Metamaterial Absorbers for Infrared Detection of Molecular Self-Assembled Monolayers.

Atsushi Ishikawa,Takuo Tanaka

doi:10.1038/srep12570

Atsushi Ishikawa, Takuo Tanaka

Open Access

https://doi.org/10.1038/srep12570

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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.

Highlights

Light absorption, which is one of the most fundamental light-matter interactions, is an essential phenomenon in a variety of the optical applications, such as photovoltaic cells and thermal management[1,2]
Recent efforts to reach atto/zeptomole sensitivity of Surface-enhanced IR absorption (SEIRA), have focused on hot-spot engineering to improve the near-field enhancement and its spatial and spectral mode overlapping between the plasmons and molecular vibrations[24,25,26]
We propose a novel spectroscopic technique based on the metamaterial IR absorber allowing for a low-background detection scheme as well as significant plasmonic enhancement

Summary

Introduction

Light absorption, which is one of the most fundamental light-matter interactions, is an essential phenomenon in a variety of the optical applications, such as photovoltaic cells and thermal management[1,2]. We experimentally demonstrate the resonant coupling of plasmonic modes of a metamaterial absorber and IR vibrational modes of a molecular self-assembled monolayer (SAM). The sensitivity is improved based on the dark reference measurement, where the vibrational signals are detected as distinct anti-resonance peaks within a strong absorption of the metamaterial.

Results

Conclusion