When a low-dimensional polaritonic material is placed in proximity to a highly conductive metal, polariton modes couple to their images in the metal, forming highly compressed image polaritons. So far, near-field mapping has been used to observe such modes in graphene and hexagonal boron nitride (hBN). However, an accurate measurement of their intrinsic loss remains challenging because of the inherent complexity of the near-field signal, particularly for the hyperbolic phonon-polaritons (HPP). Here we demonstrate that monocrystalline gold flakes, an atomically-flat low-loss substrate for image modes, provide a platform for precise near-field measurement of the complex propagation constant. As a topical example, we measure dispersion of the hyperbolic image phonon-polaritons (HIP) in hBN, revealing that their normalized propagation length exhibits a parabolic spectral dependency. At the frequency of the maximal propagation, image modes exhibit nearly two times lower normalized loss, while being 2.4 times more compressed compared to the phonon-polaritons in hBN on a dielectric substrate. We conclude that the image phonon-polaritons in van der Waals crystals provide a unique nanophotonic platform where strong light-matter interaction and wave phenomena can be harnessed at the same time.
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