Yusuke Nagasaki

Multi-spectral plasmon induced transparency via in-plane dipole and dual-quadrupole coupling

We experimentally demonstrated an approach based on dipole and dual-quadrupole coupling to construct a planar metamaterial supporting multi-spectral plasmon induced transparency. The structure consists of two short silver wires (dipole) and two long silver wires (dual-quadrupole). The in-plane coupling between the dipole and the dual-quadrupole leads to two transmission windows even in the absorbance linewidth of the dipole. This phenomenon is well described and understood by numerical analyses and a classical oscillator model.

Gap Plasmon Resonance in a Suspended Plasmonic Nanowire Coupled to a Metallic Substrate

We present an experimental demonstration of nanoscale gap plasmon resonators that consist of an individual suspended plasmonic nanowire (NW) over a metallic substrate. Our study demonstrates that the NW supports strong gap plasmon resonances of various gap sizes including single-nanometer-scale gaps. The obtained resonance features agree well with intuitive resonance models for near- and far-field regimes. We also illustrate that our suspended NW geometry is capable of constructing plasmonic coupled systems dominated by quasi-electrostatics.

All-Dielectric Dual-Color Pixel with Subwavelength Resolution

An all-dielectric optical antenna supporting Mie resonances enables light confinement below the free-space diffraction limit. The Mie scattering wavelengths of the antenna depend on the structural geometry, which allows the antennas to be used for colored imprint images. However, there is still room for improving the spatial resolution, and new polarization-dependent color functionalities are highly desirable for realizing a wider color-tuning range. Here, we show all-dielectric color printing by means of dual-color pixels with a subwavelength-scale resolution. The simple nanostructures fabricated with monocrystalline silicon exhibit various brilliant reflection color by tuning the physical dimensions of each antenna. The designed nanostructures possess polarization-dependent properties that make it possible to create overlaid color images. The pixels will generate individual color even if operating as a single element, resulting in the achievement of subwavelength-resolution encoding without color mixing. This printing strategy could be used to further extend the degree of freedom in structural color design.

Optical bistability in shape-memory nanowire metamaterial array

Non-volatile temperature-induced structural phase transitions such as those found in chalcogenide glasses are known to lead to strong changes in optical properties and are widely used in rewritable optical disk technology. Herein, we demonstrate that thermally activated optical memory can be achieved via the nanostructural reconfiguration of a metallic nanowire metamaterial array made from a shape-memory alloy: A nickel-titanium film of nanoscale thickness structured on the subwavelength scale exhibits bistability of its optical properties upon temperature cycling between 30 °C and 210 °C. The structure, comprising an array of NiTi nanowires coated with a thin film of gold to enhance its plasmonic properties, can exist in two non-volatile states presenting an optical reflectivity differential of 12% via nanoscale mutual displacements of alternating nanowires in the structure. Such all-metal shape-memory photonic gratings and metamaterials may find applications in bistable optical devices.Non-volatile temperature-induced structural phase transitions such as those found in chalcogenide glasses are known to lead to strong changes in optical properties and are widely used in rewritable optical disk technology. Herein, we demonstrate that thermally activated optical memory can be achieved via the nanostructural reconfiguration of a metallic nanowire metamaterial array made from a shape-memory alloy: A nickel-titanium film of nanoscale thickness structured on the subwavelength scale exhibits bistability of its optical properties upon temperature cycling between 30 °C and 210 °C. The structure, comprising an array of NiTi nanowires coated with a thin film of gold to enhance its plasmonic properties, can exist in two non-volatile states presenting an optical reflectivity differential of 12% via nanoscale mutual displacements of alternating nanowires in the structure. Such all-metal shape-memory photonic gratings and metamaterials may find applications in bistable optical devices.

Passive and active metasurface based on metal-insulator-metal structures

A metal-insulator-metal (MIM) structure is a fundamental plasmonic structure that has been studied widely since the early stage of plasmonics. It enables us to confine surface plasmon polariton (SPP) and concentrate light into nano-space beyond the diffraction limit. A finite-length MIM structure is considered to be a Fabry-Perot resonator of SPP as a nanocavity. Here, we review our recent studies about active metasurface based on a reconfigurable metal-air-metal (MAM) nanocavity which modify reflection or absorption spectra in scattering by changing a gap distance. Such reconfigurable MAM nanocavity becomes promising candidate for various applications such as plasmonic color or sky radiator from visible to infrared range.