Spontaneous emission from radiative chiral nematic liquid crystals at the photonic band-gap edge: An investigation into the role of the density of photon states near resonance

In this article, we investigate the spontaneous emission properties of radiating molecules embedded in a chiral nematic liquid crystal, under the assumption that the electronic transition frequency is close to the photonic edge mode of the structure, i.e., at resonance. We take into account the transition broadening and the decay of electromagnetic field modes supported by the so-called "mirrorless"cavity. We employ the Jaynes-Cummings Hamiltonian to describe the electron interaction with the electromagnetic field, focusing on the mode with the diffracting polarization in the chiral nematic layer. As known in these structures, the density of photon states, calculated via the Wigner method, has distinct peaks on either side of the photonic band gap, which manifests itself as a considerable modification of the emission spectrum. We demonstrate that, near resonance, there are notable differences between the behavior of the density of states and the spontaneous emission profile of these structures. In addition, we examine in some detail the case of the logarithmic peak exhibited in the density of states in two-dimensional photonic structures and obtain analytic relations for the Lamb shift and the broadening of the atomic transition in the emission spectrum. The dynamical behavior of the atom-field system is described by a system of two first-order differential equations, solved using the Green's-function method and the Fourier transform. The emission spectra are then calculated and compared with experimental data. © 2013 American Physical Society.

Removal of surface states and recovery of band-edge emission in InAs nanowires through surface passivation.

Surface states in semiconductor nanowires (NWs) are detrimental to the NW optical and electronic properties and to their light emission-based applications, due to the large surface-to-volume ratio of NWs and the congregation of defects states near surfaces. In this paper, we demonstrated an effective approach to eliminate surface states in InAs NWs of zinc-blende (ZB) and wurtzite (WZ) structures and a dramatic recovery of band edge emission through surface passivation with organic sulfide octadecylthiol (ODT). Microphotoluminescence (PL) measurements were carried out before and after passivation to study the dominant recombination mechanisms and surface state densities of the NWs. For WZ-NWs, we show that the passivation removed the surface states and recovered the band-edge emission, leading to a factor of ∼19 reduction of PL linewidth. For ZB-NWs, the deep surface states were removed and the PL peaks width became as narrow as ∼250 nm with some remaining emission of near band-edge surface states. The passivated NWs showed excellent stability in atmosphere, water, and heat environments. In particular, no observable changes occurred in the PL features from the passivated NWs exposed in air for more than five months.

Spontaneous emission from radiative chiral nematic liquid crystals at the photonic band-gap edge: an investigation into the role of the density of photon states near resonance.

In this article, we investigate the spontaneous emission properties of radiating molecules embedded in a chiral nematic liquid crystal, under the assumption that the electronic transition frequency is close to the photonic edge mode of the structure, i.e., at resonance. We take into account the transition broadening and the decay of electromagnetic field modes supported by the so-called "mirrorless"cavity. We employ the Jaynes-Cummings Hamiltonian to describe the electron interaction with the electromagnetic field, focusing on the mode with the diffracting polarization in the chiral nematic layer. As known in these structures, the density of photon states, calculated via the Wigner method, has distinct peaks on either side of the photonic band gap, which manifests itself as a considerable modification of the emission spectrum. We demonstrate that, near resonance, there are notable differences between the behavior of the density of states and the spontaneous emission profile of these structures. In addition, we examine in some detail the case of the logarithmic peak exhibited in the density of states in two-dimensional photonic structures and obtain analytic relations for the Lamb shift and the broadening of the atomic transition in the emission spectrum. The dynamical behavior of the atom-field system is described by a system of two first-order differential equations, solved using the Green's-function method and the Fourier transform. The emission spectra are then calculated and compared with experimental data.

A survey on technology exposure and range of abilities of elderly and disabled users in India

This paper reports a survey on people with age-related and physical impairments in India. The survey evaluates functional parameters related to human computer interaction and reports subjective attitude and exposure of users towards technology. We found a significant cognitive decline in elderly users while their functional parameters are sufficient to use existing electronic devices. However young disabled users are found to be experienced with computer but could not have access to appropriate assistive devices, which would benefit them. Most users used desktop computers and mobile phone but none used tablet, smartphone or kiosks though they are keen to learn new technologies. Overall we hope that our results will be useful for HCI practitioners in developing countries. © 2013 Springer-Verlag Berlin Heidelberg.

