Mid-infrared narrow band plasmonic perfect absorber for vibrational spectroscopy

Abstract

The introduction of narrow band perfect absorbers (PAs) operating in mid-infrared (IR) frequencies has the potential to improve the study of biomolecule monolayers using surface enhanced vibrational spectroscopy. However, wavelength dependent radiation and nonradiative losses are limiting factors to obtain narrow band absorption resonance, having full width at half maximum (FWHM) below 150 cm−1 in the mid-IR frequency range. In this work, we numerically and experimentally present an engineered narrow band PA with FWHM of 85 cm−1 operating in mid-IR frequencies. The PA is based on multi-layer metamaterial plasmonic nanoantennas. We used numerical and experimental work to fine tune the parameters of the PA in order to control its spectral response. The suggested PA system strongly provides high absorption values (100 %) in the mid-IR region and near-field enhancement factor of 103 at the corresponding resonance values. The fabricated PA shows strong narrow band resonance and all the resonance energy could be transferred to thin films (10 nm) for higher sensitivity. In addition, PA shows multi-band resonances within the mid-IR region, thus it can be effectively used for simultaneously detecting the different biomolecular fingerprints. In this sense, we experimentally observed absorption of carbonyl ν(CO) and asymmetric methyl ν(−CH3) stretching bands of thin polymethyl methacrylate (PMMA) film on the narrow band resonances of PA. The suggested system has potential to be used in live cell-membrane investigations, as multi-band structure enables investigation of both lipids and proteins.