Introduction to Nanoscale-Confined Propagating Polaritons

In recent decades, there has been an increasing interest in the search for structures that enable confining and manipulating light at the nanometer scale. However, the diffraction limit of light poses a fundamental challenge in achieving resolution beyond the wavelength of the illuminating light, restricting the fabrication and imaging of sub-wavelength structures and thus impeding progress in the field of nano-optics. The initial section of this chapter illustrates this limitation by demonstrating that waveguides—structures designed to direct light along specific directions—made of traditional dielectric materials are not suitable for confining light in nanometric volumes at visible or longer wavelengths. In the subsequent sections, we discuss considerable advances that have been made to overcome this limitation. All are based on leveraging polaritons—hybrid light matter excitations—to achieve nanoscale confinement and control of light. We will show that polaritons can propagate along interfaces between materials with permittivities of different signs as a result of the strong interaction between light and collective dipolar matter oscillations. Then, we offer a comprehensive overview of propagating polaritons. Initially, these were explored in bulk metals and polar crystals, and subsequently in graphene and hexagonal boron nitride with the advent of van der Waals (vdW) and two-dimensional (2D) materials. We also put a special focus on the mid-infrared (mid-IR) frequency range, a technologically important spectral region, and we highlight the role that anisotropy plays in the properties of propagating polaritons. Finally, we introduce alpha-phase molybdenum trioxide, the main material studied in this thesis. This vdW semiconductor supports the propagation of strongly anisotropic polaritons along its surface, promising unprecedented control of light and the flow of energy at the nanoscale.