Experimental and Modelling Techniques

Before delving into the research results of this thesis, in this chapter we provide an overview of the primary experimental and modelling approaches employed throughout it. Firstly, we present a commonly used method for producing high-quality van der Waals (vdW) materials and heterostructures based on them: mechanical exfoliation and the dry transfer technique. Subsequently, we explore various techniques widely used for exciting and characterizing polaritons and that will be central to the development of the work presented in this thesis, with a special focus on the mid-IR and far-IR ranges. We start by introducing the fundamental tool of infrared (IR) spectroscopy, FTIR (Fourier-transform infrared spectroscopy), which enables the characterization of IR optical properties of bulk materials across extensive areas and broad spectral ranges with exceptional spectral resolution. However, traditional FTIR spectroscopy is diffraction limited, and the deeply subwavelength nature of IR polaritons necessitates characterization techniques capable of achieving subdiffraction spatial resolution. Therefore, we proceed by detailing the utilization of scattering-type near-field optical microscopy (s-SNOM), which offers spatial resolution well below the diffraction limit while also collecting topographical information about a sample, offering unprecedented capabilities for investigating subwavelength polaritonic modes. Finally, we delve into the primary strategies for modeling and interpreting experimental data resulting from s-SNOM. These strategies include numerical calculations of the momentum-dependent reflection coefficient using the transfer matrix method, as well as full-wave numerical simulations based on finite element methods.