Aerodynamic noise from a poroelastic trailing edge with implications for the silent flight of owls

The interaction of a turbulent eddy with a semi-infinite, poroelastic edge is examined with respect to the effects of both elasticity and porosity on the efficiency of scattered aerodynamic noise. The scattering problem is solved using the Wiener-Hopf technique for constant plate properties to identify their scaling dependence on the resulting aerodynamic noise, including the dependence on flight velocity, where special attention is paid to the limiting cases of rigid, porous and elastic, impermeable plate conditions. Results from these analyses attempt to address how trailing edge noise may be mitigated by porosity and seek to deepen the understanding of how owls hunt in acoustic stealth. © 2012 by Justin W. Jaworski and Nigel Peake. Published by the American Institute of Aeronautics and Astronautics, Inc.

Acoustic analogy in swirling mean flow applied to predict rotor trailing-edge noise

The present study aims at accounting for swirling mean flow effects on rotor trailing-edge noise. Indeed, the mean flow in between the rotor and the stator of the fan or of a compressor stage is highly swirling. The extension of Ffowcs-Williams & Hawkings' acoustic analogy in a medium at rest with moving surfaces and of Goldstein's acoustic analogy in a circular duct with uniform mean flow to a swirling mean flow in an annular duct is introduced. It is first applied to tonal noise. In most cases, the swirl modifies the pressure distribution downstream of the fan. In several configurations, when the swirl is rather close to a solid body swirl, it is often sufficient to apply a simple Doppler effect correction when predicting the duct modes in uniform mean flow in order to predict accurately the noise radiated with swirl. However, in other realistic configurations, the swirling mean-flow effect cannot be addressed using this simple Doppler effect correction. Second, a rotor trailing-edge noise model accounting for both the effects of the annular duct and the swirling mean flow is developed and applied to a realistic fan rotor with different swirling and sheared mean flows (and as a result different associated blade stagger angles). The benchmark cases are built from the Boeing 18-inch Fan Rig Broadband Noise Test. In all cases the swirling mean flow has an effect. In some cases the a simple Doppler effect may address it, but, in other realistic configurations our acoustic analogy with swirl is needed. © 2012 by the authors. Published by the American Institute of Aeronautics and Astronautics, Inc.

The dynamic response of edge clamped plates loaded by spherically expanding sand shells

The dynamic deformation of both edge clamped stainless steel sandwich panels with a pyramidal truss core and equal mass monolithic plates loaded by spherically expanding shells of dry and water saturated sand has been investigated, both experimentally and via a particle based simulation methodology. The spherically expanding sand shell is generated by detonating a sphere of explosive surrounded by a shell of either dry or water saturated synthetic sand. The measurements show that the sandwich panel and plate deflections decrease with increasing stand-off between the center of the charge and the front of the test structures. Moreover, for the same charge and sand mass, the deflections of the plates are significantly higher in the water saturated sand case compared to that of dry sand. For a given stand-off, the mid-span deflection of the sandwich panel rear faces was substantially less than that of the corresponding monolithic plate for both the dry and water saturated sand cases. The experiments were simulated via a coupled discrete-particle/ finite element scheme wherein the high velocity impacting sand is modeled by interacting particles while the plate is modeled within a Lagrangian finite element setting. The simulations are in good agreement with the measurements for the dry sand impact of both the monolithic and sandwich structures. However, the simulations underestimate the effect of stand-off in the case of the water saturated sand explosion, i.e. the deflections decrease more sharply with increasing stand-off in the experiments compared to the simulations. The simulations reveal that the momentum transmitted into the sandwich and monolithic plate structures by the sand shell is approximately the same, consistent with a small fluid-structure interaction effect. The smaller deflection of the sandwich panels is therefore primarily due to the higher bending strength of sandwich structures. © 2013 The Authors. Published by Elsevier Ltd. All rights reserved.

