Free Space Background Research Articles

A Miniaturized Anechoic Chamber: Omnidirectional Impedance Matching Based on Truncated Spatial Kramers–Kronig Medium

AbstractSpatial Kramers–Kronig (KK) medium provides a new platform to achieve omnidirectionally reflectionless absorption without using any gain elements. However, impedance‐matched spatial KK media realized in previous experiments generally have flat geometries with large transverse dimensions. Realizing finite spatial KK absorbers in free space background is important for many practical applications but has not been demonstrated experimentally thus far. Here, the concept of spatial KK relation to cylindrical coordinates is extended and a double‐resonance space‐frequency KK permittivity profile is implemented in a circular geometry. Omnidirectional impedance matching to free space is achieved by truncating the spatial KK absorber at the boundary where the relative permittivity is unity. For the experimental verification, a two‐dimensional 6.6‐wavelength‐diameter doughnut‐shaped anechoic chamber is implemented using the designed spatial KK medium. The measured electric field distributions and experimental imaging confirm the omnidirectionally reflectionless property, demonstrating that this miniaturized anechoic chamber is suitable for electrically small scatterer measurements and imaging. The proposed finite spatial KK absorber and fabricated miniaturized anechoic chamber show great promises for electromagnetic shielding and compatibility.

Electromagnetic characteristics of a metasurface composed of uncoated and coated spherical particles

A general theory for the reflection, transmission and equivalent surface impedance of a symmetric and non-reciprocal metasurface composed of dipole scatterers has been developed under oblique incidence. The proposed theory is then used for metasurfaces composed of uncoated and coated spheres under normal illumination. The materials considered for uncoated and coated spheres are realistic, i.e., magnetodielectric and Gallium Arsenide. Some specific designs of anti-reflector, reflector and absorber metasurfaces with the free space background have been discussed. It is also observed that isorefractive background media can be used to enhance the reflection magnitude from a metasurface composed of Gallium Arsenide coated magnetodielectric spheres as compared to the free space background under some specified conditions.

Surface impedance properties of a wire grid embedded in a chiral medium

AbstractThe surface impedance characteristics of a wire grid composed of infinitely long parallel conducting wires embedded in lossless and lossy frequency-dispersive chiral background media have been investigated. Using wavefield decomposition approach for a chiral background and with the application of impedance boundary conditions for a wire grid, an analytic expression for the surface impedance of a wire grid with a chiral background has been derived. It is shown that the surface impedance magnitude of a wire grid with chiral nihility background is close to zero and almost independent of incident polar angles. A strong chiral background significantly enhances the surface impedance magnitude of a wire grid for incident polar angles closer to right angle as compared to the free space background. The same electromagnetic appearance of a wire grid with frequency-dispersive chiral and free space background media at some critical frequency has also been discussed which may find applications in electromagnetic illusions. It is also shown that if the value of incident polar angles are closer to right angle then the impedance magnitude of a wire grid embedded in the realistic chiral background is smaller as compared to the same wire grid when placed in the free space background.

Reflection characteristics of a parallel wire grid embedded in a general bi-isotropic medium.

A periodic wire grid composed of infinitely long thin parallel conducting wires with a bi-isotropic background is considered. The reflection properties of this wire grid have been investigated using numerical calculations. The analytic expressions for the surface impedance and reflection coefficient have been derived using the wave field decomposition approach and with the application of impedance boundary conditions for a wire grid. The influences of various background media upon the reflection magnitude and phase have been studied. The considered background media are taken to be bi-isotropic, Tellegen, and chiral. It is investigated if the zero reflection for a wire grid with a free space background under normal illumination vanishes for bi-isotropic, Tellegen, and chiral background media. It is found that, for a specific type of realistic bi-isotropic background medium, an equivalent wire grid of ideally conducting wires becomes nearly a perfect reflector. It is also studied if this type of perfect reflector is almost independent of incident angles.

Optical pressure control with aperiodic nanostructured material

The electromagnetic force on matter depends on both the geometry and the material properties, and for a contiguous material with a periodic boundary condition, the pressure is a useful metric. We present a statistical method with example results that allows the evaluation of pressure in relation to a nanostructured material arrangement formed by populating pixels within a region one wavelength on a side. The two example materials considered are gold and silicon, both in a free-space background. We find that both the magnitude of the pressure and the direction can be regulated, depending on the geometry, and these effects are related to the specifics of the internal structure resonances. Control of positive and negative pressure can be understood as being due to the total field, a superposition of the incident and scattered fields, where the structure regulates the local scattered field and hence the pressure through an integral of the resulting force density. The statistical analysis provides physical insight into how to constrain the design framework for applications. The application space includes biophysics, where information is obtained about biomolecules from force and torque measurements, cavity optomechanics related to basic science and sensing, and optical remote control and actuation, where regulation of the magnitude and direction and the possibility of materials with multiple functionalities provide new opportunities.

