Asymmetric Propagation Research Articles

High resolution surface plasmon resonance imaging for single cells.

BackgroundSurface plasmon resonance imaging (SPRI) is a label-free technique that can image refractive index changes at an interface. We have previously used SPRI to study the dynamics of cell-substratum interactions. However, characterization of spatial resolution in 3 dimensions is necessary to quantitatively interpret SPR images. Spatial resolution is complicated by the asymmetric propagation length of surface plasmons in the x and y dimensions leading to image degradation in one direction. Inferring the distance of intracellular organelles and other subcellular features from the interface by SPRI is complicated by uncertainties regarding the detection of the evanescent wave decay into cells. This study provides an experimental basis for characterizing the resolution of an SPR imaging system in the lateral and distal dimensions and demonstrates a novel approach for resolving sub-micrometer cellular structures by SPRI. The SPRI resolution here is distinct in its ability to visualize subcellular structures that are in proximity to a surface, which is comparable with that of total internal reflection fluorescence (TIRF) microscopy but has the advantage of no fluorescent labels.ResultsAn SPR imaging system was designed that uses a high numerical aperture objective lens to image cells and a digital light projector to pattern the angle of the incident excitation on the sample. Cellular components such as focal adhesions, nucleus, and cellular secretions are visualized. The point spread function of polymeric nanoparticle beads indicates near-diffraction limited spatial resolution. To characterize the z-axis response, we used micrometer scale polymeric beads with a refractive index similar to cells as reference materials to determine the detection limit of the SPR field as a function of distance from the substrate. Multi-wavelength measurements of these microspheres show that it is possible to tailor the effective depth of penetration of the evanescent wave into the cellular environment.ConclusionWe describe how the use of patterned incident light provides SPRI at high spatial resolution, and we characterize a finite limit of detection for penetration depth. We demonstrate the application of a novel technique that allows unprecedented subcellular detail for SPRI, and enables a quantitative interpretation of SPRI for subcellular imaging.Electronic supplementary materialThe online version of this article (doi:10.1186/1471-2121-15-35) contains supplementary material, which is available to authorized users.

Effect of 4-point bending test procedure on crack propagation in thin film stacks

An in-depth study of 4-point bending (4PB) test method had been conducted in order to determine the influence of test parameters on measured critical energy release rate Gc and fracture location. Force loading speed proved to have an influence not only on measured Gc values, but also on quality of force–displacement curve plateau, as did the loading pin distance. While the 4PB technique is used to determine the adhesion strength of a material, a study of notch depth had also been conducted in order to determine whether it is possible to trigger the cohesive failure of the tested low-k. In addition to the experimental work, the influence of crack propagation path within the sample (symmetric and asymmetric propagation) on measured force plateau was assessed using a Finite Element Method (FEM) modeling. Assuming same interfaces triggered, crack propagation path was shown to have no influence on the value of force plateau. The FEM simulation also showed good correlation with analytical results found in literature in regards to crack opening.

A study of frictional contact in dynamic fracture along bimaterial interfaces

We investigate numerically the dynamic in-plane propagation of a centered crack along bimaterial interfaces using a spectral formulation of the elastodynamic boundary integral equations. Particular attention is given to the effect of contact zones at the subsonic/intersonic transition. In a single set-up, we simulate and describe the different phenomenon observed experimentally (distinct natures of contact zones, unfavorable velocity range, asymmetric crack propagation). We show that different behaviors are observed as function of the crack propagation direction, i.e., with respect to the particle displacements of the compliant material. When the crack propagates in the same direction, the propagation velocities between $$c_\mathrm{R}$$ and $$c_\mathrm{s}$$ are forbidden and the subsonic/intersonic transition occurs with the nucleation of a daughter crack in front of the main rupture. The intersonic stress field at the crack front is compressive due to the material mismatch and a contact zone appears behind the tip. In the opposite direction, a smooth subsonic/intersonic transition occurs although crack face closure (in normal direction) is observed for speeds between $$c_\mathrm{s}$$ and $$\sqrt{2}c_\mathrm{s}$$ . In this regime, a Rayleigh disturbance is generated at the crack surface causing a contact zone which detaches from the tip. Using a contact model governed by a regularized Coulomb law, we provide a quantitative evaluation of the influence of friction on the effective fracture toughness. Finally, we show the applicability of our analysis to the description of different bimaterial situations as well as the single-material set-up.

