrication as mentioned in this paper, which is demonstrated via the excellent agreement between theoretical and experimental rugate performance.
01 Jan 1992
TL;DR: In this paper, a transient eddy-current inspection is performed, where an electromagnetic field pulse is excited in a conductor by causing a step change in the current through a coil, and part of the pulse is scattered back to the surface of the conductor where it can be observed as a transient signal in the time domain.
Abstract: In transient eddy-current inspection, an electromagnetic field pulse is excited in a conductor by causing a step change in the current through a coil. As this pulse propagates into the material, it is broadened by dispersion and scattered by discontinuities in the conductivity and permeability of the conductor. Subsurface defects cause part of the pulse to be scattered back to the surface of the conductor where it can be observed as a transient signal in the time domain, either as an EMF across the coil, or by direct measurement of the magnetic field using, for example, a Hall sensor. The observed transient is essentially the response function of the specimen with the transducer at a particular position. It contains information over a broad spectrum in contrast to time-harmonic excitation which yields information only at a single frequency.
TL;DR: Cobalt, cobalt monosilicide and rhenium thin films have been deposited on the internal walls of both n- and p-type porous silicon (PS) by metallo-organic chemical vapour deposition (MOCVD) at moderate temperatures and low pressure as discussed by the authors.
01 Dec 1992
TL;DR: The author demonstrates both theoretically and numerically how self-supervision emerges when a simple network built out of vector quantisers is optimised.
Abstract: A scheme for training multilayer unsupervised networks is presented, in which control signals propagate downwards from the higher layers to influence the optimisation of the lower layers. Because there is no external teacher involved, this is called self-supervised training. The author demonstrates both theoretically and numerically how self-supervision emerges when a simple network built out of vector quantisers is optimised.
TL;DR: In this article, a new procedure is presented for the determination of the fracture mechanics weight functions that are required for the evaluation of stress intensity factors in cracked solids, which are proportional to the displacements on the boundary of the solid when the only loading is a pair of self-equilibrated point forces at the crack front.
Abstract: A new procedure is presented for the determination of the fracture mechanics weight functions that are required for the evaluation of stress intensity factors in cracked solids. The procedure can be used with a standard three-dimensional boundary element code. The weight functions are proportional to the displacements on the boundary of the solid when the only loading is a pair of self-equilibrated point forces at the crack front. In previous work, the highly singular crack-tip fields that this loading produces have been modelled by replacing the crack front by a cylindrical cavity with appropriate displacement boundary conditions on the cavity walls. It is shown here that results are dependent on the cavity radius and that convergence of the results cannot be guaranteed.
An alternative procedure, based on the substraction of fundamental fields (SFF), is demonstrated herein. The high-order singularities are removed from the field before the reduced problem is solved numerically using a standard boundary element method. Since the reduced problem is equivalent to an unloaded crack in a solid subjected to boundary tractions, the usual quarter-point displacement elements and quarter-point traction singular elements can be used to improve the accuracy. Weight functions, so obtained, are used to evaluate stress intensity factors as a function of position on the crack front for a straight-fronted crack in a rectangular bar subjected to various loadings. Both edge and central cracks are considered and the validity of the technique is demonstrated by comparing the results with previously published values.
TL;DR: In this paper, an optical switch is demonstrated which uses a phased array of 30 closely spaced, individually addressed electrooptic AlGaAs-GaAs waveguides to focus and steer a light beam under electronic control in a planar output waveguide.
Abstract: An optical switch is demonstrated which uses a phased array of 30 closely spaced, individually addressed electrooptic AlGaAs-GaAs waveguides to focus and steer a light beam under electronic control in a planar output waveguide. A small and potentially very high speed 1-to-9 way optical switch, (with -15 dB of crosstalk) was made by placing an array of nine output waveguides at different positions in a focal plane 1 mm from the end of the electrooptic array. >
TL;DR: The thermal and photochemical decomposition of dialkyltellurium and dialkylditelluriums in t-butylbenzene solvent have been studied by, EPR spectroscopy as discussed by the authors.