Electric line source illumination of a chiral cylinder placed in another chiral background medium

An electric line source illumination of a chiral cylinder embedded in a chiral background medium is considered. The field expressions inside and outside of a chiral cylinder have been derived using the wave field decomposition approach. The effects of various chiral cylinders, chiral background media and source locations upon the scattering gain pattern have been investigated. It is observed that the chiral background reduces the backward scattering gain as compared to the free space background for a dielectric cylinder. It is also studied that by moving a line source away from a cylinder reduces the backward scattering gain for a chiral cylinder placed in a chiral background under some specific conditions. A unique phenomenon of reduced scattering gain has been observed at a specific observation angle for a chiral cylinder placed in a chiral background having an electric line source location of unity free space wavelength. An isotropic scattering gain pattern is observed for a chiral nihility background provided that if cylinder is chiral or chiral nihility type. It is also observed that this isotropic behaviour is independent of background and cylinder chirality.

Gain characteristics of a metamaterial coated slotted sphere

An analysis about the effects of various metamaterial coatings and background media upon the gain characteristics of a slotted sphere is presented using numerical simulations. It is found that the gain characteristics of a coated slotted sphere can be controlled by varying the electromagnetic parameters of coatings and background media. It is studied that the thickness of different coating layers has a significant effect upon the gain in the forward direction. It is further investigated that a double negative metamaterial coating can be used to greatly enhance the forward direction gain for certain values of the permittivity and the permeability. It is also shown that single near zero and double near zero background media enhance the forward direction gain of a dielectric-coated slotted sphere and reduce the gains for angles closer to polar directions as compared to the free space background.

Effects of bi-isotropic coatings and bi-isotropic background media upon gain characteristics of an axially slotted cylinder

An analysis about the effects of various bi-isotropic (BI) coatings and BI background media upon the gain characteristics of an axially slotted cylinder using numerical simulations is presented. It is investigated that chirality and Tellegen parameters of a coating and a background medium play a significant role in modifying the gain properties. It is further studied that an axially slotted cylinder when coated with a strong BI material and embedded in a free space background significantly enhances the gain in the forward direction. On the other hand, a strong Tellegen coating guides most of the radiated field from an axial slot toward rear side of the cylinder.

Gain properties of an uncoated and chiral coated slotted sphere embedded in a chiral background.

The gain properties of an uncoated and a chiral coated slotted sphere embedded in a chiral background have been investigated using numerical simulations. In this paper, it is found that a chiral background medium enhances the gain of an uncoated slotted sphere in the forward direction as compared to the free space background. It is shown that the forward direction gain of a chiral coated slotted sphere embedded in a chiral background increases with the increase in the background chirality. It is further determined that the maximum gain moves away from the polar direction toward the forward direction as the chirality of the coating increases for a fixed background chirality. Also, this maximum gain gradually decreases as the chirality of the coating increases. An interesting feature of an angular window is introduced for a chiral coated slotted sphere embedded in a chiral background where the gain is nearly constant for a specific range of angles.

Scattering characteristics of a chiral-coated cylindrical reflector embedded in a double-negative metamaterial

The scattering characteristics of a cylindrical reflector coated with a chiral material and embedded in a double-negative metamaterial have been studied using numerical simulation. The effects of double-negative metamaterial background media and chiral admittances of a chiral coating layer upon the co- and cross-polarized echo widths have been investigated. It is shown that a double-negative metamaterial background with relatively larger negative values of the permittivity and the permeability enhances the co-polarized echo width in the forward scattering direction and reduces this echo width in the backward scattering direction as compared to the free space background. It is further shown that by increasing the chiral admittance of a chiral-coated cylindrical reflector embedded in a double-negative metamaterial with fixed background parameters increases the co-polarized echo width in the forward and the backward scattering directions as compared to an achiral-coated cylindrical reflector embedded in the same double-negative metamaterial background.

