Design Optimisation of Plasmonic Metasurfaces for Mid-Infrared High-Sensitivity Chemical Sensing A. De Marcellis 1 & E. Palange 1 & M. Janneh 1 & C. Rizza 2,3 & A. Ciattoni 3 & S. Mengali 4 Received: 11 March 2016 /Accepted: 9 May 2016 # Springer Science+Business Media New York 2016 Abstract In this paper, we report on a general method to optimise the optical characteristics of 2D-arrays of plasmonic gold nanoantennas performing as band-pass filter functionalised metasurfaces to be used as high-sensitivity mid-infrared spectroscopic sensors. We demonstrate that it is possible to increase their sensitivity in the detection of chem- ical and biological substances when the sensors are used in the surface-enhanced infrared absorption (SEIRA) technique. This technique allows revealing the presence of a substance adsorbed on the nanoantennas by measuring its optical ab- sorption under the conditions for which the maximum value of the functionalised metasurface reflectivity occurs at the same wavelength of the substance maximum absorption peak. In particular, numerical simulations based on finite element method of the metasurface detection response demonstrate the possibility to increase the sensor sensitivity of more than four orders of magnitude with respect to that one achievable if the same amount of the substance is deposited on an unstruc- tured planar metal surface. These results can be obtained by acting on the 2D-array periodicity, nanoantenna shape (i.e. rod and cross), size and thickness independently from the wave- length at which the substance absorption occurs. Moreover, in the case of cross-shaped nanoantennas, we report a complete numerical characterisation of the dependence of the metasurface maximum reflectivity and peak wavelength on the variation of the geometrical parameters of both the nanoantennas and the 2D-array. Keywords Plasmonic sensors . Functionalised metasurfaces . SEIRA . Frequency selective surfaces Introduction Metal nanoantennas (NA) arranged in a 2D-array geometry (i.e. forming a functionalised metasurface of plasmonic NA) are generally deposited on a dielectric or semiconductor sub- strate. In principle, depending on the specific application, the optical response of the metasurfaces can be tailored in any region of the electromagnetic spectrum from visible to micro- wave by varying the periodicity of the 2D-array as well as the shape, size and metal film thickness of the basic constitutive elements (i.e. the NA). In particular, for spectroscopic and remote sensing applications in the visible and in the infrared, this allows designing high efficient sensing devices with the capability to operate as band-pass and notch filters for both the transmitted and reflected light [1–5]. In this sense, due to the scale invariance property of Maxwell’ s equations [6], moving from visible toward infrared the NA size has to be increased from a nanometre to a micrometre footprint so to maintain the same electromagnetic properties of the functionalised metasurface. However, some efforts have been also made to preserve the NA footprint and to shift the resulting resonances from visible to mid-infrared region of the electromagnetic (EM) spectrum [7, 8]. The metasurfaces can probe the pres- ence of chemical and biological substances adsorbed on their surface and, under some experimental conditions, determine also their molar concentration by using the surface-enhanced * A. De Marcellis andrea.demarcellis@univaq.it 1 Department of Industrial and Information Engineering and Economics, University of L ’Aquila, L ’Aquila, Italy 2 Department of Science and High Technology, University of Insubria, Como, Italy 3 CNR-SPIN, National Council of Research, L ’Aquila, Italy 4 Electro-Optics Research Centre, CREO Consortium, L ’Aquila, Italy Plasmonics DOI 10.1007/s11468-016-0263-9