In this paper, the unique features of periodic magneto-dielectric meta-materials in electromagnetics are addressed. These materials, which are arranged in periodic configurations, are applied for the design of novel EM structures with applications in the VHF-UHF bands. The utility of these materials is demonstrated by considering two challenging problems, namely, design of miniaturized electromagnetic band-gap (EBG) structures and antennas in the VHF-UHF bands. A woodpile EBG made up of magneto-dielectric material is proposed. It is shown that the magneto-dielectric woodpile not only exhibits band-gap rejection values much higher than the ordinary dielectric woodpile, but also for the same physical dimensions it shows a rejection band at a much lower frequency. The higher rejection is a result of higher effective impedance contrasts between consecutive layers of the magneto-dielectric woodpile structure. Composite magneto-dielectrics are also shown to provide certain advantages when used as substrates for planar antennas. These substrates are used to miniaturize antennas while maintaining a relatively high bandwidth and efficiency. An artificial anisotropic meta-substrate having /spl mu//sub r/>/spl epsiv//sub r/, made up of layered magneto-dielectric and dielectric materials is designed to maximize the bandwidth of a miniaturized patch antenna. Analytical and numerical approaches, based on the anisotropic effective medium theory (AEMT) and the finite-difference time-domain (FDTD) technique, are applied to carry out the analyzes and fully characterize the performance of finite and infinite periodic magneto-dielectric meta-materials integrated into the EBG and antenna designs.

/pdf/magneto-dielectrics-in-electromagnetics-concept-and-3epjjgbrla.pdf

Magneto-dielectrics in electromagnetics: concept and applications

Amorphous carbon nitride (a-CN) is a less-explored but promising photocatalyst for hydrogen production. Despite an extended visible light absorption (EVLA) its low quantum efficiency (QE) for water photoreduction is a long standing problem. This implies that EVLA is not proportionally translated into collection of large amounts of photogenerated electrons. Minimizing the mismatch between light-absorption and charge-collection remains a scientific challenge. Here a sponge-like hierarchical structure of a-CN that addresses this apparent mismatch is reported. Combined experimental and finite difference time domain simulations demonstrate the ability of the a-CN sponge to induce scattering for total internal light reflection that promotes localized charge carrier generation. Diffused reflectance and transient fluorescence decay studies show good agreement with simulations with a 40% enhanced light-trapping and an ≈23 times longer electron lifetime in spongy a-CN compared with that of the bulk material. The result is a new high benchmark for hydrogen production of 203.5 µmol h−1 with a QE of 6.1% at 420 nm in a reaction system of 10 vol% triethanolamine and 1 wt% Pt cocatalyst. The enhanced water photoreduction is a result of amenable photophysical and electrochemical attributes existing within the a-CN sponge.

/pdf/a-benchmark-quantum-yield-for-water-photoreduction-on-uskexf0gwk.pdf

A Benchmark Quantum Yield for Water Photoreduction on Amorphous Carbon Nitride

An efficient, elegant, and systematic formulation technique which, combining Fourier transform with matrix analysis methods, is suitable for problems related to radiation by dipole or other sources in the presence of an arbitrarily general stratified anisotropic medium has been recently developed. This technique is adapted further extended to allow the presence of general bianisotropic media described by four tensors with no limitations on their elements. Two specific applications pertaining to some canonical problems of fundamental importance are included to exemplify the method and demonstrate its usefulness: radiation by an arbitrarily oriented elementary electric dipole source located in the vicinity of a general bianisotropic slab, either grounded or ungrounded, leading to the expressions of the dyadic Green's function of the structure, and reflection and transmission of an arbitrarily polarized plane wave incident upon such a slab, leading to closed-form concise expressions for the reflection and transmission coefficient matrices. >

Interaction of electromagnetic waves with general bianisotropic slabs

An efficient and realistic electromagnetic scattering model for a tree trunk above a ground plane is presented. The trunk is modeled as a finite-length stratified dielectric cylinder with a corrugated bark layer. The ground is considered to be a smooth homogeneous dielectric with an arbitrary slope. The bistatic scattering response of the cylinder is obtained by invoking two approximations. In the microwave region, the height of the tree trunks are usually much larger than the wavelength. Therefore the interior fields in a finite length cylinder representing a tree trunk can be approximated with those of an infinite cylinder with the same physical and electrical radial characteristics. Also an approximate image theory is used to account for the presence of the dielectric ground plane which simply introduces an image excitation wave and an image scattered field. An asymptotic solution based on the physical optics approximation is derived which provides a fast algorithm with excellent accuracy when the radii of the tree trunks are large compared to the wavelength. The effect of a bark layer is also taken into account by simply replacing the bark layer with an anisotropic layer. It is shown that the corrugated layer acts as an impedance transformer which may significantly decrease the backscattering radar cross section depending on the corrugation parameters. It is also shown that for a tilted ground plane a significant cross-polarized backscattered signal is generated while the co-polarized backscattered signal is reduced. >

Electromagnetic scattering model for a tree trunk above a tilted ground plane

The problem of electromagnetic scattering by a uniaxially anisotropic sphere is studied for both dielectric and absorbing cases. The fields of the TM ro ř wave inside a uniaxial sphere can be derived to be expressed in series of Bessel functions, as in the isotropic case, but with the order of function determined by the anisotropic ratio (AR = eθ/er). For the TE to ř wave, the uniaxial medium can be treated as an isotropic material characterised by eθ only, which is the permittivity parallel to the boundary surface of the medium. Wave propagation is not affected by the f directed permittivity. The effects of uniaxial anisotropy on the radar cross section (RCS) are also considered. It is found that the uniaxial anisotropy greatly influences the scattering behaviour of a spherical dielectric object. For an absorbing sphere, however, the uniaxial anisotropy no longer plays a significant role as in a dielectric sphere, and the dependance of the RCS on uniaxial anisotropy is found to be predictable. Details of theoretical treatment and numerical results are presented.

Electromagnetic scattering by a uniaxially anisotropic sphere

An analytical formulation is presented for the computation of scattering and transmission by general anisotropic stratified material. This method employs a first-order state-vector differential equation representation of Maxwell's equations whose solution is given in terms of a 4 \times 4 transition matrix relating the tangential field components at the input and output planes of the anisotropic region. The complete diffraction problem is solved by combining impedance boundary conditions at these interfaces with the transition matrix relationship. A numerical algorithm is described which solves the state-vector equation using finite differences. The validation of the resultant computer program is discussed along with example calculations.

/pdf/electromagnetic-scattering-by-stratified-inhomogeneous-3kss2uyh1o.pdf

Electromagnetic scattering by stratified inhomogeneous anisotropic media

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Electromagnetic scattering by stratified inhomogeneous anisotropic media