Showing papers on "Metamaterial absorber published in 2005"

Electromagnetic parameter retrieval from inhomogeneous metamaterials

Abstract: We discuss the validity of standard retrieval methods that assign bulk electromagnetic properties, such as the electric permittivity « and the magnetic permeability m, from calculations of the scattering sSd parameters for finite-thickness samples. S-parameter retrieval methods have recently become the principal means of characterizing artificially structured metamaterials, which, by nature, are inherently inhomogeneous. While the unit cell of a metamaterial can be made considerably smaller than the free space wavelength, there remains a significant variation of the phase across the unit cell at operational frequencies in nearly all metamaterial structures reported to date. In this respect, metamaterials do not rigorously satisfy an effective medium limit and are closer conceptually to photonic crystals. Nevertheless, we show here that a modification of the standard S-parameter retrieval procedure yields physically reasonable values for the retrieved electromagnetic parameters, even when there is significant inhomogeneity within the unit cell of the structure. We thus distinguish a metamaterial regime, as opposed to the effective medium or photonic crystal regimes, in which a refractive index can be rigorously established but where the wave impedance can only be approximately defined. We present numerical simulations on typical metamaterial structures to illustrate the modified retrieval algorithm and the impact on the retrieved material parameters. We find that no changes to the standard retrieval procedures are necessary when the inhomogeneous unit cell is symmetric along the propagation axis; however, when the unit cell does not possess this symmetry, a modified procedure—in which a periodic structure is assumed—is required to obtain meaningful electromagnetic material parameters. DOI: 10.1103/PhysRevE.71.036617

Gradient index metamaterials.

Abstract: Metamaterials—artificially structured materials with tailored electromagnetic response—can be designed to have properties difficult or impossible to achieve with traditional materials fabrication methods. Here we present a structured metamaterial, based on conducting split ring resonators sSRRsd, which has an effective index of refraction with a constant spatial gradient. We experimentally confirm the gradient by measuring the deflection of a microwave beam by a planar slab of the composite metamaterial over a range of microwave frequencies. The gradient index metamaterial may prove an advantageous alternative approach to the development of gradient index lenses and similar optics, especially at higher frequencies. In particular, the gradient index metamaterial we propose may be suited for terahertz applications, where the magnetic resonant response of SRRs has recently been demonstrated.

Directive emissions from subwavelength metamaterial-based cavities

Abstract: We use experiment and theory to demonstrate a mechanism for directive emissions, which involves a double-plate resonance cavity made with metamaterials. In contrast to other mechanisms employing Fabry-Perot cavities, photonic crystals, or zero index materials, our system is significantly thinner than the working wavelength and requires a smaller lateral size. We show the physics to be governed by subwavelength resonance modes unique to such metamaterial-based cavities.

A brief intro to metamaterials

Abstract: Metamaterials are new artificial materials with unusual electromagnetic properties that are not found in naturally occurring materials. All "natural" materials such as glass, diamond and such have positive electrical permittivity, magnetic permeability and an index of refraction. In these new artificially fabricated materials - termed as negative index materials (NIM) or double negative (ONG) media or left handed (LH) materials or backward wave (BW) media - all these material parameters are negative. With these unusual material parameters, new kinds of miniaturized antennas and microwave components/devices can be created for the wireless communications and the defense industries. The electrical permittivity and the magnetic permeability are the main determinants of a material's response to electromagnetic (EM) waves. In metamaterials, both these material parameters are negative. Correspondingly, the refractive index of the metamaterials is also negative. Another strange property of metamaterials is its reverse Doppler effect.

Design and measurement of a four-port device using metamaterials

Abstract: A band separating device that uses an anisotropic metamaterial exhibiting negative refraction is designed and measured. The metamaterial has frequency dispersion in one component of the permeability tensor. A beam is incident onto the metamaterial and undergoes frequency dependent reflection and refraction directing different bandwidths towards three distinct measurement ports. Design issues are discussed, and measurement results are presented.

Compact leaky-wave components using metamaterial bilayers

Abstract: Here we propose and analyze theoretically the use of metamaterial planar layers for synthesizing compact sub-wavelength leaky-wave radiators in the microwave regime. In particular, the possibility of pairing "conjugate" metamaterials in order to squeeze the dimensions of microwave components is explored here for the leaky-wave operation of an open waveguide consisting of a stack of grounded metamaterial layers. We show the salient guidance and radiation properties of the anomalous natural modes supported by these bilayered structures, giving further physical insights into the phenomenon.

Planar metamaterials and applications in directive antennas

Abstract: In this paper, we reviewed some recent studies on planar metamaterials and demonstrated two new way to realize high directive antenna for the first time: 1) Quasi-periodic metamaterial surface can suppress transverse-electric (TE) surface waves and support directive emissions 2) Periodic metamaterial surface can be applied in realizing one-dimensional subwavelength Fabry-Perrot (FP) metamaterial cavity which support directive emissions We show both experimentally and theoretically that these phenomena mainly originate from the dispersive reflection phase and surface wave dispersions

Metamaterial properties, designs, and antenna applications

Abstract: Metamaterials have the potential to allow engineers to tailor the electromagnetic responses of the materials incorporated in antenna systems to enhance their performance. Several examples of the exotic properties of electromagnetic metamaterials will be given. One class of metamaterials that has already demonstrated its usefulness with antennas is the artificial magnetic conductor (AMC). It has been demonstrated that this metamaterial configuration can yield a highly resonant radiating system. The resonances occur for particular combinations of shell sizes and metamaterial properties; they result in significant enhancements of the radiated power. These resonance results have been extended to lossy DNG and epsilon negative (ENG) metamaterials. We have also demonstrated reciprocity between these source results and the scattering of plane waves from the corresponding metamaterial coated spheres. We have shown with HFSS numerical simulations that realistic center-fed electrically small dipole antennas surrounded with an ENG spherical shell and coaxially-fed monopole antennas surrounded with an ENG hemispherical shell can be made to be resonant and matched to an external source. These electrically small antenna systems have very high radiation efficiencies and Q factors near the Chu limit. A review of these metamaterial-based electrically small antenna systems will be presented and current progress toward a proof-of-principle experiment will be discussed

Miniaturization of metamaterials-based waveguide structures

Abstract: Novel metamaterial-based rectangular waveguide structures are presented Incorporating a metamaterial slab, electromagnetic properties of the structures are changed to allow propagation of backward waves

Design and testing of different metamaterial structures

Abstract: The design and testing of several metamaterials that exhibit negative permittivity and negative permeability over microwave frequency bands are presented. A transmission experiment and beam refraction experiment are carried out. The performance, especially the bandwidth of negative refraction, and the losses are compared. The sample fabricated by using a hot-press technique is shown to have a reduced loss.