Anisotropic neutrino effect on magnetar spin: constraint on inner toroidal field

The ultrastrong magnetic field of magnetars modifies the neutrino cross-section due to the parity violation of the weak interaction and can induce asymmetric propagation of neutrinos. Such an anisotropic neutrino radiation transfers not only the linear momentum of a neutron star but also the angular momentum, if a strong toroidal field is embedded inside the stellar interior. As such, the hidden toroidal field implied by recent observations potentially affects the rotational spin evolution of new-born magnetars. We analytically solve the transport equation for neutrinos and evaluate the degree of anisotropy that causes the magnetar to spin-up or spin-down during the early neutrino cooling phase. Supposing that after the neutrino cooling phase the dominant process causing the magnetar spin-down is the canonical magnetic dipole radiation, we compare the solution with the observed present rotational periods of anomalous X-ray pulsars 1E 1841−045 and 1E 2259+586, whose poloidal (dipole) fields are ∼1015 and 1014 G, respectively. Combining with the supernova remnant age associated with these magnetars, the present evaluation implies a rough constraint of global (average) toroidal field strength at Bϕ ≲ 1015 G.

Asymmetry in signal propagation between the soma and dendrites plays a key role in determining dendritic excitability in motoneurons.

It is widely recognized that propagation of electrophysiological signals between the soma and dendrites of neurons differs depending on direction, i.e. it is asymmetric. How this asymmetry influences the activation of voltage-gated dendritic channels, and consequent neuronal behavior, remains unclear. Based on the analysis of asymmetry in several types of motoneurons, we extended our previous methodology for reducing a fully reconstructed motoneuron model to a two-compartment representation that preserved asymmetric signal propagation. The reduced models accurately replicated the dendritic excitability and the dynamics of the anatomical model involving a persistent inward current (PIC) dispersed over the dendrites. The relationship between asymmetric signal propagation and dendritic excitability was investigated using the reduced models while varying the asymmetry in signal propagation between the soma and the dendrite with PIC density constant. We found that increases in signal attenuation from soma to dendrites increased the activation threshold of a PIC (hypo-excitability), whereas increases in signal attenuation from dendrites to soma decreased the activation threshold of a PIC (hyper-excitability). These effects were so strong that reversing the asymmetry in the soma-to-dendrite vs. dendrite-to-soma attenuation, reversed the correlation between PIC threshold and distance of this current source from the soma. We propose the tight relation of the asymmetric signal propagation to the input resistance in the dendrites as a mechanism underlying the influence of the asymmetric signal propagation on the dendritic excitability. All these results emphasize the importance of maintaining the physiological asymmetry in dendritic signaling not only for normal function of the cells but also for biophysically realistic simulations of dendritic excitability.

Asymmetric transmission of terahertz waves through a graphene-loaded metal grating

In this work, we theoretically investigate the propagation of terahertz (THz) waves through a graphene-loaded metal grating under external magnetic field. It is found that resonant modes in the system can be converted between transverse-electric and transverse-magnetic polarizations due to Hall conductivity of graphene. As a consequence, asymmetric transmission of THz waves through this graphene-loaded metal grating is achieved. Furthermore, by adjusting either the external magnetic field or the Fermi level of graphene, such asymmetric wave propagation can be significantly tuned. The investigations may provide a unique way to achieve the graphene-loaded optodevices for THz waves.

