Probability Distributions of Random Electromagnetic Fields in the Presence of a Semi-Infinite Isotropic Medium

Using a TE/TM decomposition for an angular plane-wave spectrum of free random electromagnetic waves and matched boundary conditions, we derive the probability density function for the energy density of the vector electric field in the presence of a semi-infinite isotropic medium. The theoretical analysis is illustrated with calculations and results for good electric conductors and for a lossless dielectric half-space. The influence of the permittivity and conductivity on the intensity, random polarization, statistical distribution and standard deviation of the field is investigated, both for incident plus reflected fields and for refracted fields. External refraction is found to result in compression of the fluctuations of the random field.Comment: 23 pages, 11 figures, accepted for publication in Radio Scienc

Probability Distribution of the Quality Factor of a Mode-Stirred Reverberation Chamber

We derive a probability distribution, confidence intervals and statistics of the quality (Q) factor of an arbitrarily shaped mode-stirred reverberation chamber, based on ensemble distributions of the idealized random cavity field with assumed perfect stir efficiency. It is shown that Q exhibits a Fisher-Snedecor F-distribution whose degrees of freedom are governed by the number of simultaneously excited cavity modes per stir state. The most probable value of Q is between a fraction 2/9 and 1 of its mean value, and between a fraction 4/9 and 1 of its asymptotic (composite Q) value. The arithmetic mean value is found to always exceed the values of all other theoretical metrics for centrality of Q. For a rectangular cavity, we retrieve the known asymptotic Q in the limit of highly overmoded regime.Comment: accepted for publication in IEEE Trans. Electromagn. Compat., 201

Nonstationary random acoustic and electromagnetic fields as wave diffusion processes

We investigate the effects of relatively rapid variations of the boundaries of an overmoded cavity on the stochastic properties of its interior acoustic or electromagnetic field. For quasi-static variations, this field can be represented as an ideal incoherent and statistically homogeneous isotropic random scalar or vector field, respectively. A physical model is constructed showing that the field dynamics can be characterized as a generalized diffusion process. The Langevin--It\^{o} and Fokker--Planck equations are derived and their associated statistics and distributions for the complex analytic field, its magnitude and energy density are computed. The energy diffusion parameter is found to be proportional to the square of the ratio of the standard deviation of the source field to the characteristic time constant of the dynamic process, but is independent of the initial energy density, to first order. The energy drift vanishes in the asymptotic limit. The time-energy probability distribution is in general not separable, as a result of nonstationarity. A general solution of the Fokker--Planck equation is obtained in integral form, together with explicit closed-form solutions for several asymptotic cases. The findings extend known results on statistics and distributions of quasi-stationary ideal random fields (pure diffusions), which are retrieved as special cases.Comment: 54 pages, 8 figures, to appear in J. Phys. A: Math. Theo

Photochemistry and photophysics of thienocarbazoles

Two methylated thienocarbazoles and two of their synthetic nitro-precursors have been examined by absorption, luminescence, laser flash photolysis and photoacoustic techniques. Their spectroscopic and photophysical characterization involves fluorescence spectra, fluorescence quantum yields and lifetimes, and phosphorescence spectra and phosphorescence lifetimes for all the compounds. Triplet-singlet difference absorption spectra, triplet molar absorption coefficients, triplet lifetimes, intersystem crossing S-1 similar tosimilar to--> T-1 and singlet molecular oxygen yields were obtained for the thienocarbazoles. In the case of the thienocarbazoles it was found that the lowest-lying singlet and triplet excited states, S, and T-1, are of pi,pi* origin, whereas for their precursors S-1 is n,pi*, and T-1 is pi,pi*. In both thienocarbazoles it appears that the thianaphthene ring dictates the S, T, yield, albeit there is less predominance of that ring in the triplet state of the linear thienocarbazole, which leads to a decrease in the observed Phi(T) value.info:eu-repo/semantics/publishedVersio

Postcolonial manifestations of African spatiality in Europe : the invisible 'public' spaces of Ghent

The focus of this chapter is on everyday spaces of African migration in the mid-sized city of Ghent. One manifestation of African spatiality is discussed in-depth in relation to its (in)visibility and publicity: an African shop located in an ordinary terraced house. With no less than 12 activities taking place in the building, the shop is rather a “public” place than solely a space of commercial transactions, although this is not signaled in very visible ways. By analyzing the modest stylistic appropriations of the façade and the significant re-arrangements of the buildings’ interior spaces that challenge more conventional usages of spaces in Ghent’s ordinary houses, this chapter puts this African shop to the fore as emblematic of how the process of materialization of transnational lifestyles and connections is always a balancing act between the visibility necessary for functioning as a (semi-)pubic place and the invisibility required to circumvent hegemonic regulatory regimes