DESIGN AND FABRICATION OF OPTICALLY TRANSPARENT SMART METASURFACES BASED ON CVD GRAPHENE

Next generation networks need intelligent surfaces that, jointly with the usage of Artificial Intelligence (AI) and Machine Learning, smartly reflect impinging electromagnetic waves in the microwave region. It is difficult to find in literature such devices based on the exploitation of a tunable material like graphene, due to its purely resistive nature. Furthermore, the difficulties about the patterning operations does not allow simple implementation of the aforementioned solutions and often lead to asymmetric devices which work properly only with a single polarization of the incident wave. In this work we propose an innovative solution for the design of an optically transparent smart metasurface based on CVD graphene, regardless of the polarization of the incident wave, working in X and Ka bands depending on the specific size design. In particular, we propose the design of a 1-bit coding digital metasurface which exploits two different unit cell designs so that a $\pi$ phase shift between their responses is obtained. The design is optimized for an incident TM-polarized wave, but acceptable results are obtained with a TE-polarized one, as well. It is worth underlining that a smart metasurface with optical transparency and tunability characteristics as the one we will present is unprecedented in the scientific literature. Chapter 1 aims at first providing a general overview about metamaterials and metasurfaces, which are remarkable devices that will be massively exploited in next generation networks thanks to their unique characteristics. In Chapter 2, some notions about graphene will be introduced, with a focus on its electronic structure which is the reason for its exceptional properties. More specifically, the manufacturing process, which I followed my internship at CNR-NANOTEC in Bari, will be widely analysed. In particular, the following processes will be discussed: - growth of graphene on a copper substrate; - transfer on other substrates; - Raman characterization of graphene; - patterning techniques; - doping processes. After that, in Chapter 3 the design of an optically transparent 1-bit coding smart metasurface based on CVD graphene will be conducted, exploiting graphene tunability in order to provide a more flexible architecture. Three geometries will be proposed, with two different size designs in order for them to work both in X band and in mm-wave region. Then, after an evaluation of pros and cons of each geometry, the best one will be chosen and a numerical analysis of the behaviour of the digital metasurface working in the mm-wave region will be carried out, based on the previously chosen unit cell. Eventually, some preliminary experimental results about graphene patterning will be presented in Section 3.7.