Revealing Nanoscale Confinement Effects on Hyperbolic Phonon Polaritons with an Electron Beam

TL;DR

This study uses electron energy-loss spectroscopy to investigate nanoscale confinement effects on hyperbolic phonon polaritons in hexagonal boron nitride, revealing complex phenomena and guiding mid-infrared device design.

Contribution

It demonstrates a novel application of EELS to directly observe and analyze nanoscale hyperbolic polariton confinement and dispersion in hBN.

Findings

Localized polaritons induced by environmental heterogeneity

Enhanced and suppressed excitations due to two-dimensional interference

Modification of edge-confined polaritons by nanoscale heterogeneity

Abstract

Hyperbolic phonon polaritons (HPhPs) in hexagonal boron nitride (hBN) enable the direct manipulation of mid-infrared light at nanometer scales, many orders of magnitude below the free-space light wavelength. High resolution monochromated electron energy-loss spectroscopy (EELS) facilitates measurement of excitations with energies extending into the mid-infrared while maintaining nanoscale spatial resolution, making it ideal for detecting HPhPs. The electron beam is a precise source and probe of HPhPs, that allows us to perform novel experiments to observe nanoscale confinement in HPhP structures and directly extract hBN polariton dispersions for both modes in the bulk of the flake and modes along the edge. Our measurements reveal technologically important non-trivial phenomena, such as localized polaritons induced by environmental heterogeneity, enhanced and suppressed excitation due to two-dimensional interference, and strong modification of high-momenta excitations of edge-confined polaritons by nanoscale heterogeneity on edge boundaries. Our work opens exciting prospects for the design of real-world optical mid-infrared devices based on hyperbolic polaritons.