Background-suppressed surface-enhanced molecular detection by metamaterial infrared absorber

A background-suppressed surface-enhanced molecular detection technique is experimentally demonstrated by utilizing the resonant coupling of plasmonic modes of a metamaterial absorber and Infrared (IR) vibrational modes of molecules. The fabricated metamaterial consisted of one-dimensional (1D) array of Au micro-ribbons on a thick Au film separated by an MgF2 gap layer. The surface structures were designed to exhibit an anomalous IR absorption at ∼ 3000 cm^-1, which spectrally overlapped with C-H stretching vibrational modes. 16-Mercaptohexadecanoic acid (16-MHDA) was used as a test molecule, which formed a 2-nm thick self-assembled monolayer (SAM) with their thiol head-group chemisorbed on the Au surface. In the FTIR measurements, the symmetric and asymmetric C-H stretching modes of the 16-MHDA were clearly observed as Fano-like anti-resonance peaks within a broad plasmonic absorption of the metamaterial. The lowbackground detection scheme with tailored plasmonic enhancement by the metamaterial absorber dramatically improved the sensitivity down to ∼ 1.8 attomoles within the diffraction-limited IR beam spot. Our metamaterial approach thus may open up new avenues for realizing ultrasensitive IR inspection technologies.