In this paper, we describe the strain-dependent behavior of an electric-LC ͑ELC͒ resonator unit cell, commonly used in metamaterial designs. We leverage analytic expression to understand the way strain manifests itself in a change in electromagnetic ͑EM͒ response. We verify the simplified physical models using full-wave simulations and generalize the trends to accommodate the strain profile for any arbitrary plane-stress loading scenario. © 2010 American Institute of Physics. ͓doi:10.1063/1.3507892͔ Metamaterials can greatly expand man's ability to control interactions with electromagnetic radiation and enable such phenomena as cloaking, 1,2 beyond diffraction-limited imaging, 3 gradient negative-index lenses, 4 and perfect absorbers. 5 They are a powerful concept by allowing designers to utilize geometry, and not just material properties, to engineer a structure's electromagnetic response; often providing properties not found in nature.However, transitioning metamaterials into real, operational systems requires knowledge of their behavior in relevant environments. Of significance is the role mechanical loading/strain plays in the electromagnetic response of a metamaterial. Mechanical strain is by definition, a deformation of the geometry of a structure. Since metamaterials rely so heavily on geometry for the desired response, it implies a direct causal relationship between applied strain and electromagnetic performance.Previous efforts investigated the strain 6 and temperature 7 dependent response of magnetic resonant elements; Melik et al. 6 even proposes using metamaterials as wireless strain gauges. Our efforts focused on a critical missing piece, the electric-LC resonator, depicted in Fig. 1. This structure operates at x-band, utilizing two parallel capacitors for enhanced resonant response. Figure 1 depicts the S-parameter curves for the cell.The mechanical model assumes the metamaterial unit cell is part of a large ͑Ͼ10 ͒, load-bearing structure. Mechanical loading on the cell is homogeneous, and the copper contributes insignificantly to the overall stiffness of the composite; therefore, the in-plane strain profile is approximately uniform across the unit cell ͑no local stiffening effects from the adhered copper͒. Due to the uniformity of the strain profile and the resultant absence of higher order differential terms, the linear system of Eq. ͑1͒ can be utilized to describe the deformed geometry of the unit cell as follows: 8The superscript 0 and 1 refer to the undeformed and deformed geometries, respectively, and the 3 ϫ 3 matrix is the mechanical strain tensor. The model accommodates different values of E ZZ in the substrate and copper, due to the differing mechanical properties in the materials. Additionally, the analysis was restricted to plane stress because 1,2 demonstrated that changes in EM performance due to curvature in the unit cell can be neglected. Therefore, terms E XZ and E YZ are equal to zero.The unit cell was modeled in ANSYS-HFSS, 9 utilizing equation surfaces that integrate the strain ...
In this letter, we discuss the design, fabrication, and experimental characterization of a bi-layer fully functional near-perfect metamaterial absorber (MMA) in the long-wavelength infrared (LWIR), which is broadband and generally insensitive to polarization up to a 60 incidence angle. A spectral absorptance of !99% was attained simultaneously at multiple LWIR wavelengths, with a bandwidth of 2 lm where the absorptance is !90%. This remarkable behavior is attributed to the strong mixing of coupling modes between the two resonators and the ground plane in the presence of a lossy dielectric, in which single layer structures do not exhibit. Furthermore, we show, by comparing two different MMA structures, how the absorption can be tailored by design within and across several IR subdivisions through a slight change in geometrical parameters. The bi-layer MMA has the immediate application of a functionally versatile, low-profile thermal sensor or emitter. Perfect absorbers in the infrared (IR) are particularly exciting due to their applicability as thermal emission control surfaces, among other applications. Efforts have been made to tailor the thermal emission of structures through the use of 1-D and 2-D gratings, 1,2 photonic crystals, 3,4 and microstrip patch arrays.5 However, many of these solutions are narrowband and lack functionality at certain polarizations or incidence angles, all of which are critical to practical thermal applications where maximum absorption of light is desired. 6 Since the experimental demonstration of a near-unity metamaterial absorber (MMA) in the microwave spectrum in 2008, 7 a great deal of research has been invested into establishing the utility of a perfect absorber for real-world application. Work rapidly expanded into the terahertz, 8 short-wave IR, 9 mid-wave IR, 10 near-infrared, 11,12 and visible range; 13 and with it came a flood of attempts to increase bandwidth and insensitivities to polarization and incidence angles (see Ref.6 for a comprehensive review). Initially, most of these designs suffered in one area or another: multi-band and wideangle designs were still narrow-band 14-17 or polarization-dependent, 7,8 while these wholly insensitive broadband designs did not attain a consistent absorptance over 90%. 18,19 However, significant advancements have been made in creating "fully functional" near-perfect absorbers: those which exhibit strong broadband responses while being simultaneously angle and polarization insensitive. In the past year alone, fully functional MMAs have been demonstrated in the microwave, 20-25 terahertz, 26-28 and optical 29 regimes. Despite this advancement, we found a lack in development of fully functional IR MMAs. [30][31][32][33][34][35][36][37][38] In this letter, we demonstrate, both computationally and experimentally, a fully functional near-perfect MMA in the long wavelength IR (LWIR) range. We used a bi-layer pattern to achieve a wide-angle, polarization-insensitive, broadband performance which is superior overall to many single layer 30,31,33...
This paper investigates three-dimensional cut wire pair (CWP) behavior in vertically oriented meta-atoms. We first analyze CWP metamaterial inclusions using full-wave electromagnetic simulations. The scattering behavior of the vertical CWP differs substantially from that of the planar version of the same structure. In particular, we show that the vertical CWP supports a magnetic resonance that is solely excited by the incident magnetic field. This is in stark contrast to the bianisotropic resonant excitation of in-plane CWPs. We further show that this CWP behavior can occur in other vertical metamaterial resonators, such as back-to-back linear dipoles and back-to-back split ring resonators (SRRs), due to the strong coupling between the closely spaced metallic elements in the back-to-back configuration. In the case of SRRs, the vertical CWP mode (unexplored in previous literature) can be excited with a magnetic field that is parallel to both SRR loops, and exists in addition to the familiar fundamental resonances of the individual SRRs. In order to fully describe the scattering behavior from such dense arrays of three-dimensional structures, coupling effects between the close-packed inclusions must be included. The new flexibility afforded by using vertical resonators allows us to controllably create purely electric inclusions, purely magnetic inclusions, as well as bianisotropic inclusions, and vastly increases the degrees of freedom for the design of metafilms.
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