In-plane anisotropic and ultra-low-loss polaritons in a natural van der Waals crystal

Polaritons-hybrid light-matter excitations-enable nanoscale control of light. Particularly large polariton field confinement and long lifetimes can be found in graphene and materials consisting of two-dimensional layers bound by weak van der Waals forces^1,2 (vdW materials). These polaritons can be tuned by electric fields^3,4 or by material thickness⁵, leading to applications including nanolasers⁶, tunable infrared and terahertz detectors⁷, and molecular sensors⁸. Polaritons with anisotropic propagation along the surface of vdW materials have been predicted, caused by in-plane anisotropic structural and electronic properties⁹. In such materials, elliptic and hyperbolic in-plane polariton dispersion can be expected (for example, plasmon polaritons in black phosphorus⁹), the latter leading to an enhanced density of optical states and ray-like directional propagation along the surface. However, observation of anisotropic polariton propagation in natural materials has so far remained elusive. Here we report anisotropic polariton propagation along the surface of α-MoO₃, a natural vdW material. By infrared nano-imaging and nano-spectroscopy of semiconducting α-MoO₃ flakes and disks, we visualize and verify phonon polaritons with elliptic and hyperbolic in-plane dispersion, and with wavelengths (up to 60 times smaller than the corresponding photon wavelengths) comparable to those of graphene plasmon polaritons and boron nitride phonon polaritons^3-5. From signal oscillations in real-space images we measure polariton amplitude lifetimes of 8 picoseconds, which is more than ten times larger than that of graphene plasmon polaritons at room temperature¹⁰. They are also a factor of about four larger than the best values so far reported for phonon polaritons in isotopically engineered boron nitride¹¹ and for graphene plasmon polaritons at low temperatures¹². In-plane anisotropic and ultra-low-loss polaritons in vdW materials could enable directional and strong light-matter interactions, nanoscale directional energy transfer and integrated flat optics in applications ranging from bio-sensing to quantum nanophotonics.