Excitation and Manipulation of In-Plane Hyperbolic Phonon Polaritons in α-MoO₃ Flakes via Plasmonic Antennas

Hyperbolic phonon polaritons (HPhPs) in polar van der Waals crystals, such as α-MoO₃ and α-V₂O₅, enable deep subwavelength light confinement and in-plane manipulation via hyperbolic dispersion, yet momentum mismatch hinders efficient excitation. We present a far-field method to excite in-plane HPhPs using plasmonic gold nanorods. Surface plasmon resonance (SPPR) in nanorods mediates strong interactions between free-space waves and HPhPs in α-MoO₃. Excitation efficiency is highly sensitive to nanorod length and angular orientation relative to the [100] crystal axis of α-MoO₃. Maximum HPhP near-field intensity is achieved when the nanorods are aligned to optimize coupling with the anisotropic polaritonic modes, with efficiency closely linked to the resonance conditions of the plasmonic antennas. Numerical designs are corroborated well with near-field optical characterizations. Therefore, our approach establishes a platform to study interactions between plasmonic antennas and anisotropic polaritons, advancing controlled manipulation of in-plane polaritonic modes in van der Waals materials.