Takuo Tanaka

Few-layer HfS2 transistors.

2D materials are expected to be favorable channel materials for field-effect transistor (FET) with extremely short channel length because of their superior immunity to short-channel effects (SCE). Graphene, which is the most famous 2D material, has no bandgap without additional techniques and this property is major hindrance in reducing the drain leakage. Therefore, 2D materials with finite band gap, such as transition metal dichalcogenides (TMDs, e.g. MoS2 WSe2) or phosphorene, are required for the low power consumption FETs. Hafnium disulfide (HfS2) is a novel TMD, which has not been investigated as channel material. We focused on its potential for well-balanced mobility and bandgap properties. The higher electron affinity of Hf dichalcogenides compared with Mo or W chalcogenides facilitates the formation of low resistance contact and staggered heterojunction with other 2D materials. Here we demonstrate the first few layer HfS2 FET with robust current saturation and high current on/off ratio of more than 10^4.HfS2 is the novel transition metal dichalcogenide, which has not been experimentally investigated as the material for electron devices. As per the theoretical calculations, HfS2 has the potential for well-balanced mobility (1,800u2009cm2/V·s) and bandgap (1.2u2009eV) and hence it can be a good candidate for realizing low-power devices. In this paper, the fundamental properties of few-layer HfS2 flakes were experimentally evaluated. Micromechanical exfoliation using scotch tape extracted atomically thin HfS2 flakes with varying colour contrasts associated with the number of layers and resonant Raman peaks. We demonstrated the I-V characteristics of the back-gated few-layer (3.8u2009nm) HfS2 transistor with the robust current saturation. The on/off ratio was more than 104 and the maximum drain current of 0.2u2009μA/μm was observed. Moreover, using the electric double-layer gate structure with LiClO4:PEO electrolyte, the drain current of the HfS2 transistor significantly increased to 0.75u2009mA/μm and the mobility was estimated to be 45u2009cm2/V·s at least. This improved current seemed to indicate superior intrinsic properties of HfS2. These results provides the basic information for the experimental researches of electron devices based on HfS2.

Metamaterial Absorbers for Infrared Detection of Molecular Self-Assembled Monolayers

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 ~3000u2009cm−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.

Controlling bi-anisotropy in infrared metamaterials using three-dimensional split-ring-resonators for purely magnetic resonance

We propose and demonstrate the strategy to control bi-anisotropic response in three-dimensional split-ring-resonators (3D-SRRs) array for purely magnetic resonance in the mid-infrared region. By using a metal-stress-driven self-folding method, inversion symmetry along a propagation axis of 3D-SRRs was controlled. The inversion symmetry of 3D-SRRs realized non-bi-anisotropic response of a magnetic resonant mode at around 10 μm in wavelength resulting in purely magnetic resonance with high transmission of 70%. Highly transparent purely magnetic artificial elements demonstrated in this study will be a key component for functional applications using artificial magnetism at the optical frequencies.

Up Scalable Full Colour Plasmonic Pixels with Controllable Hue, Brightness and Saturation

It has long been the interests of scientists to develop ink free colour printing technique using nano structured materials inspired by brilliant colours found in many creatures like butterflies and peacocks. Recently isolated metal nano structures exhibiting preferential light absorption and scattering have been explored as a promising candidate for this emerging field. Applying such structures in practical use, however, demands the production of individual colours with distinct reflective peaks, tunable across the visible wavelength region combined with controllable colour attributes and economically feasible fabrication. Herein, we present a simple yet efficient colour printing approach employing sub-micrometer scale plasmonic pixels of single constituent metal structure which supports near unity broadband light absorption at two distinct wavelengths, facilitating the creation of saturated colours. The dependence of these resonances on two different parameters of the same pixel enables controllable colour attributes such as hue, brightness and saturation across the visible spectrum. The linear dependence of colour attributes on the pixel parameters eases the automation; which combined with the use of inexpensive and stable aluminum as functional material will make this colour design strategy relevant for use in various commercial applications like printing micro images for security purposes, consumer product colouration and functionalized decoration to name a few.

Cross-Polarized Surface-Enhanced Infrared Spectroscopy by Fano-Resonant Asymmetric Metamaterials

Plasmonic metamaterials have overcome fundamental limitations in conventional optics by their capability to engineer material resonances and dispersions at will, holding great promise for sensing applications. Recent demonstrations of metamaterial sensors, however, have mainly relied on their resonant nature for strong optical interactions with molecules, but few examples fully exploit their functionality to manipulate the polarization of light. Here, we present cross-polarized surface-enhanced infrared absorption (SEIRA) by the Fano-resonant asymmetric metamaterial allowing for strong background suppression as well as significant field enhancement. The metamaterial is designed to exhibit the controlled Fano resonance with the cross-polarization conversion property at 1730u2009cm−1, which spectrally overlaps with the C=O vibrational mode. In the cross-polarized SEIRA measurement, the C=O mode of poly(methyl methacrylate) molecules is clearly observed as a distinct dip within a Fano-resonant transmission peak of the metamaterial. The vibrational signal contrast is then improved based on the cross-polarized detection scheme where only the light interacting with the metamaterial-molecular coupled system is detected by totally eliminating the unwanted background light. Our metamaterial approach achieves the zeptomole sensitivity with a large signal-to-noise ratio in the far-field measurement, paving the way toward the realization of ultrasensitive IR inspection technologies.

