Near-field probing of image phonon-polaritons in hexagonal boron nitride on gold crystals

TL;DR

This paper demonstrates that using monocrystalline gold flakes as substrates allows for precise near-field measurement of phonon-polaritons in hexagonal boron nitride, revealing lower losses and higher compression compared to dielectric substrates.

Contribution

The study introduces monocrystalline gold flakes as an effective low-loss substrate for near-field probing of image phonon-polaritons in van der Waals materials, enabling more accurate loss measurements.

Findings

Normalized propagation length of phonon-polaritons shows a parabolic spectral dependence.

Image phonon-polaritons exhibit up to twice lower propagation loss on gold flakes.

Polaritons are 2.4 times more compressed on gold flakes compared to dielectric substrates.

Abstract

Near-field mapping has been widely used to study hyperbolic phonon-polaritons in van der Waals crystals. However, an accurate measurement of the polaritonic loss remains challenging because of the inherent complexity of the near-field signal and the substrate-mediated loss. Here we demonstrate that large-area monocrystalline gold flakes, an atomically-flat low-loss substrate for image polaritons, provide a platform for precise near-field measurement of the complex propagation constant of polaritons in van der Waals crystals. As a topical example, we measure propagation loss of the image phonon-polaritons in hexagonal boron nitride, revealing that their normalized propagation length exhibits a parabolic spectral dependency. Further, we show that image phonon-polaritons exhibit up to a twice lower normalized propagation loss, while being 2.4 times more compressed compared to the case of dielectric substrate. We conclude that the monocrystalline gold flakes provide a unique nanophotonic platform for probing and exploitation of the image modes in low-dimensional materials.