New asymmetric propagation invariant beams obtained by amplitude and phase modulation in frequency space

In this paper, we demonstrate, numerically and experimentally that using the mask-lens setup used by Durnin to generate Bessel beams Durnin [Phys. Rev. Lett. 58, 1499 (1987)], it is possible to generate different kinds of propagation invariant beams. A modification in the amplitude or phase of the field that illuminates the annular slit is proposed that corresponds to modulation in frequency space. In particular, we characterize the new invariant beams that were obtained by modulating the amplitude of the annular mask and when the incident field was modulated with a one-dimensional quadratic or cubic phase. Experimental results using an amplitude mask are shown in order to corroborate the numerical predictions.

A mathematical model of the unidirectional block caused by the pulmonary veins for anatomically induced atrial reentry.

It is widely believed that the pulmonary veins (PVs) of the left atrium play the central role in the generation of anatomically induced atrial reentry but its mechanism has not been analytically explained. To understand this mechanism, a new analytic approach is proposed by adapting the geometric relative acceleration analysis from spacetime physics based on the hypothesis that a large relative acceleration can translate to a dramatic increase in the curvature of a wavefront and subsequently to conduction failure. By verifying the strong dependency of the propagational direction and the magnitude of anisotropy for conduction failure, this analytic method reveals that a unidirectional block can be generated by asymmetric propagation toward the PVs. This model is validated by computational tests in a T-shaped domain, computational simulations for three-dimensional atrial reentry and previous in-silico reports for anatomically induced atrial reentry.

Asymmetrical Dynamic Propagation Problem on the Edges of Mode I Crack Subjected to Superimposed Loads

By application of the theory of complex functions, asymmetrical dynamic propagation problem on the edges of mode I crack subjected to superimpose loads was researched. Analytical solutions of the stresses, displacements, and stress intensity factors are obtained using the methods of self-similar functions. The problems studied can be facilely transformed into Riemann-Hilbert problems and their closed solutions are attained rather simply according to this measure. In terms of correlative material properties, the variable rule of dynamic stress intensity factor was depicted very well.

Non-Hermitian nanophotonic and plasmonic waveguides

Non-Hermitian Hamiltonians arising from parity-time (PT)-symmetric potentials have been extensively explored in optical systems, owing to their ability to generate asymmetric and even nonreciprocal light propagation. In this paper, we investigate such PT potentials in plasmonic systems, demonstrating asymmetric optical propagation in deeply subwavelength waveguides. In particular, we investigate a five layer plasmonic waveguide composed of metallic layers separated by dielectric media containing either loss or gain in equal quantities. Through an analytic solution of Maxwell's equations, we identify the four lowest order modes of the waveguide, including two positive index modes and two negative index modes, and investigate their evolution with increasing but balanced gain and loss, $\ensuremath{\kappa}$. Both the exact analytic approach and an approximate one based on Rayleigh-Schrodinger perturbation theory demonstrate eigenvalue merging and state coalescence with increasing $\ensuremath{\kappa}$, unlike the familiar energy-level splitting observed in conventional coupled systems. The state coalescence always occurs between modes of opposite parity. Also, by changing the coupling between the waveguide layers, state coalescence can occur between modes with opposite refractive indices, resulting in the merging of a positive index mode with a negative index mode at the exceptional point. We use dispersion diagrams and field profiles to illustrate the asymmetric plasmon propagation properties with increasing $\ensuremath{\kappa}$. We also show that at the system's exceptional point, the modal power varies quadratically along the waveguide. This study represents a spectral analysis of deeply subwavelength PT-symmetric plasmonic and multimodal photonic waveguides and provides a foundation for designing asymmetric and unidirectional nanophotonic devices.

