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

TL;DR

This paper demonstrates a method to use STEM-EELS for nanoscale mapping of low-energy hyperbolic phonon polaritons in van der Waals crystals, revealing new insights into their excitations.

Contribution

The authors adapt STEM-EELS to probe energy losses down to 100 meV and develop a classical response theory to interpret hyperbolic phonon polaritons in vdW materials.

Findings

STEM-EELS can map hyperbolic phonon polaritons in h-BN.

Electron loss is dominated by hyperbolic phonon polaritons, not bulk phonons.

Theoretical models match experimental spectra accurately.

Abstract

Van der Waals (vdW) materials exhibit intriguing structural, electronic and photonic properties. Electron Energy Loss Spectroscopy (EELS) within Scanning Transmission Electron Microscope (STEM) allows for nanoscale mapping of such properties. However, typical STEM-EELS detection is limited to energy losses in the eV range, which are too large for probing important low-energy excitation such as phonons or mid-IR plasmons. Here we adapt a conventional STEM-EELS system to probe energy loss down to 100 meV, and apply it to map phononic states in h-BN, a representative van der Waals (vdW) material. The h-BN EELS spectra, surprisingly, depend on the h-BN thickness and the distance of the electron beam to h-BN flake edges. To explain this observation, we developed a classical response theory describing the interaction of fast electrons with (anisotropic) slabs of vdW materials. Theoretical and numerical calculations perfectly match the experimental h-BN spectra, and reveal that the electron loss is dominated by the excitation of hyperbolic phonon polaritons, and not of bulk phonons as often reported. Thus, STEM-EELS offers the possibility to probe low-energy polaritons in vdW materials, with our theory being of fundamental importance for interpreting future experiments.