Low‐Loss Anisotropic Image Polaritons in van der Waals Crystal α‐MoO ₃

Abstract Orthorhombic molybdenum trioxide (α‐MoO 3 ), a newly discovered polaritonic van der Waals crystal, is attracting significant attention due to its strongly anisotropic mid‐infrared phonon‐polaritons. At the same time, coupling of polariton with its mirror image in an adjacent metal gives rise to a significantly more confined image mode. Here, monocrystalline gold flakes—an atomically flat low‐loss substrate for mid‐infrared image polaritons—are employed to measure the full complex‐valued propagation constant of the hyperbolic image phonon‐polaritons in α‐MoO 3 by near‐field probing. The anisotropic dispersion is mapped and the damping of the polaritons propagating at different angles to the crystallographic directions of α‐MoO 3 is analyzed. These experiments demonstrate the strongly confined image phonon‐polaritons in α‐MoO 3 exhibiting intrinsic limiting lifetime of 4.2 ps and a propagation length of 4.5 times the polariton wavelength, owing to the negligible substrate‐mediated loss. Furthermore, it is shown that the image modes with positive group velocity have simultaneously larger momentum and lifetime compared to their counterparts on a dielectric substrate, while the image modes with negative group velocity possess a smaller momentum. These results spotlight the hyperbolic image phonon‐polaritons as a superior platform for unconventional light manipulation at the nanoscale.