Near and far field

About: Near and far field is a research topic. Over the lifetime, 15922 publications have been published within this topic receiving 220571 citations.

All-metal wideband metasurface for near-field transformation of medium-to-high gain electromagnetic sources

Abstract: Electromagnetic (EM) metasurfaces are essential in a wide range of EM engineering applications, from incorporated into antenna designs to separate devices like radome. Near-field manipulators are a class of metasurfaces engineered to tailor an EM source’s radiation patterns by manipulating its near-field components. They can be made of all-dielectric, hybrid, or all-metal materials; however, simultaneously delivering a set of desired specifications by an all-metal structure is more challenging due to limitations of a substrate-less configuration. The existing near-field phase manipulators have at least one of the following limitations; expensive dielectric-based prototyping, subject to ray tracing approximation and conditions, narrowband performance, costly manufacturing, and polarization dependence. In contrast, we propose an all-metal wideband phase correcting structure (AWPCS) with none of these limitations and is designed based on the relative phase error extracted by post-processing the actual near-field distributions of any EM sources. Hence, it is applicable to any antennas, including those that cannot be accurately analyzed with ray-tracing, particularly for near-field analysis. To experimentally verify the wideband performance of the AWPCS, a shortened horn antenna with a large apex angle and a non-uniform near-field phase distribution is used as an EM source for the AWPCS. The measured results verify a significant improvement in the antenna’s aperture phase distribution in a large frequency band of 25%.

Validity of Simplified Approaches for the Evaluation of Lightning Electromagnetic Fields Above a Horizontally Stratified Ground

Abstract: We review in this paper simplified analytical expressions derived by Wait using the concept of attenuation function for the analysis of the propagation of lightning radiated electromagnetic fields over a horizontally stratified ground. Considerations regarding the use of these formulations as well as their domain of applicability are given. For the case, where the upper ground layer has a lower conductivity than the lower layer, the magnitude of the attenuation function can take values greater than unity. Time-domain waveforms of the vertical electric field along a horizontally stratified ground, obtained using the simplified formulations feature, an oscillatory behavior in their early-time response. The peak value of the field is also found to be larger than that corresponding to the case of a perfect ground. The accuracy of the Wait's formulations is examined taking as reference full-wave simulations obtained using the finite-difference time domain (FDTD) technique. FDTD simulations confirm the oscillatory waveform of the far field above a horizontally stratified ground (with an upper layer characterized by a lower conductivity than that of the lower layer), as well as the enhancement of the field peak compared to the case of a homogeneous, perfectly conducting ground.

Semi-infinite arrays of isotropic point scatterers. A unified approach

Abstract: We solve the two-dimensional problem of acoustic scattering by a semi-infinite pe- riodic array of identical isotropic point scatterers, i.e., objects whose size is negligible compared to the incident wavelength and which are assumed to scatter incident waves uniformly in all directions. This model is appropriate for scatterers on which Dirichlet boundary conditions are applied in the limit as the ratio of wavelength to body size tends to infinity. The problem is also relevant to the scattering of an E-polarized electromagnetic wave by an array of highly conducting wires. The actual geometry of each scatterer is characterized by a single parameter in the equations, related to the single-body scattering problem and determined from a harmonic boundary-value problem. Using a mixture of analytical and numerical techniques, we confirm that a number of phenomena reported for specific geometries are in fact present in the general case (such as the presence of shadow boundaries in the far field and the vanishing of the circular wave scattered by the end of the array in certain specific directions). We show that the semi-infinite array problem is equivalent to that of inverting an infinite Toeplitz matrix, which in turn can be formulated as a discrete Wiener-Hopf problem. Numerical results are presented which compare the amplitude of the wave diffracted by the end of the array for scatterers having different shapes.

Electromagnetic field correlations near a surface with a nonlocal optical response

Abstract: The coherence length of the thermal electromagnetic field near a planar surface has a minimum value related to the nonlocal dielectric response of the material. We perform two model calculations of the electric energy density and the field’s degree of spatial coherence. Above a polar crystal, the lattice constant gives the minimum coherence length. It also gives the upper limit to the near field energy density, cutting off its 1/z3 divergence. Near an electron plasma described by the semiclassical Lindhard dielectric function, the corresponding length scale is fixed by plasma screening to the Thomas–Fermi length. The electron mean free path, however, sets a larger scale where significant deviations from the local description are visible.

Thermally excited near-field radiation and far-field interference.

Abstract: Thermal radiation from samples of Au layers patterned on GaAs, SiO2, and SiC at 300 K are studied with a scattering-type scanning near-field optical microscope (wavelength: ~14.5 μm), without applying external illumination. Clear near-field images are obtained with a spatial resolution of ~60 nm. All the near field signals derived from different demodulation procedures decrease rapidly with increasing probe height h with characteristic decay lengths of 40 ~60 nm. Near-field images are free from any signature of in-plane spatial interference. The findings are accounted for by theoretically expected surface evanescent waves, which are thermally excited in the close vicinity of material surfaces. Outside the near-field zone (1 μm < h), signals reappear and vary as a sinusoidal function of h, exhibiting a standing wave-like interference pattern. These far-field signals are ascribed to the effect of weak ambient radiation.