Directive gain from an axially slotted dielectric clad cylinder embedded in a metamaterial

The gain properties of an axially slotted cylinder coated with a lossless dielectric layer and embedded in a lossless metamaterial have been studied analytically. It is studied that the gain pattern of this cylinder when embedded in Epsilon near-zero or double-near-zero metamaterial loses its directional characteristics as compared with the same cylinder embedded in the free space. It is also found that the dielectric coated slotted cylinder embedded in a single-negative metamaterial has a directive gain pattern as compared with the gain pattern of dielectric coated slotted cylinder when embedded in the free space background.

Study of the impedance mismatch at the output end of a THz parallel-plate waveguide

We study the reflection of terahertz (THz) radiation at the end of a parallel-plate waveguide (PPWG), due to the impedance mismatch between the propagating transverse-electromagnetic mode and the free-space background. We find that, for a PPWG with uniformly spaced plates, the reflection coefficient at the output face increases as the plate separation decreases, consistent with predictions by early low frequency ray optical theory. We also study the reflection coefficient for tapered PPWGs where the plate spacing is tapered, for which no analytical theory exists, and quantify the reflection coefficient as a function of the plate separation and the THz frequency.

Generalized nihility media from transformation optics

Nihility media in the previous literature are usually understood as media withϵ = μ = 0. Transformation optics opens a new perspective for capturing the essence ofsuch media. From this perspective, we generalize the definition of nihilitymedia as transformation media derived from volumeless geometrical elements.A volumeless geometrical elements can be either a point (P), a line (L), or asurface (S). Their corresponding transformation media are therefore called P-, L-,or S-type nihility media, respectively. The previous defined nihility media withϵ = μ = 0 is a special case under the P-type nihility media. The constructions of nihility media bymetamaterials are discussed. The eigenfields in different types of nihility media are derived.The interactions between an externally incident wave and a slab of nihility media in a freespace background are analyzed. Furthermore, we discuss compensated bilayerscomposed of nihility media. It is shown that for a slab of P-type nihility media, anormally incident wave can perfectly transmit through, while all obliquely incidentwaves are completely blocked; for a slab made of L-type nihility media, bothnormally and obliquely incident waves can transmit with some reflections, whichcan be eliminated by adding a compensating L-type nihility media; for a slabof S-type nihility media, all field components can perfectly transmit through.

Design and performance of a planar-array MIT system with normal sensor alignment

In this study the performance of a planar array for magnetic induction tomography (MIT) was investigated and the results of measurements to determine the precision and sensitivity of the sensor were undertaken. A planar-array MIT system utilizing flux-linkage minimization for the primary field has been constructed and evaluated. The system comprises 4 printed excitation coils of 4 turns which were shielded, 8 surface-mount inductors of inductance 10 µH as sensor, mounted such that in principle no primary-field flux threads them, and a calibration coil to produce a strong primary field. The excitation current was multiplexed via relays to drive the excitation and reference coils. The noise values were similar in real and imaginary components in the lower frequencies and the factor to which the primary field could be reduced was greatest in the nearest coil. Methods for determining the true real and imaginary components and for flux-linkage minimization for the primary field for variations in channel sensitivities are described and the results of measurements of the system's noise and drift are given. A SNR of 47 dB was observed at 4 MHz when a 0.3 Sm−1 saline filled tank of dimensions 20 cm × 20 cm × 10 cm was placed centrally over the array. Finally, images were reconstructed from measurements of saline samples in a free space background, with the samples moved past the array in 21 1 cm steps to emulate mechanical scanning of the array. The image reconstruction characteristics of the planar array in conjunction with the reconstruction technique employed are discussed.

Numerical Testing of Dielectric Mixing Rules By Fdtd Method

The present paper reports the results of an extensive numerical analysis of electromagnetic fields in random dielectric materials. The effective permittivity of the mixture is calculated by FDTD simulations of such a sample in a TEM-waveguide. The mixture consists of a homogeneous free space background in which circular cylinders are embedded in random positions. Clustering of the inclusions is allowed. The choice of two-dimensional spheres allows us to make much more use of the numerical capacity than what would be the case in three-dimensional inclusions. Various dielectric contrasts are studied from permittivity ratio 2 to 50. Numerical results are compared with classical mixing rules, and they show moderate agreement with the Bruggeman rule rather than the Maxwell Garnett approach. However, another key finding of this paper is that obtained mixing results are not symmetric with respect to inclusions and background. This makes the exact validity of the symmetric Bruggeman formula questionable.