Conclusions and Outlook

The work that comprises this thesis undertakes an extensive exploration into the fundamental principles of nano-optics in anisotropic 2D, or quasi-2D, materials. Initiated in 2018, this research was spurred by the discovery of \(\alpha \text{-MoO}_{3}\) as the first natural material supporting in-plane anisotropic phonon polaritons. An extensive revision of the field, as well as a discussion of the experimental and modelling techniques used throughout this thesis is carried out in the introductory Part I. The exploration of \(\alpha \text{-MoO}_{3}\) , which upon the discovery of in-plane hyperbolic polaritons became one of the prototypical mid-infrared 2D materials, constitutes a central block of this thesis. Part II therefore ranges from the comprehensive analytical framework of polaritons in thin slabs of \(\alpha \text{-MoO}_{3}\) and other biaxial crystals, in Chap. 3 , to the precise experimental characterization of this material’s infrared optical response, combining far- and near-field methods, in Chap. 4 . Part II therefore establishes the building blocks for the study of hyperbolic-related nano-optical phenomena using \(\alpha \text{-MoO}_{3}\) , which not only are exploited in the subsequent chapters of the thesis, but also have been widely used in the scientific community working on nano-optics in 2D materials.