OU-MRU Vol.15 September 2015

Okayama University Medical Research Updates （OU-MRU）

2015.09 Vol.15

Source: Okayama University (Japan), Center for Public Affairs and Communications
For immediate release: 28 Sept 2015
Okayama University research: Metamaterial absorbers for infrared inspection technologies(Okayama, 28 September) A metamaterial absorber capable of highly sensitive infrared
detection enhances the spectroscopy of tiny molecular details
Plasmonic metamaterials are man-made substances whose structure can be manipulated toinfluence the way they interact with light. As such, metamaterials offer an attractiveplatform for sensing applications, including infrared (IR) absorption spectroscopy – atechnique used to uncover details of the chemical make-up and structure of substances.
Now, Atsushi Ishikawa at Okayama University and his colleague (or collaborator), Takuo
Tanaka at RIKEN’s Metamaterials Laboratory, Japan, (Note: I just feel, not necessary toemphasize “Tanaka at RIKEN”.) have fabricated a novel plasmonic metamaterial absorbercomprised of gold and magnesium fluorine capable of high sensitivity IR detection1. The
metamaterial could prove invaluable in the development of next-generation IR inspection
technologies.
The researchers carefully designed their absorber to maximise the IR signal and minimisebackground noise. The metamaterial consists of 50 nm gold ribbons on a thick gold film,
separated by a layer of magnesium fluorine (see image).
The wavelength of IR is longer and has less energy than visible light, meaning that it is notstrong enough to excite electrons, unlike other types of spectroscopy. IR absorption
spectroscopy therefore exploits the ability of IR to induce vibrations in bonded atoms.
Organic compounds will absorb IR radiation corresponding to the different types ofmolecular vibrations present; the resulting absorption spectra tell scientists about theunique chemical structure of the compounds.
To test the capabilities of the new metamaterial, the team decided to identify the stretchingvibrational modes of carbon-hydrogen bonds in 16-Mercaptohexadecanoic (16-MHDA) acid.
They dipped the absorber in 16-MHDA ethanol solution to encourage a self-assembling
monolayer of the acid molecules to develop. Under IR radiation at different incident angles,
the metamaterial-monolayer spectral output displayed distinct peaks corresponding tocarbon-hydrogen stretching, with the most pronounced peaks under IR at an angle of 40°.
The new metamaterial approach gave highly-detailed measurements pertaining to tinymolecular details (at the attamole level) in the 16-MHDA monolayer. The researchers hope
their new technique will open doors to the development of ultrasensitive IR inspection
technologies for material science and security applications.

OKAYAMA University
1-1-1 Tsushima-naka, Kita-ku, Okayama 700-8530 Japan© OKAYAMA University

OU-MRU Vol.15 September 2015

BackgroundMetamaterials
The ability to manipulate the light absorption of materials could revolutionize manytechnologies, such as photovoltaic cells and thermal devices. Research into the design anddevelopment of plasmonic metamaterials is still relatively new. These materials aresynthetic, and scientists can design their surface structures to exploit the behavior ofsurface plasmons – quasiparticles that exist on metal surfaces and interact with light – toachieve tuneable optical properties.
Infrared absorption spectroscopy could be dramatically enhanced by the introduction oftuneable metamaterial-based absorbers designed to enable high-resolution detection oftiny molecular details.

Methodology
The metamaterial absorber built by the team comprised gold nano-ribbons (measuring 50nm thick) on a gold film base, with a thin layer of magnesium fluorine separating the twogold layers. As molecular monolayers self-assemble on noble metal surfaces, they
hypothesized that the gold-based metamaterial would prove a strong candidate for enabling
the high-resolution measurement of IR-induced vibrational modes in self-assembling
monolayers.
Their approach involved covering their metamaterial absorber with an ultrathin selfassembling monolayer of 16-MDHA acid molecules. They also covered a bare gold film
sample with the same monolayer for comparison.
The researchers subjected the two monolayers to IR radiation at different incident angles.
The monolayer on bare gold exhibited a low signal-noise ratio, and it was very difficult to
see the absorption dips on the spectral output corresponding to the IR-induced carbonhydrogen stretching in the monolayer.
In contrast, the absorption dips were very well-pronounced in the spectral read-out for themetamaterial-monolayer, because the vibrational modes of the 16-MHDA molecules
resonated with the plasmonic modes of the metamaterial. This so-called ‘resonant coupling’
produced distinct peaks corresponding to IR-induced carbon-hydrogen stretching in the 16MHDA molecular structure. The resonant coupling was dependent on the angle of theincident light, with the clearest, strongest signal at an angle of 40°.
Future work
The researchers believe their absorber may open doors to new ultra-sensitive IR detection
technologies. Further, their technique could be exploited in other ways – by optimizing thesurface structure of other metamaterials, they could enhance resonant coupling still furtherand enable sensitivities down to the zeptomole level.

OKAYAMA University
1-1-1 Tsushima-naka, Kita-ku, Okayama 700-8530 Japan© OKAYAMA University

OU-MRU Vol.15 September 2015

Figure caption
Researchers at Okayama University have created a new IR spectroscopic technique utilizing
the properties of a metamaterial-based absorber to enhance spectral output. Trials on aself-assembling monolayer of 16-MHDA acid showed distinct peaks corresponding tocarbon-hydrogen stretching in the monolayer.

ReferenceAtsushi Ishikawa & Takuo Tanaka. Metamaterial absorbers for infrared detection ofmolecular self-assembled monolayers. Nature Scientific Reports 5 (12570) Published online,
31 July 2015.
doi: 10.1038/srep12570.
http://www.nature.com/articles/srep12570

Correspondence toAssistant Professor Atsushi Ishikawa, Ph.D.
Department of Electrical and Electronic Engineering,
Okayama University, 3-1-1 Tsushimanaka, Kitaku,
Okayama, 700-8530, Japan.
E-mail: a-ishikawa@okayama-u.ac.jp
Further information
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OU-MRU Vol.15 September 2015

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About Okayama UniversityOkayama University is one of the largest
comprehensive universities in Japan withroots going back to the Medical Training
Place sponsored by the Lord of Okayamaand established in 1870. Now with 1,300
faculty and 14,000 students, the University
offers courses in specialties ranging frommedicine and pharmacy to humanities
and physical sciences. Okayama
University is located in the heart of Japan
approximately 3 hours west of Tokyo byShinkansen.

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