Asymmetric light propagation based on semi-circular photonic crystals

A new structure based on a semi-circular photonic crystal is proposed to achieve asymmetric light propagation. The semi-circular photonic crystal structure proposed in this paper is a deformation of a two-dimensional conventional square photonic crystal. Through the directional bandgap of the semi-circular photonic crystal, the light from one direction can transfer to the other side, but the light from the opposite direction cannot. A high contrast ratio is obtained by designing the constitutive parameters of the photonic crystal and choosing the suitable light frequency. This structure promises a significant potential in optical integration and other areas.

Observation of unidirectional negative refraction in an acoustic metafluid prism

We theoretically and experimentally demonstrate the unidirectional negative refraction in an acoustic metafluid prism. The acoustic metafluid is designed to be uniaxial and highly anisotropic in inertia. Since the directions of phase and group velocities do not coincide in the anisotropic medium, it is possible to realize the unidirectional negative refraction by the broken symmetry of phase velocity vectors with respect to the acoustic axis. Being a distinct case of the asymmetric sound propagation, the unidirectional negative refracting effect could be potentially significant in various areas such as underwater sensing, and medical ultrasonics.

Theoretical and experimental investigations of asymmetric light transport in graded index photonic crystal waveguides

To provide asymmetric propagation of light, we propose a graded index photonic crystal (GRIN PC) based waveguide configuration that is formed by introducing line and point defects as well as intentional perturbations inside the structure. The designed system utilizes isotropic materials and is purely reciprocal, linear, and time-independent, since neither magneto-optical materials are used nor time-reversal symmetry is broken. The numerical results show that the proposed scheme based on the spatial-inversion symmetry breaking has different forward (with a peak value of 49.8%) and backward transmissions (4.11% at most) as well as relatively small round-trip transmission (at most 7.11%) in a large operational bandwidth of 52.6 nm. The signal contrast ratio of the designed configuration is above 0.80 in the telecom wavelengths of 1523.5–1576.1 nm. An experimental measurement is also conducted in the microwave regime: A strong asymmetric propagation characteristic is observed within the frequency interval of 12.8 GHz–13.3 GHz. The numerical and experimental results confirm the asymmetric transmission behavior of the proposed GRIN PC waveguide.

Light and/or atomic beams to detect ultraweak gravitational effects

We shall review the opportunities lent by ring lasers and atomic beams interferometry in order to reveal gravitomagnetic effects on Earth. Both techniques are based on the asymmetric propagation of waves in the gravitational field of a rotating mass; actually the times of flight for co- or counter-rotating closed paths turn out to be different. After discussing properties and limitations of the two approaches we shall describe the proposed GINGER experiment which is being developed for the Gran Sasso National Laboratories in Italy. The experimental apparatus will consist of a three-dimensional array of square rings, 6m × 6m, that is planned to reach a sensitivity in the order of 1prad/√Hertz or better. This sensitivity would be one order of magnitude better than the best existing ring, which is the G-ring in Wettzell, Bavaria, and would allow for the terrestrial detection of the Lense-Thirring effect and possibly of deviations from General Relativity. The possibility of using either the ring laser approach or atomic interferometry in a space mission will also be considered. The technology problems are under experimental study using both the German G-ring and the smaller G-Pisa ring, located at the Gran Sasso

Asymmetric Light Propagation Based on Graded Photonic Crystals

This study proposes a novel graded photonic crystal structure that can be used to achieve asymmetric light propagation. This structure is obtained by changing the radius of a conventional square lattice photonic crystal. A higher contrast ratio is obtained by extending the numbers of the periods of the graded photonic crystal in the horizontal direction. The best contrast ratio almost reaches 1. The intensity at the spot where the light propagated forward converges is two orders of magnitude greater than the intensity at the optical axis of the backward transmission. Numerical results are obtained using the finite-difference time domain and plane wave methods. The proposed structure has significant application potential in optical integration.

