センサ・マイクロマシン部門誌（論文誌E）では，メタマテリアルおよびプラズモニクスに関連した論文・レターを集めた特集号を企画し，本誌の2017年11月号で今回の特集号を発行する運びとなりました。

メタマテリアルやプラズモニック構造は，入射波長より

Extraordinary optical transmission through multi-lattice metal hole arrays (MLMHA) in the terahertz region is investigated numerically by using coupled-mode analysis. MLMHA consist of two kinds of metal hole arrays (MHA) layer whose lattice constants are incommensurate with each other. By comparing the transmittance spectra of single lattice constant MHA and that of MLMHA, we observe the splitting and new peaks in the transmittance spectra for MLMHA. To study the origin of the new peaks, the transmittance spectra for MLHAL is compared with that of a system in which we filled the holes of MHA sandwiched between other MHA's. We found that in the transmittance spectra for the filled holes MHA, only peaks associated with MHA sandwiched between other MHA's vanish and the new peaks remained. Therefore, it is confirmed that the appearance of the new peaks is not due to the interaction between the surface modes of different lattice constant MHA's. To clarify the mode pattern for split peaks we investigate the distributions of electric field outside MHA by rotating the MLMHA. The result reveals that the split peaks come out from the same peak mode. Consequently, it is found that the interaction does not occur between surface modes of different lattice constant MHAs.

We proposed and demonstrated an electro-mechanically actuated optical metamaterial for dynamic spectral control of Fano resonance in the optical region. Dynamic modulation was achieved by the change in interaction among metamaterial unit cells on beam structures actuated by parallel plate type actuators. Fano resonance at wavelengths around 1300 nm was achieved by complementary asymmetric-double-bar structures based on Babinet's principle. The MEMS actuated optical metamaterial showed 10% modulation of Fano resonant spectra. This result is the first demonstration of dynamic control of Fano resonance in electro-mechanically actuated metamaterials operating in the optical region. Since arbitrary metamaterial structures are applicable to the proposed device and fabrication is simple, the device will provide opportunities to realize dynamic spectral control of various kinds of metamaterials.

We fabricated out-of-plane asymmetric double bar metamaterial with metal/insulator/metal structures. Au and SiO₂ were used as metal and insulator, respectively. Coupling efficiency between upper and lower metal bars through surface plasmon waves was controlled by the thickness of SiO₂ between the metal bars. The optical characteristics were measured and compared with calculated values. Both the measured and calculated spectra showed transmittance peak caused by Fano resonance at wavelengths between 1500 and 1600 nm.

Metamaterials, which can show novel optical functionalities beyond conventional optics, has been actively studied in recent years. As one of approaches to add tunability on resonant characteristics of metamaterials, slightly shifting relative position of stacked metamaterials or metasurfaces has been reported. In this study, based on numerical simulation we have designed stacked closed-ring resonator (CRR) arrays that show relatively large spectral shift with a small lateral shift of the stacked layers in terahertz (THz) range. We also show that changing dielectric constant of the dielectric media between CRR arrays is also effective to obtain large spectral shift of the resonant responses. Our findings may be utilized to develop novel type of THz active devices and also sensing devices.

Arrayed plasmonic color filters integrated on Si photodiodes with 21 elements were experimentally demonstrated. The plasmonic color filters consisted of nanodot subwavelength periodic structures. Optical characteristics of the color filters were measured and compared with calculated results. In order to obtain spectrum of an incident light from the signal of photodiodes, spectrum reconstruction using Tikhonov regularization was adapted. In the spectral sensitivity of each photodiode, the peak wavelength was adjusted between 400 and 850 nm in wavelength by the period of nanodot structure on each photodiode. The measured peak wavelengths roughly agreed with calculated values with errors less than 30 nm in wavelength. Spectral characteristics of a monochromatic light were measured by fabricated device and spectrum reconstruction was carried out. The position of the center wavelength could be successfully detected.

We developed a near-infrared photodetector using nano-pillars formed on an n-typed Si wafer. Au nano-pillars performed as optical antennas, which absorb the infrared light effectively. Each nano-pillar had several hundreds nm of diameter, 1.0-µm-height, and was arrayed with 2-µm-pitch over 100 µm ×100 µm area, fabricated using a MEMS 8-inch line. The absorbed photo energy was transduced to photocurrent by a Schottky barrier formed at the interface between the Au/Cr and n-typed Si. The photocurrent sensitivity (responsivity) of the nano-pillar photodetector exhibited around 10-times increase in average, and 23-times in the maximum, compared with a control Schottky typed photodetector with a flat Au surface. The detectable wavelength limit was extended to 1.8 µm. Because this method was clarified to be compatible with a standard Si fabrication process, it will serve as a fundamental way to construct a photodetector for multi-wavelength photodetection.

Solar selective absorbers that function at temperatures up to 700°C and possess shallow honeycomb cylindrical microcavities coated with a metal-dielectric multi-layer are investigated. Even shallow microcavities, absorptance at solar spectrum range is largely increased due to multilayer showing spectrally selective absorption property. To obtain high spectral selective property, design strategy for the multilayer coated microstructure is established. Honeycomb array cylindrical microcavities were fabricated on tungsten substrate and multi-layers consisting of platinum nano-film sandwiched by aluminum oxide layers were created for a uniform coating via atomic layer deposition. Thermal stability of the fabricated sample at 700°C in vacuum is verified through annealing test for 100 h.

A reconfigurable transmittance modulation device for Au nanograting retarder that consisted of out-of-plane thermally-driven grating was designed, fabricated, and demonstrated. In this study, an Au nanograting that is known to exhibit giant birefringence was constituted by integrating MEMS thermal actuators. The thermally-driven grating was made of Au/SiO₂ bimorph beams that period and slit width are 800 nm and 400 nm, respectively. The device is fabricated on a glass substrate with embedded Si as a sacrificial layer. The gap between the grating and the substrate is changed by applying driving current, and transmittance was changed affected by Fabry-Pérot interference. Transmittance peak wavelength for TE polarization was shifted up to 40 nm keeping the retardation of the Au nanograting. A calculation model of transmittance applying multilayer interference was proposed and demonstrated its effectiveness in TE polarization. By optimizing the design dimensions of the Au nanograting, it is expected to realize a microscale waveplate capable of transmittance modulation.

Micro-electromechanical-systems driven wire grid polarizers (MEMS-WGPs) were proposed and fabricated. MEMS-WGPs mainly consisted of an Au cantilever array and two kinds of electrical leads. One lead was connected with cantilevers and the other lead was located under the cantilevers. Because the cantilevers were electrostatically actuated downward by applying voltage between the two leads and made contact with the next cantilever, transmittance could be changed. Static wire grid polarizers were also fabricated and measured their optical characteristics in THz region, as preliminary step toward MEMS-WGPs. MEMS-WGP was successfully fabricated and the cantilever array was electrostatically actuated. In THz region, transmittance of the fabricated MEMS-WGP was changed from 14.7% to 10.6% on average by applied voltage of 200V.

Refractive index sensors using polarization-independent metamaterial are experimentally and numerically demonstrated in the near-infrared region, which shows potential applications in high sensitivity biological and chemical sensing. Fabricated metamaterial shows highly sensitive response, as high as 585 nm/RIU, to surrounding dielectric media.