Swirling mean flow effect on fan-trailing edge broadband noise in a lined annular duct

The present study aims at investigating the effect of a swirling mean flow and a lined annular duct on rotor trailing-edge noise. The objectives are to investigate these effects on the eigenvalues and a tailored Green's function on one hand and on the realistic case of the fan trailing-edge noise on the other hand. Indeed, the mean flow in between the rotor and the stator of the fan is highly swirling. Moreover, interstage liners are used to reduce the noise produced by the fan stage. The extension of Ffowcs-Williams & Hawkings' acoustic analogy in a medium at rest with moving surfaces, of Goldstein's acoustic analogy in a hardwall circular duct with uniform mean flow and of Rienstra & Tester's Green's function in an annular lined duct with uniform mean flow to a swirling mean flow in an annular duct with liner is introduced. First, the eigenvalues and the Green's function are investigated showing a strong effect of the swirl and of the liner. Second, a rotor trailing-edge noise model accounting for both the effects of the annular duct with lined walls and the swirling mean flow is developed and applied to a realistic fan rotor with different swirling mean flows (and as a result different associated blade stagger angles). The benchmark cases are built from the Boeing 18-inch Fan Rig Broadband Noise Test. In all cases the swirling mean flow has a strong effect on the absolute noise level. The overall liner insertion loss is little changed by the swirl in the studied cases.

Handbook of nanoindentation with biological applications

Nanoindentation is ideal for the characterization of inhomogeneous biological materials. However, the use of nanoindentation techniques in biological systems is associated with some distinctively different techniques and challenges. For example, engineering materials used in the microelectronics industry (e.g. ceramics and metals) for which the technique was developed, are relatively stiff and exhibit time-independent mechanical responses. Biological materials, on the other hand, exhibit time-dependent behavior, and can span a range of stiffness regimes from moduli of Pa to GPa - eight to nine orders of magnitude. As such, there are differences in the selection of instrumentation, tip geometry, and data analysis in comparison with the "black box" nanoindentation techniques as sold by commercial manufacturers. The use of scanning probe equipment (atomic force miscroscopy) is also common for small-scale indentation of soft materials in biology. The book is broadly divided into two parts. The first part presents the "basic science" of nanoindentation including the background of contact mechanics underlying indentation technique, and the instrumentation used to gather mechanical data. Both the mechanics background and the instrumentation overview provide perspectives that are optimized for biological applications, including discussions on hydrated materials and adaptations for low-stiffness materials. The second part of the book covers the applications of nanoindentation technique in biological materials. Included in the coverage are mineralized and nonmineralized tissues, wood and plant tissues, tissue-engineering substitute materials, cells and membranes, and cutting-edge applications at molecular level including the use of functionalized tips to probe specific molecular interactions (e.g. the ligand-receptor binding). The book concludes with a concise summary and an insightful forecast of the future highlighting the current challenges. © 2011 by Pan Stanford Publishing Pte. Ltd. All rights reserved.

Climb-enabled discrete dislocation plasticity

A small strain two-dimensional discrete dislocation plasticity framework coupled to vacancy diffusion is developed wherein the motion of edge dislocations is by a combination of glide and climb. The dislocations are modelled as line defects in a linear elastic medium and the mechanical boundary value problem is solved by the superposition of the infinite medium elastic fields of the dislocations and a complimentary non-singular solution that enforces the boundary conditions. Similarly, the climbing dislocations are modelled as line sources/sinks of vacancies and the vacancy diffusion boundary value problem is also solved by a superposition of the fields of the line sources/sinks in an infinite medium and a complementary non-singular solution that enforces the boundary conditions. The vacancy concentration field along with the stress field provides the climb rate of the dislocations. Other short-range interactions of the dislocations are incorporated via a set of constitutive rules. We first employ this formulation to investigate the climb of a single edge dislocation in an infinite medium and illustrate the existence of diffusion-limited and sink-limited climb regimes. Next, results are presented for the pure bending and uniaxial tension of single crystals oriented for single slip. These calculations show that plasticity size effects are reduced when dislocation climb is permitted. Finally, we contrast predictions of this coupled framework with an ad hoc model in which dislocation climb is modelled by a drag-type relation based on a quasi steady-state solution. © 2013 Elsevier Ltd. All rights reserved.

Unsteady simulation paradigms for trailing edge ejection

Tucker