Electromagnetic parameter retrieval from inhomogeneous metamaterials. Phys Rev E 71:036617

Department of Electrical and Computer Engineering, Duke University, Durham, North Carolina 27708, USA.
Physical Review E (Impact Factor: 2.29). 04/2005; 71(3 Pt 2B):036617. DOI: 10.1103/PhysRevE.71.036617
Source: PubMed


We discuss the validity of standard retrieval methods that assign bulk electromagnetic properties, such as the electric permittivity epsilon and the magnetic permeability mu, from calculations of the scattering (S) 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.

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    • "Recently, instead of parasitic elements, metamaterials have been investigated as spatial decoupling resonators [14]–[18]. However, the planar antennas in [12], [17], and [18] exhibited a high mutual coupling level and narrow bandwidth. "
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    ABSTRACT: One-dimensional electromagnetic bandgap (1-D EBG) and split ring resonator (SRR) structures were inserted between two closely located monopole antennas to suppress mutual coupling. The 1-D EBG and SRR structures in these planar multiple antennas function as a reflector and wave trap, respectively. With the effect of these two structures, the mutual coupling between the two antennas is reduced by more than 42 dB and the back lobes are reduced by 6 dB. Thereby, the radiation efficiency of the antenna is also improved. The two fabricated antennas with 0.19λ 0 spacing exhibit mutual coupling (S 21 , S 12) of less than −30 dB from 2.43 to 2.54 GHz. A minimum correlation coefficient of 0.002 and maximum radiation efficiency of 82% are also demonstrated. Index Terms—Electromagnetic bandgap (EBG) materials, isolation technology, multiple-input multiple-output (MIMO) antennas, mutual coupling, split ring resonator (SRR).
    IEEE Transactions on Antennas and Propagation 09/2015; 63(9):4194-4198. DOI:10.1109/TAP.2015.2447052 · 2.18 Impact Factor
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    • "where k=/c and c is the speed of light in the vacuum [10]. "
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    ABSTRACT: In this paper the design for an absorbing metamaterial with near unity absorbance in terahertz region is presented. The absorber's unit cell structure consists of two metamaterial resonators that couple to electric and magnetic fields separately. The structure allows us to maximize absorption by varying dielectric material and thickness and, hence the effective electrical permittivity and magnetic permeability.
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    • "In particular, Dr. Kuester and Dr. Baker-Jarvis pointed out that near the resonance region of MM slabs, macroscopic properties of MM slabs cannot be anymore directly related to electric and magnetic fields, but their averaged values through electric and magnetic susceptibilities [10], [27], [28]. They also demonstrated that due to these susceptibilities, (semi-infinite) reflection coefficients at the air-slab interface and at the slab-air interface become two independent quantities near the resonance region of MM slabs, resulting in electromagnetic properties different than those obtained from the methods [14]–[18]. Although the method in [10] is effective in taking into account higher-order Bloch modes at two MM boundaries in determination of accurate electromagnetic properties of isotropic MM structures from S-parameters, the applied formalism assumes that effective thicknesses of isotropic MM structures and their locations within reference-planes are known (if the MM structure is not a cascade connection of a finite but large number of MM slabs). "
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    ABSTRACT: It is well recognized that near-field effects become dominant when the metamaterial (MM) is in resonance. In addition, any inaccurate information of the location of reference planes, and the effective length can seriously affect the accuracy of retrieved electromagnetic properties of MMs. By considering all these issues, in this research paper, we propose a retrieval method for reference-plane invariant and thickness-independent determination of electromagnetic parameters of MM slabs involving boundary effects. Our method first accomplishes determination of effective length of MMs and calibration-plane factors using scattering parameter measurements, aside the resonance region, of two identical MMs with different lengths. Our method then incorporates near-field effects in accurate retrieval of electromagnetic properties of MMs. The method is verified by scattering parameters simulated for a homogeneous conventional material and a weakly or negligibly coupled inhomogeneous MM slab made by two metallic concentric split-ring-resonators. Consequences of an inaccurate information of reference-plane transformation factors and the value of effective lengths and of noninclusion of near field effects on the retrieved electromagnetic properties are thoroughly discussed by way of few examples to substantiate the accuracy of the proposed method.
    IEEE Journal of Selected Topics in Quantum Electronics 07/2015; 21(4):1-11. DOI:10.1109/JSTQE.2014.2358565 · 2.83 Impact Factor
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