Non-local optical plasmon supported by Fermi arc states in thin film Weyl semimetal*

The Fermi surface of a thin-film Weyl semimetal consists of highly anisotropic contours, each of which contains topologically protected Fermi arc states on the thin-film surfaces, as well as states with dominant support in the bulk of the system. We study collective charge oscillations for this system, uncovering a single optical plasmonic mode originating from collective oscillations on the two surfaces. Remarkably, these lowest frequency modes involve surface charges which oscillate out-of-phase, suggesting the resulting electric fields are relatively confined to the interior of the thin film. We discuss possible implications of our findings for plasmonic applications.