title: Metamaterial Absorbers for Infrared Detection of Molecular Self-Assembled Monolayers.
abstract: 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.
authors.affiliation: 1] Department of Electrical and Electronic Engineering, Okayama University, 3-1-1 Tsushimanaka, Kitaku, Okayama, Okayama 700-8530, Japan [2] Metamaterials Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan.
authors.forename: Atsushi
authors.initials: A
authors.lastname: Ishikawa
authors.affiliation: 1] Metamaterials Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan [2] Research Institute for Electronic Science, Hokkaido University, N21W10 Kitaku, Sapporo, Hokkaido 001-0020, Japan [3] Department of Innovative and Engineered Materials, Tokyo Institute of Technology, 4259 Nagatsutacho, Midoriku, Yokohama, Kanagawa 226-8503, Japan.
authors.forename: Takuo
authors.initials: T
authors.lastname: Tanaka