Ultra-Confinement of Polaritons in Single Atomic Layer Ag Photonic Quantum Dots.

Light scattering by two-dimensional (2D) van der Waals heterostructures (vdWHs) is immense, especially given their infinitesimal volume, thus enabling strong light-matter interactions. Surface 2D polariton waves manifest through a large concentration of electromagnetic field in the vertical direction, normal to their propagation. By confining vdWH materials into 2D photonic shapes, one can manipulate and compress light in lateral directions. Scattering-type scanning near-field optical microscopy is a perfect tool for direct imaging of the propagating polaritons and studying the properties of confined polaritons in nanostructures. Though, thus far, the quantitative analysis, such as the wavelength extraction, has been challenged for confined polaritons by incapability of mapping of the wave period on a sub-wavelength scale and the difficulty of identifying an adequate substrate's "background" to subtract. Here, an analytical approach is developed to reveal the local propagation constant of confined polaritons under the above-mentioned constraints and map it with sub-wavelength resolution. Applied to the analysis of the SiC/2D-Ag/EG (epitaxial graphene) photonic nanostructures, the technique uncovered that the polaritons are highly confined in both vertical (∼λ/50) and lateral directions (∼λ/40) by 2D metal.