Mid-Infrared Surface Plasmon Polariton Sensors Resonant with the Vibrational Modes of Phospholipid Layers

We present an experimental study of subwavelength hole arrays in thin metal films employed as surface-enhanced optical sensors operating in the mid-infrared. The extremely narrow surface plasmon polariton spectral resonances are fitted to an analytical Fano interference model in the wavelength range 2-10 mu m. In general, the resonance frequency shifts after deposition of few-molecule layers (3.2-24 nm thickness) according to electrodynamic polarization models, hence allowing for label-free sensing. The absolute value of the shift is shown to depend on the overlap between the electric field distribution of the specific surface plasmon mode and the molecular layer, as verified by electromagnetic simulations. Biochemical sensor application is finally demonstrated by determining, from a single mid-infrared measurement, both the thickness and the absorption spectrum of phospholipid monolayers and trilayers, obtained by liposome adsorption.