Bypassing Nonlocal Phenomena in Metals Using Phonon-Polaritons

Electromagnetic design relies on an accurate understanding of light-matter interactions yet often overlooks electronic length scales. Under extreme confinement, this omission can lead to nonclassical effects such as nonlocal response. Here, we use mid-infrared phonon-polaritons in hexagonal boron nitride (hBN) screened by monocrystalline gold flakes to push the limits of nanolight confinement unobstructed by nonlocal phenomena, even when the polariton phase velocity approaches the Fermi velocities of electrons in gold. We employ near-field imaging to probe polaritons in nanometer-thin crystals of hBN on gold and extract their complex propagation constant, observing effective indices exceeding 94. We further show the importance of sample characterization by revealing a thin low-index interfacial layer naturally forming on monocrystalline gold. Our experiments address a fundamental limitation posed by nonlocal effects in van der Waals heterostructures and outline a pathway to bypass their impact in high-confinement regimes.