A one-dimensional acoustic negative refractive index metamaterial based on the transmission line approach is presented. This structure implements the dual transmission line concept extensively investigated in microwave engineering. It consists of an acoustic waveguide periodically loaded with membranes realizing the function of series "capacitances" and transversally connected open channels realizing shunt "inductances." Transmission line based metamaterials can exhibit a negative refractive index without relying on resonance phenomena, which results in a bandwidth of operation much broader than that observed in resonant devices. In the present case, the negative refractive index band extends over almost one octave, from 0.6 to 1 kHz. The developed structure also exhibits a seamless transition between the negative and positive refractive index bands with a zero index at the transition frequency of 1 kHz. At this frequency, the unit cell is only one tenth of the wavelength. Simple acoustic circuit models are introduced, which allow efficient designs both in terms of dispersion and impedance, while accurately describing all the physical phenomena. Using this approach, a good matching at the structure terminations is achieved. Full-wave simulations, made for a 10-cell-long structure, confirm the good performances in terms of dispersion diagram, Bloch impedance, and reflection and transmission coefficients.

Cloaking using a volumetric structure composed of stacked two-dimensional transmission-line networks is verified with measurements. The measurements are done in a waveguide, in which an array of metallic cylinders is inserted causing a short-circuit in the waveguide. The metal cylinders are cloaked using a previously designed and simulated cloak that "hides" the cylinders and thus enables wave propagation inside the waveguide.

Abstract-We present a space-domain integral-equation method for the analysis of periodic structures formed by three-dimensional (3-D) metallic objects arranged in a general skewed two-dimensional lattice. The computation of the space-domain Green's function is accelerated using the Ewald transformation. The method is validated on several periodic structures ranging from planar frequency-selective surfaces to 3-D photonic crystals and metamaterials. For these structures, our technique shows a clear advantage in terms of computational speed when compared with available commercial softwares.Index Terms-Frequency-selective surfaces (FSSs), Green's functions (GFs), integral equations (IEs), metamaterials, periodic structures, photonic-bandgap (PBG) materials.

Abstract-This paper presents new contributions to the modeling and design of reflecting cells embedding discrete control elements such as microelectromechanical system (MEMS) or diodes. First, a rigorous assessment of the different possibilities to simulate and measure the reconfigurable cell in a periodic environment is proposed. Strategies to efficiently model a cell comprising discrete control elements are then presented and discussed in terms of versatility, required assumptions, and computational effort. The most efficient method allows computing all reconfigurable states cell parameters, including information such as the total and dissipated power in each MEMS or diode, in a few minutes using a commercial full-wave solver and adequate post-processing. Finally, the benefit of such an efficient modeling is illustrated by the optimization of an element phase states distribution using a particle swarm optimizer. The concepts presented are also directly applicable to reconfigurable transmitting cells.

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