In-Plane Hyperbolic Phonon-Polaritons in van der Waals Nanocrystals

Anisotropic hyperbolic phonon-polaritons (HPhPs) in a van der Waals material, orthorhombic-phase molybdenum trioxide (α-MoO₃), provide strategies in multiple dimensions to manipulate photons at the nanoscale. However, the nano-imaging studies of the in-plane HPhPs along orthogonal crystal orientations are still rare. In this work, the launching and propagating properties of HPhPs are studied in α-MoO₃ upon a holey silicon nitride microcavity through a photon-induced force microscope (PiFM) over a broad tunable mid-infrared region of 780–1010 cm⁻¹. Near-field patterns stemming from the orthogonal polariton momenta are observed in this spectral band. A simple model based on the geometrical optics of elliptic resonance cones is used to explain the near-field phonon-polariton features. These near-field optical properties indicate abundantly possible operations and applications for nanophotonics.