Article High Quality Factor Silicon Membrane Metasurface for Intensity-Based Refractive Index Sensing Andrea Tognazzi 1,2 , Davide Rocco 1,2 , Marco Gandolfi 1,2 , Andrea Locatelli 1,2 , Luca Carletti 1,2 and Costantino De Angelis 1,2, *   Citation: Tognazzi, A.; Rocco, D.; Gandolfi, M.; Locatelli, A.; Carletti, L.; De Angelis, C. High Quality Factor Silicon Membrane Metasurface for Intensity-Based Refractive Index Sensing. Optics 2021, 2, 193–199. https://doi.org/10.3390/opt2030018 Academic Editor: Mario Bertolotti Received: 3 August 2021 Accepted: 2 September 2021 Published: 6 September 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Dipartimento di Ingegneria dell’Informazione, Università degli Studi di Brescia, Via Branze 38, 25123 Brescia, Italy; a.tognazzi007@unibs.it (A.T.); davide.rocco@unibs.it (D.R.); marco.gandolfi1@unibs.it (M.G.); andrea.locatelli@unibs.it (A.L.); luca.carletti@unibs.it (L.C.) 2 Istituto Nazionale di Ottica (INO)-Consiglio Nazionale delle Ricerche (CNR), Via Branze 45, 25123 Brescia, Italy * Correspondence: costantino.deangelis@unibs.it; Tel.: +39-0303715901 Abstract: We propose a new sensing device based on all-optical nano-objects placed in a suspended periodic array. We demonstrate that the intensity-based sensing mechanism can measure envi- ronment refractive index change of the order of 1.8 × 10 −6 , which is close to record efficiencies in plasmonic devices. Keywords: sensing; metasurface; BIC 1. Introduction Optical sensing has become very important in applications ranging from chemistry to biology, since it is a noninvasive approach and can provide a very fast response time [1–3]. In particular, metallic nanoparticles (NPs) have been extensively used for optical sensing because of their localized surface plasmon resonance [4,5]. Indeed, the latter property offers a strong enhancement of the electric field in proximity of the NPs, magnifying the system sensitivity to the environment refractive index changes. Hence, the nature of the analyte placed in proximity of the metallic NP can be disclosed by analyzing the sensor optical response. Recently, a plethora of sensors based on plasmonic structures have been presented [4,6,7] and proved to selectively detect cadmium ions and bacteria when jointly operated with optical fibers [8–10]. Although plasmonic sensors can offer a very good performance in terms of sensitivity, analyte selectivity and versatility, they may be plagued by strong ohmic losses due to light absorption in the metallic parts. This fact could yield an important temperature increase in the system, ultimately triggering unwanted chemical reactions or altering the analytes, which may provoke serious consequences for inflammable detection of gases such as methane or propane. Keeping the sensor temperature under control, by avoiding Joule heating occurring in plasmonic devices, is also of paramount importance in biological applications since even temperatures below 80 ◦ C may cause permanent damages in in-vivo applications. In order to contain the losses, a valuable alternative is the design of optical sensors based on dielectric materials [11]. Nanostructures made with dielectric materials with high refractive index support Mie-type resonances that provide localized field enhancement. This resonant behavior can be obtained in the spectral region where the optical absorption of the material is negligible. In this regard, optical sensing in the environment of all- dielectric nanostructures was reported in [12–17]. In particular, silicon nanoresonator-based biosensors with good sensitivity were devised. The latest results show that all-dielectric optical sensing—a relatively unexplored field up to now—is promising for the conception of new generation sensors. In this context, we propose a new sensing device based on all-optical nano-objects arranged in a suspended periodic array similar to what was reported in [18]. In particular, Optics 2021, 2, 193–199. https://doi.org/10.3390/opt2030018 https://www.mdpi.com/journal/optics