Optical metasurfaces: from reconfigurable polymer-based platforms to sensing applications

Abstract

Research is of paramount importance for the well-being and improvement of life, and this is often supported by material research. It is enough to remember that in the past man could greatly improve his own existence by the discovery of stone, and later he could do so by the discovery of bronze and still later iron. Technology today leads us to have increasingly compact devices that are capable of transmitting data as quickly as possible. Among the greatest challenges for scientists is research into materials for microprocessors, optical fibers, and the optimal use of renewable energy sources. The study of light-matter interaction is of fundamental importance since most electromagnetic devices and phenomena originate from it. In this case, the creation of different structures and geometries makes it possible to modify electromagnetic radiation for the intended scientific purposes. Metamaterials offer the possibility to overcome the limits of the physical and chemical properties of materials. With technical processes, it is possible to create structures that show a unique response thanks to their dimensions, which are smaller than the characteristic incident wavelength. One of the most important challenges in the biomedical, environmental, and chemical fields is the biorecognition of analytes in the surrounding environment with high sensitivity and specificity. One possible solution to achieve this goal is to study the change in refractive index that correlates with the specific molecule or biomolecule that needs to be detected in a fluid. The aim of this thesis is to develop an optical approach for various metasurfaces with high sensitivity, which can be used for biosensors and thus for the detection of biological material such as cells, proteins, bacteria etc. Another important aspect is the study of metasurfaces capable of opti-cal reconfiguration by external stimuli, useful to tune the focus of metalenses. This thesis is divided into four chapters, one appendix, and conclusion and perspective. In the first chapter, the concept of chirality is introduced, the interaction between chiral light and chiral matter. The focus is on extrinsic chiral metasurfaces, the study of 3D out-of-plane helices. A modeling study of various helix parameters and an analysis of the modes and their sensitivity have been performed. The second chapter presents Fano Resonance Optical Coating (FROC). After a brief introduction, relevant theoretical references are given. After accurate simulations on the FROCs, several samples were fabricated and analyzed by spectrophotometry and ellipsometry to provide a few applications for these samples. In the third chapter, metalenses supplied by the Capasso group are presented, infiltrated with various liquid crystals according to Cassie-Baxter theory. The purpose is to tune the metasurface to allow in-depth optical investigation. The goal is to tune the metalenses using photonics to excite the gold nanoparticles inside the liquid crystal. In the fourth chapter, a technique is presented for the low-cost reproduction of metalenses, which focus visible light and can be thermally tuned. The goal is to provide materials that do not degrade over time and that retain their properties for focusing. It has been experimentally demonstrated that due to the thermal effect, it is possible to tune the focus of the lens with a shift of 150 μm. In the appendix, we present a metasurface consisting of a polymer matrix containing gold nanoparticles. These substrates are analyzed from a thermoplasmonic point of view, obtaining excellent results useful, for example, for the purification of materials from bacteria. In addition, these membranes are analyzed from the point of view of sensing by stretching. In the second appendix metasurfaces based on the MIMI nanocavities used as a platform for refractive index sensing are presented.