Testing for asymmetric financial contagion: New evidence from the Asian crisis

This paper investigates financial contagion as an asymmetric propagation mechanism across both equity and foreign exchange markets. In order to provide a robust analysis of the contagion dynamics, we apply an asymmetric generalized dynamic conditional correlation (AG-DCC) model. This specification allows examining the presence of asymmetric responses in correlations to negative returns, focusing on four countries affected by a specific emerging-market crisis (Asian crisis in 1997–1998). We find that conditional correlations among stock (currency) markets increase significantly during the crisis period, supporting the presence of asymmetric responses to negative shocks and the contagion phenomenon. The results also support the regional nature of this crisis, which is also spread with a higher magnitude among equity rather than currency markets. This evidence has important implications for portfolio diversification strategies and the effectiveness of policy responses to prevent the spread of the crisis among countries.

Crack propagation behaviors at Cu/SiC interface by molecular dynamics simulation

The propagation of the interfacial cracks at Cu/SiC interface under tensile (mode I) loadings and combination of tensile and shear (mixed mode) loadings are studied by molecular dynamics (MD) simulations. For the mode I, the asymmetrical interfacial crack propagation is observed at the interface, and the stress concentration is found both at the crack tip and somewhere of interface due to the lattice mismatch. Six loading methods with different loading angles are considered in this work, the behaviors of the crack propagation are found to be dependent on the loading angles. In addition, the Rice and Thomson (R–T) model is also used to predict the behaviors of interfacial crack growth theoretically. With pure tensile loading, the energies necessary for dislocation nucleation at the two crack tips are found to be different, which leads to asymmetrical crack propagation. For the mixed modes, the behaviors of the crack propagation are predicted by comparing the dislocation nucleation energy and the decohesion energy. The predictions of the R–T model are consistent with the MD results qualitatively. This research is intended to provide a fundamental explanation of the asymmetrical crack propagation at interface from atomscale.

Broadband asymmetric waveguiding of light without polarization limitations

Optical diodes are fundamental elements for optical computing and information processing. Attempts to realize such non-reciprocal propagation of light by breaking the time-reversal symmetry include using indirect interband photonic transitions, the magneto-optical effect, optical nonlinearity or photonic crystals. Alternatively, asymmetric reciprocal transmission of light has been proposed in photonic metamaterial structures for either circularly or linearly polarized waves. Here we employ the recent concept of gradient index metamaterials to demonstrate a waveguide with asymmetric propagation of light, independent of polarization. The device blocks both transverse electric and magnetic polarized modes in one direction but transmits them in the other for a broadband spectrum. Unlike previous works using chiral properties of metamaterials, our device is based on the principle of momentum symmetry breaking at interfaces with phase discontinuities. Experiments in the microwave region verify our findings, which may pave the way to feasible passive optical diodes.

A 4H-SiC Bipolar Technology for High-Temperature Integrated Circuits

A 4H-SiC bipolar technology suitable for high-temperature integrated circuits is tested with two interconnect systems based on aluminum and platinum. Successful operation of low-voltage bipolar transistors and digital integrated circuits based on emitter coupled logic (ECL) is reported from 27°C up to 500°C for both the metallization systems. When operated on −15 V supply voltage, aluminum and platinum interconnect OR-NOR gates showed stable noise margins of about 1 V and asymmetric propagation delays of about 200 and 700 ns in the whole temperature range for both OR and NOR output. The performance of aluminum and platinum interconnects was evaluated by performing accelerated electromigration tests at 300°C with current density of about 1 MA/cm2 on contact chains consisting of 10 integrated resistors. Although in both cases the contact chains failed after less than one hour, different failure mechanisms were observed for the two metallization systems: electromigration for the aluminum system and poor step coverage and via filling for the platinum system.

Asymmetric light propagation in transverse separation modulated photonic lattices

We investigate, both theoretically and numerically, the asymmetric light propagation in transverse separation modulated photonic lattices. The theoretical results show that the transmission contrast η of the structure is determined only by the coupling strengths between the chirped lattice and the two boundary uniform lattice portions. The numerical studies demonstrate that η is independent of the separation modulation function of the chirped lattice, which is in good agreement with the theory.