Comparative analysis of two models for the optical response of a thin slab: a film of finite thickness versus a surface current sheet.

An accurate description of the electromagnetic properties of materials is fundamental to optical and electric devices. As a current research hotspot, thin slabs generally are modeled as a film of finite thickness with a dielectric function. However, inspired by two-dimensional materials, thin slabs can be regarded as surface current sheets with conductivity. Due to the convenience of the latter in simplifying the calculations, it becomes increasingly significant to determine the equivalent conditions of the two models. In this work, we compare the differences between the thin film and surface current models in calculating the transmissivity, reflectivity, and absorptivity of a SiC film. For normal incidence, the difference between the calculations of the two models is only non-negligible when the thickness is large (500 nm), because of the invalidation of surface current models and the excitation of Fabry-Perot resonance. In particular, we derive analytical formulas for the relative error in transmittance versus phase difference, which can be used to predict the difference between the two models as a function of film thickness. For oblique incidence, the two models have significant differences in the vicinity of epsilon-near-zero (ENZ) frequency. The excitation of the Berreman leaky mode in a thin film model causes a narrow blank absorption peak close to the ENZ frequency. However, we found that the surface current model is unable to form this resonance mode and further demonstrate it theoretically. In addition, it is found that the two models are equivalent in the case of a transverse electric wave even though the incidence is oblique. This work can enhance the awareness of the light-matter interaction and open unprecedented avenues for designing ultrathin optical devices.

Surface and volume phonon polaritons in a uniaxial hyperbolic material: optic axis parallel versus perpendicular to the surface.

Uniaxial hyperbolic materials enable excitation of phonon polaritons with utrahigh wavevectors that have been shown to be promising for many optical and thermal radiative applications and thus have attracted much attention recently. However, the characteristics of surface and volume phonon polaritons excited with uniaxial hyperbolic materials that exhibit in-plane anisotropy or in-plane isotropy have not been discussed thoroughly and some issues have so far remained elusive. In this paper, we conducted a comprehensive investigation on surface and volume phonon polaritons in a bulk or a thin slab of hexagonal boron nitride (hBN). We clarified the excitation, characteristics and topology of surface and volume phonon polaritons in such a uniaxial hyperbolic material. In particular, we showed that hyperbolic surface phonon polaritons (HSPhPs) can exist in the Type I hyperbolic band of hBN with confined wavevectors when the optic axis (OA) is parallel to the surface. For a thin hBN slab, we revealed a split of HSPhPs and a smooth transition between HSPhPs and HVPhPs in the Type II hyperbolic band. Furthermore, we also identified non-Dyakonov surface phonon polaritons excited without evanescent ordinary waves. These findings may extend the understanding of phonon polaritons in hyperbolic materials and offer new theoretical guidance for the design of infrared optical devices with hyperbolic materials.

Multiband selective absorbers made of 1D periodic Ag/SiO2/Ag core/shell coaxial cylinders horizontally lying on a planar substrate.

In this paper, we present a one-dimensional periodic microstructure for multiband selective absorbers of thermal radiation. The microstructure is made of Ag/SiO2/Ag core/shell coaxial cylinders horizontally lying on top of a SiO2 dielectric spacer and an opaque silver substrate. The spectral-directional absorptivity of the proposed structure was numerically investigated with the finite element based Comsol Multiphysics software. Multiband selective absorption in the wavenumber range from 2500 to 20000 cm-1 for TM-wave incidence was obtained. Physical mechanisms responsible for the multiband selective absorption were elucidated due to the resonance of magnetic polaritons in the SiO2 spacer shell, excitation of surface plasmon polaritons at the SiO2/Ag interface, and the effect of Wood's anomaly. Furthermore, the effects of a silver core radius, spacer shell thickness, a confocal elliptical core/shell cylinder on the property of multiband absorption, and the absorptivity of the structure with one core/four shells coaxial cylinders were explored.

[Hemodynamic analyses of large intracranial aneurysms].

OBJECTIVE: To simulate the computational hemodynamics of large intracranial aneurysms and analyze the hemodynamics of three types of large intracranial aneurysms. METHODS: A total of 32 patient-specific models of large intracranial aneurysms were constructed with the data of DSA (digital subtraction angiography). According to the location of outflow vessel, plane of main vortex and impact zone, large intracranial aneurysms were classified into type A (outflow vessel in the plane of main vortex), type B1 (outflow vessel out of plane of main vortex, impact zone at the lateral wall of aneurysm) and type B2 (outflow vessel out of plane of main vortex, impact zone at the dome of aneurysm). Blood flow was assumed to be laminar and incompressible and blood Newtonian fluid. The time-dependent pulsatile boundary condition was deployed at inlet. CFD ICEM and Fluent software packages were used to simulate the computational hemodynamics of large intracranial aneurysms. RESULTS: The distributions of hemodynamic variables during the cardiac cycle were analyzed for wall shear stress, velocity and streamlines. The velocity ratio (ratio of aneurysmal flow velocity to parent artery flow velocity) of type A, B1 and B2 was 0.186 ± 0.019, 0.706 ± 0.077 and 0.208 ± 0.041 respectively. The wall shear stress ratio (ratio of aneurysmal wall shear stress to parent artery wall shear stress) of types A, B1 and B2 was 0.081 ± 0.029, 1.019 ± 0.139 and 0.103 ± 0.031 respectively. The flow velocity and wall shear stress were the highest in type B1 group, followed by those in type B2 group and the lowest in type A group. CONCLUSION: As reflected by the location of impact zone, the location of outflow vessel and inflow-angle can influence the level of blood flow in aneurysm sac.

