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16 September 2016Controlled Fano resonances via symmetry breaking in metamaterials for high-sensitive infrared spectroscopy
A high-sensitive polarized surface-enhanced infrared absorption (polarized SEIRA) is proposed and demonstrated by
utilizing the resonant coupling of Fano-resonant mode of the asymmetric metamaterials and IR vibrational mode of a
polymer nano-film. The asymmetric metamaterials consisting of an Au nano-rod pair with a coupling nano-antenna were
fabricated and characterized to demonstrate the controlled Fano resonances at 1730 cm-1, which spectrally overlapped
with the C=O stretching vibrational mode. In the co-polarized SEIRA measurement, the C=O mode of the PMMA nano-film
was clearly observed as a conventional anti-resonant peak within the Fano line-shape of the metamaterial. For the
cross-polarized case, on the other hand, a distinctive dip appeared within a cross-polarized transmission peak of the
metamaterial. Since the unwanted background is strongly suppressed in the cross-polarized detection scheme, the signal
contrast was dramatically improved, allowing for the attomole detection of the C=O bond in the far-field measurement.
Our metamaterial approach achieves the significant improvement of signal-to-background ratio in the far-field
measurement, thus paving the way toward the high-sensitive analysis of functional group in direct IR spectroscopy.