Probing and steering bulk and surface phonon polaritons in uniaxial materials using fast electrons: hexagonal boron nitride

TL;DR

This paper theoretically investigates how fast electrons excite and control bulk and surface phonon polaritons in uniaxial hyperbolic materials like hexagonal boron nitride, revealing new excitation mechanisms and control strategies.

Contribution

It introduces a systematic theoretical framework for understanding electron-induced polariton excitation and demonstrates control of polariton propagation via electron velocity and direction in uniaxial materials.

Findings

Bulk modes can be excited by electrons outside the uniaxial medium.

Excitation depends strongly on electron velocity and angle relative to the optical axis.

Surface and bulk phonon polaritons can be selectively excited and steered.

Abstract

We theoretically describe how fast electrons couple to polaritonic modes in uniaxial materials by analyzing the electron energy loss (EEL) spectra. We show that in the case of an uniaxial medium with hyperbolic dispersion, bulk and surface modes can be excited by a fast electron traveling through the volume or along an infinite interface between the material and vacuum. Interestingly, and in contrast to the excitations in isotropic materials, bulk modes can be excited by fast electrons traveling outside the uniaxial medium. We demonstrate our findings with the representative uniaxial material hexagonal boron nitride. We show that the excitation of bulk and surface phonon polariton modes is strongly related to the electron velocity and highly dependent on the angle between the electron beam trajectory and the optical axis of the material. Our work provides a systematic study for understanding bulk and surface polaritons excited by a fast electron beam in hyperbolic materials and sets a way to steer and control the propagation of the polaritonic waves by changing the electron velocity and its direction.