Comparative analysis of two models for phonon polaritons in van der Waals materials: 2D and 3D.

Phonon polaritons with ultralow losses and high confinement in extremely anisotropic media have opened up new avenues for manipulating the flow of light at the nanoscale. Recent advances in var der Waals (vdW) materials reveal unprecedented dispersion characteristics of polaritons using a two-dimensional (2D) model, treating the slab as a surface without thickness. However, the difference between the 2D and three-dimensional (3D) models of hyperbolic polaritons remains largely unexplored. Herein, we compare the polaritonic difference between these two models for biaxial vdW slabs. In addition, we demonstrate that the fundamental mode in slab configuration corresponds to the polaritonic mode in surface sheet and higher-order modes vanish in the latter configuration. In particular, we reveal that the difference in in-plane polaritons along the [100] and [001] crystal directions between the two models is associated with the inverse of the dielectric function along these two directions. For example, we compare the near-field radiative heat transfer (NFRHT) between two vdW slabs based on these two models. It is found that when the attenuation length of the higher-order hyperbolic mode is less than the gap distance, the enhancement achieved using the 3D model comes from only the fundamental mode, resulting in a negligible difference between these two models. Therefore, our findings may help to understand in-plane anisotropic polaritons and provide guidance for the application of the 2D model in the analysis of vdW materials.

Blood flow and macromolecular transport in complex blood vessels.

Numerical simulations of pulsatile flows and macromolecular (such as LDL) transport in complex blood vessels, including the cerebral artery, are carried out using the FLUENT software. The hemodynamic factors such as axial velocity, secondary flow as well as LDL concentration distribution in the complex vessel are obtained. It is found that in the case of pulsatile flow, the LDL concentration is higher in the central region of the flow than on the wall. Under the precondition of impermeability, the numerical results indicate that the blood flow is quite complicated in complex blood vessel. The complex flow can reduce the LDL concentration on the vessel wall, which is helpful to prevent the concentration polarization.

Modeling calcium wave based on anomalous subdiffusion of calcium sparks in cardiac myocytes.

Ca(2+) sparks and Ca(2+) waves play important roles in calcium release and calcium propagation during the excitation-contraction (EC) coupling process in cardiac myocytes. Although the classical Fick's law is widely used to model Ca(2+) sparks and Ca(2+) waves in cardiac myocytes, it fails to reasonably explain the full-width at half maximum(FWHM) paradox. However, the anomalous subdiffusion model successfully reproduces Ca(2+) sparks of experimental results. In this paper, in the light of anomalous subdiffusion of Ca(2+) sparks, we develop a mathematical model of calcium wave in cardiac myocytes by using stochastic Ca(2+) release of Ca(2+) release units (CRUs). Our model successfully reproduces calcium waves with physiological parameters. The results reveal how Ca(2+) concentration waves propagate from an initial firing of one CRU at a corner or in the middle of considered region, answer how large in magnitude of an anomalous Ca(2+) spark can induce a Ca(2+) wave. With physiological Ca(2+) currents (2pA) through CRUs, it is shown that an initial firing of four adjacent CRUs can form a Ca(2+) wave. Furthermore, the phenomenon of calcium waves collision is also investigated.

Slip-flow and heat transfer of a non-newtonian nanofluid in a microtube.

The slip-flow and heat transfer of a non-Newtonian nanofluid in a microtube is theoretically studied. The power-law rheology is adopted to describe the non-Newtonian characteristics of the flow, in which the fluid consistency coefficient and the flow behavior index depend on the nanoparticle volume fraction. The velocity profile, volumetric flow rate and local Nusselt number are calculated for different values of nanoparticle volume fraction and slip length. The results show that the influence of nanoparticle volume fraction on the flow of the nanofluid depends on the pressure gradient, which is quite different from that of the Newtonian nanofluid. Increase of the nanoparticle volume fraction has the effect to impede the flow at a small pressure gradient, but it changes to facilitate the flow when the pressure gradient is large enough. This remarkable phenomenon is observed when the tube radius shrinks to micrometer scale. On the other hand, we find that increase of the slip length always results in larger flow rate of the nanofluid. Furthermore, the heat transfer rate of the nanofluid in the microtube can be enhanced due to the non-Newtonian rheology and slip boundary effects. The thermally fully developed heat transfer rate under constant wall temperature and constant heat flux boundary conditions is also compared.

Brewster angle with a negative-index material.

The demonstration and confirmation of metamaterials with simultaneous negative permittivity and permeability, and thus a negative refractive index, has resulted in a surge of interest in the reflection and refraction phenomena at the interfaces of these so-called negative-index materials (NIMs). We present a systematic study of the Brewster angle, i.e., the angle of incidence at which no reflection occurs, for both TE and TM waves scattering at the interface between two semi-infinite planar media, one of which may be a NIM. Detailed physical explanations that account for the Brewster angle for a plane wave incident upon a NIM are provided under the framework of the Ewald-Oseen extinction theorem, considering the reemission of induced electric and magnetic dipoles. The conditions under which the Brewster angle exists are concisely summarized in a map of different material parameter regimes.