There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Metamaterials are artificial structures that can be designed to exhibit specific electromagnetic properties not commonly found in nature. Recently, metamaterials with simultaneously negative permittivity (/spl epsiv/) and permeability (/spl mu/), more commonly referred to as left-handed (LH) materials, have received substantial attention in the scientific and engineering communities. The unique properties of LHMs have allowed novel applications, concepts, and devices to be developed. In this article, the fundamental electromagnetic properties of LHMs and the physical realization of these materials are reviewed based on a general transmission line (TL) approach. The general TL approach provides insight into the physical phenomena of LHMs and provides an efficient design tool for LH applications. LHMs are considered to be a more general model of composite right/left hand (CRLH) structures, which also include right-handed (RH) effects that occur naturally in practical LHMs. Characterization, design, and implementation of one-dimensional and two-dimensional CRLH TLs are examined. In addition, microwave devices based on CRLH TLs and their applications are presented.

Composite right/left-handed transmission line metamaterials

Metasurfaces: From microwaves to visible

This paper gives a basic review and a summary of recent developments for leaky-wave antennas (LWAs). An LWA uses a guiding structure that supports wave propagation along the length of the structure, with the wave radiating or “leaking” continuously along the structure. Such antennas may be uniform, quasi-uniform, or periodic. After reviewing the basic physics and operating principles, a summary of some recent advances for these types of structures is given. Recent advances include structures that can scan to endfire, structures that can scan through broadside, structures that are conformal to surfaces, and structures that incorporate power recycling or include active elements. Some of these novel structures are inspired by recent advances in the metamaterials area.

Leaky-Wave Antennas

Metamaterials, artificial composite structures with exotic material properties, have emerged as a new frontier of science involving physics, material science, engineering and chemistry. This critical review focuses on the fundamentals, recent progresses and future directions in the research of electromagnetic metamaterials. An introduction to metamaterials followed by a detailed elaboration on how to design unprecedented electromagnetic properties of metamaterials is presented. A number of intriguing phenomena and applications associated with metamaterials are discussed, including negative refraction, sub-diffraction-limited imaging, strong optical activities in chiral metamaterials, interaction of meta-atoms and transformation optics. Finally, we offer an outlook on future directions of metamaterials research including but not limited to three-dimensional optical metamaterials, nonlinear metamaterials and “quantum” perspectives of metamaterials (142 references).

Metamaterials: a new frontier of science and technology

A novel composite right-/left-handed (CRLH) backward-wave coupled-line directional coupler with arbitrary coupling level and broad bandwidth is presented, explained by even/odd-mode analysis, validated by full-wave simulations, and demonstrated by experiments. First, the CRLH-transmission-line (CRLH TL) theory is given, and a microstrip implementation of a CRLH TL is described. A simple circuit model is then proposed both for the understanding and design of the coupler. The coupler exhibits very unusual characteristics, such as zero electrical length, imaginary even/odd-mode characteristic impedances, and coupling dependence on even/odd attenuation length (instead of propagation length). Both a quasi-0and 3-dB coupler are demonstrated experimentally. The 3-dB coupler exhibits amplitude balance of /spl plusmn/2 dB over a huge bandwidth of 50% (3.5-5.8 GHz), phase balance of 90/spl deg//spl plusmn/5/spl deg/ from 3.0 to 4.0 GHz and directivity of 20 dB.

A novel composite right-/left-handed coupled-line directional coupler with arbitrary coupling level and broad bandwidth

The composite right/left-handed (CRLH) transmission line (TL) is presented as a general TL possessing both left-handed (LH) and right-handed (RH) natures. The dispersion diagram of the two-dimensional (2-D) CRLH-TL is obtained analytically based on the Bloch-Floquet theory, and the unique phenomenon of infinite guided-wavelength propagation, with nonzero group velocity at a nonzero frequency, is explained. It is shown analytically that when the CRLH TL is "balanced", two eigenfrequencies at the /spl Gamma/ point (/spl beta/=0) of the Brillouin zone (BZ) degenerate at /spl omega//sub /spl Gamma// and a seamless transition from the LH to the RH modes without a bandgap can be achieved. In addition, these phenomena are demonstrated experimentally in the case of a CRLH microstrip line.

Characteristics of the composite right/left-handed transmission lines

Planar distributed periodic structures of microstrip-line and stripline types, which support left-handed (LH) waves are presented and their negative refractive index (NRI) properties are shown theoretically, numerically, and experimentally. The supported LH wave is fully characterized based on the composite right/left-handed transmission-line theory and the dispersion characteristics, refractive indexes, and Bloch impedance are derived theoretically. In addition, formulas to extract equivalent-circuit parameters from full-wave simulation are given. Open (microstrip) and closed (stripline) structures with a 5/spl times/5 mm/sup 2/ unit cell operating at approximately 4 GHz are designed and characterized by full-wave finite-element-method simulations. A 20 /spl times/ 6 unit-cell NRI lens structure interfaced with two parallel-plate waveguides is designed. The focusing/refocusing effect of the lens is observed by both circuit theory and full-wave simulations. Focusing in the NRI lens is also observed experimentally in excellent agreement with circuit theory and numerical predictions. This result represents the first experimental demonstration of NRI property using a purely distributed planar structure.

Planar distributed structures with negative refractive index

A novel resonator using a composite right/left-handed (CRLH) transmission line (TL) is presented and its extraordinary resonant characteristics with an infinite-wavelength wave are discussed. The resonance corresponds to the zeroth-order on the analogy of the conventional mode numbering. The zeroth-order resonant characteristics and loss mechanism are fully described by the circuit theory and it is shown that the resonant frequency of the zeroth-order resonator (ZOR) is independent of the physical length so that the resonator can be arbitrarily small under the condition that sufficient reactance is squeezed in a small length. Full-wave simulations and experiments for meta-structured ZORs implemented with the microstrip line technology agree well with the theoretical results and hence the validity of the theory is confirmed. A 61% size reduction from the conventional resonator with a good unloaded Q of 250 is obtained for a prototype ZOR made of 1.5-cell CRLH TL at 1.9GHz in the experiment.

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Novel zeroth-order resonance in composite right/left- handed transmission line resonators

A novel planar antenna based on the zeroth order resonator (ZOR) which resonates with an infinite wavelength is studied. Since the resonant frequency of the ZOR antenna is determined not by the physical resonator length hut by LCvalues loaded in the resonator, the antenna size can he arbitrary regardless of the operation frequency and, therefore, one can have I freedom in radiation pattern design. A small-sued four-unitsell ZQR antenna with 75% foot print area reduction compared with a conventional patch antenna with a broad radiation pattero is demonstrated at 4.88 GHz with a broad radiation.

Atsushi Sanada

Papers

A novel composite right-/left-handed coupled-line directional coupler with arbitrary coupling level and broad bandwidth

Characteristics of the composite right/left-handed transmission lines

Planar distributed structures with negative refractive index

Novel zeroth-order resonance in composite right/left- handed transmission line resonators

A planar zeroth-order resonator antenna using a left-handed transmission line