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 nanometre-thin crystals of hBN on gold and extract their complex propagation constant, observing effective indices exceeding 90. We further show the importance of sample characterisation 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.
@article{arxiv.2503.16863,
title = {Prospects to bypass nonlocal phenomena in metals using phonon-polaritons},
author = {Jacob T. Heiden and Eduardo J. C. Dias and Minhyuk Kim and Martin Nørgaard and Vladimir A. Zenin and Sergey G. Menabde and Hu Young Jeong and N. Asger Mortensen and Min Seok Jang},
journal= {arXiv preprint arXiv:2503.16863},
year = {2026}
}