Bi-anisotropic Fano resonance in three-dimensional metamaterials

We experimentally investigated the bi-anisotropic properties of Fano resonance in three-dimensional (3D) metamaterials. Fano resonance in 3D metamaterials arises from the interference of in-phase and anti-phase modes that originate from mode hybridization in coupled 3D split ring resonators (SRRs) with detuned resonant wavelengths. At Fano resonance, not only permittivity and permeability but also the bi-anisotropic parameter show doubly dispersive response. Manipulation of the bi-anisotropic response at Fano resonance was demonstrated through controlling the inversion symmetry of the 3D-SRRs. Improvement of inversion symmetry due to rotation of 3D-SRRs results in enhancement of magnetic response and inhibition of electric and bi-anisotropy responses at Fano resonance. Negligible electric and bi-anisotropic responses at Fano resonance were achieved due to the small radiative nature of the anti-phase mode. This bi-anisotropic Fano metamaterials with rich and tunable bi-anisotropy will extend the capabilities of new optical phenomena and broaden the applications of bi-anisotropic metamaterials.

Controlling coulomb interactions in infrared stereometamaterials for unity light absorption

We investigate the influence of near field interactions between the constituent 3D split ring resonators on the absorbance and resonance frequency of a stereo metamaterial based perfect light absorber. The experimental and theoretical analyses reveal that the magnetic resonance red shifts and broadens for both the decreasing vertical and lateral separations of the constituents within the metamaterial lattice, analogous to plasmon hybridization. The strong interparticle interactions for higher density reduce the effective cross-section per resonator, which results in weak light absorption observed in both experimental and theoretical analyses. The red shift of the magnetic resonance with increasing lattice density is an indication of the dominating electric dipole interactions and we analyzed the metamaterial system in an electrostatic point of view to explain the observed resonance shift and decreasing absorption peak. From these analyses, we found that the fill factor introduces two competing factors determining the absorption efficiency such as coulomb interactions between the constituent resonators and their number density in a given array structure. We predicted unity light absorption for a fill factor of 0.17 balancing these two opposing factors and demonstrate an experimental absorbance of 99.5% at resonance with our 3D device realized using residual stress induced bending of 2D patterns.We investigate the influence of near field interactions between the constituent 3D split ring resonators on the absorbance and resonance frequency of a stereo metamaterial based perfect light absorber. The experimental and theoretical analyses reveal that the magnetic resonance red shifts and broadens for both the decreasing vertical and lateral separations of the constituents within the metamaterial lattice, analogous to plasmon hybridization. The strong interparticle interactions for higher density reduce the effective cross-section per resonator, which results in weak light absorption observed in both experimental and theoretical analyses. The red shift of the magnetic resonance with increasing lattice density is an indication of the dominating electric dipole interactions and we analyzed the metamaterial system in an electrostatic point of view to explain the observed resonance shift and decreasing absorption peak. From these analyses, we found that the fill factor introduces two competing factors det...

Design of a colorimetric sensing platform using reflection mode plasmonic colour filters

Plasmonic nano structures fabricated using inexpensive and abundant aluminum metal shows intense narrow reflection peaks with strong response to the external stimuli, provides a simple yet powerful detection mechanism that is well suited for the development of low cost and low power sensors, such as colorimetric sensors, which transduces external stimuli or environmental changes in to visible colour changes. Such low cost and disposable sensors have huge demands in the point-of-care and home health care diagnostic applications. We present the design of a colorimetric sensing platform based on reflection mode plasmonic colour filters on both silicon and glass substrate, which demonstrate a sharp colour change for varying ambient refractive index. The sensor is essentially a plasmonic metamaterial in which the aluminum square plate hovering on a PMMA nano pillar in the background of a perforated aluminum reflector forms the unit cell which is arranged periodically in a 2D square lattice. The meta-surface has two distinct absorption peaks in the visible region leaving a strong reflection band, which strongly responds to the ambient refractive index change, provides a means for the realization of low cost colorimetric sensing platform.

Isotropic metamaterial perfect light absorber using 3D split ring resonator in the mid IR region

We present the design of a perfect light absorber using 3D metamaterial for operation in the mid IR region. The 3D metamaterial is a metal half ring projecting normal to the substrate plane, which ensures wide-angle operation for the direct magnetic coupling. The absorber is essentially a 3-layer architecture having the 3D metamaterial on top acting as impedance matching layer to the surrounding medium, which ensures near zero reflection. A ground metal plane of 120nm thickness at the bottom layer cancels any transmission through the structure for incident electromagnetic field. A dielectric spacer layer of thickness 100nm separates the top and bottom layers. The metal parts of the absorber are realized using gold and the dielectric spacer layer is defined by SiN thin film deposited using PECVD. The 3D half rings are formed from the lithographically defined 2D template by releasing residual stress in the thermally evaporated gold thin film using ICP RIE of SiN sacrificial layer. We report an absorbance of more than 90% at a peak wavelength of 12.5μm with a FWHM of 2μm.

Robust plasmonic hot-spots in a metamaterial lattice for enhanced sensitivity of infrared molecular detection

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.