Okayama University Acta Medica Okayama 15 2015 Metamaterial absorbers for infrared inspection technologies EN Atsushi Ishikawa No potential conflict of interest relevant to this article was reported.

Nature Publishing Group Acta Medica Okayama 2045-2322 5 2015 Metamaterial Absorbers for Infrared Detection of Molecular Self-Assembled Monolayers 12570 EN Atsushi Ishikawa Takuo Tanaka The emerging field of plasmonic metamaterials has introduced new degree of freedom to manipulate optical field from nano to macroscopic scale, offering an attractive platform for sensing applications. So far, metamaterial sensor concepts, however, have focused on hot-spot engineering to improve the near-field enhancement, rather than fully exploiting tailored material properties. Here, we present a novel spectroscopic technique based on the metamaterial infrared (IR) absorber allowing for a low-background detection scheme as well as significant plasmonic enhancement. Specifically, we experimentally demonstrate the resonant coupling of plasmonic modes of a metamaterial absorber and IR vibrational modes of a molecular self-assembled monolayer. The metamaterial consisting of an array of Au/MgF2/Au structures exhibits an anomalous absorption at ~3000 cm�|1, which spectrally overlaps with C-H stretching vibrational modes. Symmetric/asymmetric C-H stretching modes of a 16-Mercaptohexadecanoic acid monolayer are clearly observed as Fano-like anti-resonance peaks within a broad plasmonic absorption of the metamaterial. Spectral analysis using Fano line-shape fitting reveals the underlying resonant interference in plasmon-molecular coupled systems. Our metamaterial approach achieves the attomole sensitivity with a large signal-to-noise ratio in the far-field measurement, thus may open up new avenues for realizing ultrasensitive IR inspection technologies. No potential conflict of interest relevant to this article was reported.

American Institute of Physics Acta Medica Okayama 0003-6951 111 18 2017 Selective electroless plating of 3D-printed plastic structures for three-dimensional microwave metamaterials 183102 EN Atsushi Ishikawa Department of Electrical and Electronic Engineering, Okayama University Taiki Kato Department of Electrical and Electronic Engineering, Okayama University Nobuyuki Takeyasu Department of Chemistry, Okayama University Kazuhiro Fujimori Department of Electrical and Electronic Engineering, Okayama University Kenji Tsuruta Department of Electrical and Electronic Engineering, Okayama University A technique of selective electroless plating onto PLA-ABS (Polylactic Acid-Acrylonitrile Butadiene Styrene) composite structures fabricated by three-dimensional (3D) printing is demonstrated to construct 3D microwave metamaterials. The reducing activity of the PLA surface is selectively enhanced by the chemical modification involving Sn2+ in a simple wet process, thereby forming a highly conductive Ag-plated membrane only onto the PLA surface. The fabricated metamaterial composed of Ag-plated PLA and non-plated ABS parts is characterized experimentally and numerically to demonstrate the important bi-anisotropic microwave responses arising from the 3D nature of metallodielectric structures. Our approach based on a simple wet chemical process allows for the creation of highly complex 3D metal-insulator structures, thus paving the way toward the sophisticated microwave applications of the 3D printing technology. Published by AIP Publishing. No potential conflict of interest relevant to this article was reported.

American Institute of Physics Acta Medica Okayama 0003-6951 111 24 2017 Robust plasmonic hot-spots in a metamaterial lattice for enhanced sensitivity of infrared molecular detection 243106 EN Atsushi Ishikawa Department of Electrical and Electronic Engineering, Okayama University Shuhei Hara Department of Electrical and Electronic Engineering, Okayama University Takuo Tanaka Metamaterials Laboratory, RIKEN Xiang Zhan NSF Nanoscale Science and Engineering Center, University of California Kenji Tsuruta Department of Electrical and Electronic Engineering, Okayama University High-density and long-lived plasmonic hot-spots are an ideal system for high-sensitive surface-enhanced infrared absorption (SEIRA), but these conditions arc usually incompatible due to unwanted near-field coupling between the adjacent unit structures. Here, by fully controlling plasmonic interference in a metamaterial lattice, we experimentally demonstrate densely packed long-lived quadrupole plasmons for high-sensitive SEIRA. The metamaterial consists of a strongly coupled array of super-and sub-radiant plasmonic elements to exhibit an electromagnetic transparency mode at 1730 cm(-1), which spectrally overlaps with the C=O vibrational mode. In the SEIRA measurement, the C=O mode of poly(methyl methacrylate) molecules is clearly observed as a distinct dip within a transmission peak of the metamaterial. The corresponding numerical simulations reveal that constructive interference uniformly forms coherent quadrupole plasmons over the metamaterial lattice, leading to a stronger molecular signal from the system. Our metamaterial approach provides a robust way to construct ideal hot-spots over the sample, paving the way toward a reliable sensing platform of advanced infrared inspection technologies. Published by AIP Publishing. No potential conflict of interest relevant to this article was reported.

