Mid-infrared nanoantenna arrays on silicon and CaF2 substrates for sensing applications

We report on the fabrication and systematic characterization of nanoantenna arrays with several different geometries realized both on standard silicon (Si) substrates and Calcium Fluoride (CaF2) substrates aimed at the realization of a mid-Infrared protein detector. In particular, we present a novel nanofabrication procedure which allows the adoption of CaF2 in a standard lithographic process with results comparable to the ones obtained on silicon wafers. The transmittance and reflectance spectra of the nanoantennas, were acquired by means of an Infrared microscope coupled to a Michelson Interferometer. In all the nanoantenna devices, the plasmonic resonance follows a linear scaling relation: a lattice parameter change of a ±(5–10)%, indeed, results in a shift of the Si (1, 0) plasmonic resonance frequency which is proportional to 1/a. This scaling behavior offers a useful tool for device frequency tuning, which can be used to obtain a fine spectral overlap with the protein amide-I and amide-II bands. A Lorentzian analysis of the resonance peaks reveals that our nanostructures have an high Q factor (Q = ν0/Δν), demonstrating the effectiveness of our fabrication procedures.