  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.
