Engineering Phonon Polaritons in van der Waals Heterostructures to Enhance In-Plane Optical Anisotropy

TL;DR

This study demonstrates enhanced in-plane optical anisotropy of phonon polaritons in van der Waals heterostructures, revealing potential for advanced nanophotonic applications and novel characterization methods.

Contribution

First direct observation of in-plane anisotropic phonon polariton modes in vdW heterostructures, showing anisotropy exceeds the material's refractive index ratio.

Findings

Maximum in-plane optical anisotropy of 1.25 at 1405-1440 cm-1

Anisotropy enhancement due to high phonon polariton confinement in h-BN

Provides insights into polariton behavior in vdW heterostructures

Abstract

Van der Waals heterostructures assembled from layers of 2D materials have attracted considerable interest due to their novel optical and electrical properties. Here we report a scattering-type scanning near field optical microscopy study of hexagonal boron nitride on black phosphorous (h-BN/BP) heterostructures, demonstrating the first direct observation of in-plane anisotropic phonon polariton modes in vdW heterostructures. Strikingly, the measured in-plane optical anisotropy along armchair and zigzag crystal axes exceeds the ratio of refractive indices of BP in the x-y plane. We explain that this enhancement is due to the high confinement of the phonon polaritons in h-BN. We observe a maximum in-plane optical anisotropy of {\alpha}_max=1.25 in the 1405-1440 cm-1 frequency spectrum. These results provide new insights on the behavior of polaritons in vdW heterostructures, and the observed anisotropy enhancement paves the way to novel nanophotonic devices and to a new way to characterize optical anisotropy in thin films.