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

Abstract 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.