Surface enhanced spectroscopy on organic nanofilms using engineered metamaterials

Metallic nanostructures composed of nano-sized interparticle distances have been proven to exhibit extreme optical properties due to high near-electric field enhancements. Tuning the interparticle distances also enables a broad range of operating resonance frequencies. In this study, we both numerically and experimentally present a multi-directional (clover-like) bowtie nanoantenna-based perfect absorber (PA) operating in the mid-infrared frequencies. The PA is based on a multilayer sandwich metamaterial with a nanoantenna configuration on top and exhibits double, narrow-band (130 cm−1) resonances. The multi-directional bowtie nanoantennas are fabricated with gap sizes ranging between 50 and 200 nm. The total PA system demonstrates near-unity (>90%) absorbance at each resonance and exhibits a near-field enhancement of 104, which is an order of magnitude improvement compared to a simple antenna. The small gaps and narrow bands enable highly sensitive detection of organic thin films (10 nm) using surface-enhanced infrared absorption (SEIRA) spectroscopy. We experimentally observed carbonyl (C[dbnd]O), methyl (C-H), and (C-O) stretching absorptions of thin polymethyl methacrylate (PMMA) film on the narrow-band resonances of the PA. Given that the measured signal of (C-H) stretching absorption is very weak compared to other molecular signals, even on thick PMMA samples, our suggested PA offers one of the highest sensitivities demonstrated so far (5% absorption difference) due to the introduction of multiple gaps on narrow-band perfect absorbers