Probing low-energy hyperbolic polaritons in van der Waals crystals with an electron microscope

Abstract

Hexagonal Boron Nitride (hBN) is a representative material of a wide class of two-dimensional (2D) systems in which individual atomic layers are only weakly coupled by van der Waals interaction, resulting in extreme optical anisotropy. The latter gives rise to hBN's hyperbolic phonon polaritons (h-PhPs) at mid-infrared (mid-IR) energies, in the range of 90-200 meV. Hyperbolic polaritons might be key to emerging technologies that rely on nanoscale light confinement and manipulation. Here we perform an experimental mapping of the spectral signature of a 2D material by analyzing the electron energy loss spectroscopy (EELS) near a hBN edge, as a function of electron beam position. We understand these excitations and the dispersion of the EELS peaks in terms of the excitation of surface h-PhPs. A theoretical description of the polaritonic losses that also considers the experimental resolution, provides an excellent agreement between theory and experiment, proving that fast electrons can couple to hyperbolic polaritons. Further technical improvements might enable this tool to become a versatile technique for infrared vibrational spectroscopy of polaritons.