Nature Acta Medica Okayama 2045-2322 10 1 2020 Whitish daytime radiative cooling using diffuse reflection of non-resonant silica nanoshells 6486 EN Takahiro Suichi Department of Electrical and Electronic Engineering, Okayama University Atsushi Ishikawa Department of Electrical and Electronic Engineering, Okayama University Takuo Tanaka Metamaterials Laboratory, RIKEN Cluster for Pioneering Research Yasuhiko Hayashi Department of Electrical and Electronic Engineering, Okayama University Kenji Tsuruta Department of Electrical and Electronic Engineering, Okayama University Daytime radiative cooling offers efficient passive cooling of objects by tailoring their spectral responses, holding great promise for green photonics applications. A specular reflector has been utilized in cooling devices to minimize sunlight absorption, but such a glaring surface is visually less appealing, thus undesirable for public use. Here, by exploiting strong diffuse reflection of silica nanoshells in a polymer matrix, daytime radiative cooling below the ambient temperature is experimentally demonstrated, while showing whitish color under sunlight. The cooling device consists of a poly(methyl methacrylate) layer with randomly distributed silica nanoshells and a polydimethylsiloxane (PDMS) layer on an Ag mirror. The non-resonant nanoshells exhibit uniform diffuse reflection over the solar spectrum, while fully transparent for a selective thermal radiation from the underneath PDMS layer. In the temperature measurement under the sunlight irradiation, the device shows 2.3 degrees C cooler than the ambient, which is comparable to or even better than the conventional device without the nanoshells. Our approach provides a simple yet powerful nanophotonic structure for realizing a scalable and practical daytime radiative cooling device without a glaring reflective surface. No potential conflict of interest relevant to this article was reported.

American Institute of Physics Acta Medica Okayama 0003-6951 117 10 2020 Super-chiral vibrational spectroscopy with metasurfaces for high-sensitive identification of alanine enantiomers 101103 EN Takumi Iida Department of Electrical and Electronic Engineering, Okayama University Atsushi Ishikawa Department of Electrical and Electronic Engineering, Okayama University Takuo Tanaka Metamaterials Laboratory, RIKEN Cluster for Pioneering Research Atsuya Muranaka Advanced Elements Chemistry Laboratory, RIKEN Cluster for Pioneering Research Masanobu Uchiyama Advanced Elements Chemistry Laboratory, RIKEN Cluster for Pioneering Research Yasuhiko Hayashi Department of Electrical and Electronic Engineering, Okayama University Kenji Tsuruta Department of Electrical and Electronic Engineering, Okayama University Chiral nature of an enantiomer can be characterized by circular dichroism (CD) spectroscopy, but such a technique usually suffers from weak signal even with a sophisticated optical instrument. Recent demonstrations of plasmonic metasurfaces showed that chiroptical interaction of molecules can be engineered, thereby greatly simplifying a measurement system with high sensing capability. Here, by exploiting super-chiral field in a metasurface, we experimentally demonstrate high-sensitive vibrational CD spectroscopy of alanine enantiomers, the smallest chiral amino acid. Under linearly polarized excitation, the metasurface consisting of an array of staggered Au nano-rods selectively produces the left- and right-handed super-chiral fields at 1600 cm�|1, which spectrally overlaps with the functional group vibrations of alanine. In the Fourier-transform infrared spectrometer measurements, the mirror symmetric CD spectra of D- and L-alanine are clearly observed depending on the handedness of the metasurface, realizing the reliable identification of small chiral molecules. The corresponding numerical simulations reveal the underlying resonant chiroptical interaction of plasmonic modes of the metasurface and vibrational modes of alanine. Our approach demonstrates a high-sensitive vibrational CD spectroscopic technique, opening up a reliable chiral sensing platform for advanced infrared inspection technologies. No potential conflict of interest relevant to this article was reported.