Ultra-Low-Loss Anisotropic Image Polaritons in α-MoO₃

Polaritons in van der Waals crystals possess a high degree of field confinement, allowing for the strong light-matter interaction and light manipulation at nanoscale. At the same time, when a polaritonic van der Waals crystal is placed in proximity to a highly conductive metal, the coupling between the collective charge oscillations and their images in the metal results in a manifestation of an even more compressed low-dimensional mode – the image polariton.<br/>We use the near-field optical microscopy and the photo-induced force microscopy to study the hyperbolic image phonon-polaritons (HIP) in orthorhombic molybdenum trioxide (α-MoO₃) – a recently discovered polaritonic van der Waals crystal that supports strongly anisotropic mid-infrared phonon-polaritons. As a low-loss substrate for the image modes, we employ the atomically-flat monocrystalline gold flakes. By leveraging the unique physical properties of the gold crystals, we systematically probe different momentum states on the isofrequency surface and measure the full complex-valued propagation constant of the HIP propagating at different angles to the crystallographic axes of α-MoO₃.<br/>We report an exceptionally long lifetime of phonon-polaritons in α-MoO₃: 4.2 ps and 9.7 ps in the second and the third Reststrahlen bands, respectively, owing to the suppression of all substrate-mediated loss channels in our samples. In contrast, the less confined phonon-polaritons in the α-MoO₃ on SiO₂ substrate have been shown to have a significantly shorter lifetime of ~2 ps in the second Reststrahlen band.<br/>Furthermore, our study reveals the drastically different dispersion properties of the HIP with opposite signs of the group velocity: only the modes with positive group velocity (in the second Reststrahlen band) have both significantly larger momentum and lifetime compared to their counterparts on a dielectric substrate, while the image modes with anomalous dispersion (in the third Reststrahlen band) possess smaller momentum.<br/>Summarizing, our near-field study demonstrates a unique combination of the in-plane anisotropic phonon-polaritons in α-MoO₃, the image mode, and the monocrystalline gold substrate, leading to a manifestation of the hyperbolic image phonon-polaritons with simultaneously greater field compression and larger lifetime compared to the phonon-polaritons in α-MoO₃ on a dielectric substrate. Our results spotlight the HIP in α-MoO₃ as an appealing platform for versatile nanophotonic applications.