In this special issue, we review and report the most recent results of metamaterials research achieved mainly in Japan. For optical frequencies, they include meta-surface fabrication and its application to wavelength-selective thermal emitters, highly efficient absorbers, and devices for optical telecommunication. For THz frequencies, we report high-performance components for THz spectroscopy. For microwave frequencies, we describe the CRLH transmission line theory and its various applications including non-reciprocal CRLH transmission lines. We also review the Dirac cone formation by periodic metamaterials.

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By designing structures smaller than the wavelength, it is possible to endow the structure with an optical response on demand. Negative index of refraction material can be constructed by combining components of negative permittivity and negative permeability. Negative refractive index can make a perfect lens, while a stack of metal and dielectric layers have hyperbolic dispersion, which assures super resolution. In optical region, negative permeability is achieved by using a magnetic resonance in artificial structure such as a pair of metallic platelets. With epsilon nearly zero materials, it is possible to manipulate phase. By designing spatial distribution of permittivity and permeability, light propagation can be controlled at will. Recent achievement of metasurface is also introduced.

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A new class of optical metamaterials is attracting great attention: metasurfaces, which are ultrathin twodimensional structures that diversely manipulate light. Metasurfaces ease fabrication difficulty and avoid the influence of absorption loss in metals more than three-dimensional optical metamaterials. In particular, the recent development of nanoimprint lithography technologies is increasing the possibility of practical applications of metasurfaces. Metasurfaces manipulate light waves with a huge degree of freedom, and also enhance various optical processes through the high local density of the states of radiation fields. We introduce recent progress in light-emitting optical metasurfaces that exhibit a large enhancement of thermal emission and fluorescence.

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Stereoscopic structure is essential to realize three-dimensional functional metamaterials. Several fabrication techniques for such a stereoscopic metamaterial structure are reviewed, and as a novel and promising technique for the large area 3D metamaterials, metal-stress driven self-folding method, which uses electron beam lithography and self-organized formation of resonator rings using residual stress of metal thin films, is introduced. As an example of 3D functions of metamterial, isotropic effective index of 0.35 was experimentally demonstrated at 30 THz.

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Dirac cones on the Γ point of periodic metamaterials have a unique feature in which the electromagnetic wave propagates in the medium without a phase change, which realizes such novel optical phenomena as cloaking, scatter-free waveguides, and the lenses of arbitrary shapes. In this article, we analyze the mechanism of Dirac-cone formation by tight binding approximation and k・p perturbation to show that its essence is the accidental degeneracy of two modes with particular symmetry combinations. The Dirac cone can be materialized for any waves in the periodic structures, which include electromagnetic, electronic, phonon, and magnon waves.

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Various types of optical devices such as lasers and light modulators support the advanced information society of today. Most of them are active and light-emitting devices. In these days, however, light absorber metamaterials are also paid attention as elements of photonics devices because they can be used for optical filters, emitters and blackbodies. This paper briefly describes mechanism of light absorption and reviews recent works on the metamaterial light-absorbers.

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The concept and applications of composite right/left-handed (CRLH) metamaterials are overviewed. First, based on equivalent circuit models, the composite nature of left- and right-handedness of CRLH metamaterials are shown theoretically. Then, the concept of balanced CRLH metamaterial is presented. It is shown that the CRLH metamaterials possess three unusual phenomena under the balanced condition, i.e., 1) vanishing the bandgap between the left and right-handed bands, 2) a Dirac cone with linear dispersion characteristics at the Γ-point, and 3) a continuous Bloch impedance at the Γ-point. Finally, typical microwave applications of CRLH metamaterials of wide range beam scanning leaky-wave antennas, zeroth order resonators, and absorbing Dirac cone metamaterials are introduced.

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THz technologies are attractive from the viewpoints of material characterization, sensing, imaging, and high-speed communication. For further expansion of applications, the development of novel optics based on metamaterials is crucial. Here, we introduce the recent development of such simple metamaterial optics as a polarizer, a band-pass filter, and an achromatic wave plate in which the metallic structures of several tens μm cause functional optical responses. They have advantages of usability, tunability, easy design, and cost.

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Plasmonic metamaterials with artificial subwavelength structures are attracting much attention. Metal nano-rods are typical plasmonic metamaterials and function as nanophotonic resonators. At the resonant conditions in the visible and near-infrared regions, the electromagnetic waves at the resonant wavelengths strongly couple with the nano-rods and are absorbed due to Joule and radiative losses. Therefore, ultrasmall wavelength-selective absorbers can be made that consist of metal nano-rods. We fabricated wavelength-selective optical attenuators, which are comprised of Au nano-rods that are integrated with silicon waveguides for optical communications. The nano-rods and silicon waveguide were fabricated on silicon-on-insulator substrates with micromachining technologies. An evanescent wave of a propagating light into the silicon waveguide was coupled with a surface-plasmon-polariton mode of the Au nano-rods at a resonant wavelength and efficiently absorbed. At a wavelength of 1565 nm, we observed optical attenuation of－17.7dB.

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Nonreciprocal Composite Right/Left Handed (CRLH) metamaterials are reviewed showing combined eigen-mode and transmission–line approach along with a formula for the phase nonreciprocity. Pseudotraveling wave resonators based on the nonreciprocal CRLH metamaterials and their unique characteristics are also presented. In addition, enhancement of nonreciprocity using coplanar waveguide-based metamaterials are proposed and demonstrated in the experiment.

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This paper presents the new concept of optical coherent receiver front end which can obtain optical phase error signals in PSK (Phase Shift Keying) communication data as well as spatial tracking error without any extra tracking sensor, leading to downsizing of inter-satellite laser communication terminals. This new receiver front end consists an optical 90 deg hybrid mixer with free space optical components, segmental photo detectors and RF electronics, to realize not only balanced detection of both in-phase and quadrature components but also heterodyning detection of tip/ tilt error signals.

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