Influence of stagnant zones on transient and asymptotic dispersion in macroscopically homogeneous porous media

The role of stagnant zones in hydrodynamic dispersion is studied for creeping flow through a fixed bed of spherical permeable particles, covering several orders of characteristic time and length scales associated with fluid transport. Numerical simulations employ a hierarchical model to cope with the different temporal and spatial scales, showing good agreement with our experimental results on diffusion-limited mass transfer, transient, and asymptotic longitudinal dispersion. These data demonstrate that intraparticle liquid holdup in macroscopically homogeneous porous media clearly dominates over contributions caused by the intrinsic flow field heterogeneity and boundary-layer mass transfer.     

窄禁带半导体的非线性极化率和光学双稳性

本文改进了半导体两带模型,计算了窄禁带半导体的非线性极化率.运用所求得的极化率,研究了InSb的光学双稳性,所得到的稳态曲线、临界开关光强与入射光频率的关系以及动态特性都与实验结果有较好的符合.     

Atom Trap Trace Analysis of Ca Isotopes

In our experiment we aim at the detection of the rarest, naturally occuring calcium isotope Ca by means of atom trap trace analysis. On basis of single-atom detection of Ca our present sensitivity for Ca is estimated to be 1 atom per hour at an abundance of 10. To reach a sensitivity at the level of natural abundance, which is 10, we need to reduce atomic beam losses. To achieve this, optical compression of the atomic beam is a promising option. We use Monte Carlo Simulations to demonstrate that optical compression of the atomic beam increases throughput of the atomic beam as well as isotope selectivity.     

Joint cosmic microwave background and weak lensing analysis: constraints on cosmological parameters

We use cosmic microwave background (CMB) observations together with the red-sequence cluster survey weak lensing results to derive constraints on a range of cosmological parameters. This particular choice of observations is motivated by their robust physical interpretation and complementarity. Our combined analysis, including a weak nucleosynthesis constraint, yields accurate determinations of a number of parameters including the amplitude of fluctuations sigma(8)=0.89+/-0.05 and matter density Omega(m)=0.30+/-0.03. We also find a value for the Hubble parameter of H(0)=70+/-3 km s(-1) Mpc(-1), in good agreement with the Hubble Space Telescope key-project result. We conclude that the combination of CMB and weak lensing data provides some of the most powerful constraints available in cosmology today.     

耗散系统的准热力学模型

以具有自治系统一般形式的运动方程及其相应的Fokker-Planck方程为出发点,确立了一种适应面比较广的耗散系统的准热力学模型.定义了与驱动项f相共轭的函数X,取代作为上述运动方程的自田变量x(x可代表某种输出或化学反应的产物).以状态参量f,X,C(C为控制参量)对照参量p,V,T,得到一系列涉及准热力学函数及其导数的基本关系式,它们与传统热力学是一一对应的,最后给出一个应用范例. 关键词：     

Optimisation of digital optical switches

A generally applicable procedure is given for shaping of 1 × 2 digital optical switches in order to reduce the cross-talk (CT) or, equivalently, the required device length. The method is based on coupled mode theory and assumes that the two coupled mode equations may be decoupled in good approximation. Doing so it follows that the cross talk may be expressed in terms of a Fourier component of a function, which depends on the shape of the device. Based on this a procedure for optimising the Y-junction may be formulated. Calculations, using the beam propagation method and coupled mode theory, on a thus obtained structure are presented and show that a length reduction by a factor of 2 can be obtained at a CT level of –38 dB.     

Semi-guided plane wave reflection by thin-film transitions for angled incidence

The non-normal incidence of semi-guided plane waves on step-like or tapered transitions between thin film regions with different thicknesses, an early problem of integrated optics, is being reconsidered. As a step beyond the common effective index picture, we compare two approaches on how this problem can be tackled—at least approximately—by nowadays readily available simulation tools for integrated optics design. Accepting the scalar approximation, using an ansatz of harmonic field dependence on the position along the interface, the 3-D problem reduces to a 2-D Helmholtz problem, for guided wave input and transparent-influx boundary conditions, with an effective permittivity that depends on the incidence angle. Alternatively, one complements the structure with a second mirrored interface, such that the 2-D cross section of a wide multimode rib waveguide emerges. Constraints for transverse resonance then permit to translate the propagation constants of its polarized modes into discrete samples of the phase changes experienced by an in-plane guided wave upon total internal reflection at the sidewalls.