Differential Fano interference spectroscopy of subwavelength hole arrays for mid-infrared mass sensors

We studied mid-infrared sensors based on the wavelength shift of Surface Plasmon Polariton resonances upon solid substance deposition on subwavelength hole arrays in a thin metal film (metal meshes). We present an experimental and numerical investigation of the mid-infrared transmission of metal meshes with and without a dielectric substrate, and we develop an analytical model which describes the Fano interference between the Bethe continuum and the SPP resonances. Fitting the differential transmission signal, measured before and after deposition of a target solid film, we demonstrate sensitivity down to few molecular monolayers, at least one order of magnitude better than mid-infrared vibrational spectroscopy. Sensor calibration was performed on thin polymer films and an example of real application is then provided by measuring the optical density of phospholipid membrane complexes with thickness in the range 2-10 nm. © 2013 SPIE.