Fano resonances in metasurfaces are important due to their low loss subradiant behavior that allows excitation of high-quality (Q) factor resonances extending from the microwave to the optical regime. High-Q Fano resonances have recently enabled applications in the areas of sensing, modulation, filtering, and efficient cavities for lasing spasers. Highly conducting metals are the most commonly used materials for fabricating the metasurfaces, especially at the low-frequency terahertz region where the DC, Drude, and perfect electric conductivity show similar resonant behavior of the subwavelength meta-atoms. Here, it is experimentally and theoretically demontrated that the Q factor of a low asymmetry Fano resonance is extremely sensitive to the conducting properties of the metal at terahertz frequencies. Large differences in the Q factor and figure of merit of the Fano resonance is observed for perfect electric conductors, Drude metal, and a DC-conducting metal, which is in sharp contrast to the behavior of the inductive–capacitive resonance of meta-atoms at terahertz frequency. Identification of such a low asymmetry regime in Fano resonances is the key to engineer the radiative and nonradiative losses in plasmonic and metamaterial-based devices that have potential applications in the microwave, terahertz, infrared, and the optical regimes.

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Ultrahigh-Q Fano Resonances in Terahertz Metasurfaces: Strong Influence of Metallic Conductivity at Extremely Low Asymmetry

The progress of technology in consumer electronics demand an antenna having a compact size, high gain and bandwidth, and multiple antennas at transmitter and receiver to enhance the channel capacity. Over the last decade, numerous techniques are proposed to improve the performance of the antenna. One such technique is the use of metamaterials (MTMs) in antenna design. MTMs are artificial structures to provide unique electromagnetic properties that are not available in natural materials. The unique properties of these materials allow the design of high-performance antennas, filters, and microwave devices which cannot be obtained using traditional antennas. Loading antenna with the one, two, and three-dimensional MTM structures comprised of a periodic subwavelength unit cell exhibits RLC resonant structures and allows to manipulate electromagnetic waves in the antenna system. These structures offer low resonant frequency compared to the antenna resonant frequency resulting in antenna miniaturization and manipulation of electromagnetic waves helps in enhancing the gain and bandwidth, and achieving circular polarization (CP) of an antenna system. Also, metamaterial loading enhances isolation between the antenna elements in the multiple-input-multiple output (MIMO) system by suppressing the surface waves. In this paper, the electromagnetics of MTM with analytical expressions and its application in antenna design are discussed in detail. The MTM-based antennas are classified into MTM loading, MTM inspired antenna, metasurface loading, and composite right/left hand (CRLH) based antennas. The recent development in MTM inspired antenna and its application in antenna miniaturization, enhancing gain and bandwidth, achieving CP and mutual coupling suppression in MIMO antenna systems are discussed to make it useful for further research.

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Electromagnetic Metamaterials: A New Paradigm of Antenna Design

We demonstrate a nonradiative flat band for spoof surface plasmon polaritons bounded on a structured surface with Lieb lattice symmetry in the terahertz regime. First, we theoretically derive the dispersion relation of spoof plasmons in a metallic Lieb lattice based on the electrical circuit model. We obtain three bands, one of which is independent of wave vector. To confirm the theoretical result, we numerically and experimentally observe the flat band in transmission and attenuated total reflection configurations. We reveal that the quality factor of the nonradiative flat-band mode decoupled from the propagating wave is higher than that of the radiative flat-band mode. This indicates that the nonradiative flat-band mode is three-dimensionally confined in the lattice.

Observation of a nonradiative flat band for spoof surface plasmons in a metallic Lieb lattice

One of the fascinating topological phenomena is the edge state in one-dimensional system. In this work, the topological photonics in the dimer chains composed by the split ring resonators are revealed based on the Su-Schrieffer-Heeger model. The topologically protected photonic edge state is observed directly with the in situ measurements of the local density of states in the topological nontrivial chain. Moreover, we experimentally demonstrate that the edge state localized at both ends is robust against a varied of perturbations, such as losses and disorder. Our results not only provide a versatile platform to study the topological physics in photonics but also may have potential applications in the robust power transfer.

Experimental demonstration of the robust edge states in a split-ring-resonator chain.

Magnetic resonance coupling between connected split ring resonators (SRRs) and magnetic plasmon (MP) excitations in the connected SRR chains were theoretically studied. By changing the connection configuration, two different coupling behaviors were observed, and therefore two kinds of MP bands were formed in the connected ring chains, accordingly. These MPs were revealed with positive and negative dispersion for the homo- and anti-connected chain, respectively. Notably, these two MP modes both have wide bandwidth due to the conductive coupling. Moreover, the anti-connected chain is found supporting a novel negative propagating wave with a wide band starting from zero frequency, which is a fancy phenomenon in one-dimensional system.

Structural-configurated magnetic plasmon bands in connected ring chains

The coupling between arbitrarily positioned and oriented split ring resonators is investigated up to THz frequencies. Two different analytical approaches are used, one based on circuits and the other on field quantities that includes retardation. These are supplemented by numerical simulations and experiments in the GHz range, and by simulations in the THz range. The field approach makes it possible to determine separately the electric and magnetic coupling coefficients which, depending on orientation, may reinforce or may cancel each other. Maps of coupling are produced for arbitrary orientations of two co-planar split rings resonant at around 2 GHz and then with the geometry scaled down to be resonant at around 100 THz. We prove that the inertia of electrons at high frequencies results in a dramatic change in the maps of coupling, due to reduction of the magnetic contribution. Our approach could facilitate the design of metamaterials in a wide frequency range up to the saturation of the resonant frequency.

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F. Hesmer

Papers

Mapping inter-element coupling in metamaterials: Scaling down to infrared