Quantum Surface-Response of Metals Revealed by Acoustic Graphene Plasmons

TL;DR

This paper proposes using ultra-confined acoustic graphene plasmons to probe the quantum surface-response functions of metals with subnanometer resolution, enabling precise characterization of electronic length scales at interfaces.

Contribution

It introduces a theoretical framework and experimental proposal for inferring the quantum response of metals via shifts in graphene plasmon dispersion caused by quantum surface effects.

Findings

AGPs can resolve quantum electronic length scales with subnanometer accuracy.

Quantum shifts in AGP dispersion encode information about metal surface-response functions.

The method enables plasmon-based measurement of quantum surface properties.

Abstract

A quantitative understanding of the electromagnetic response of materials is essential for the precise engineering of maximal, versatile, and controllable light--matter interactions. Material surfaces, in particular, are prominent platforms for enhancing electromagnetic interactions and for tailoring chemical processes. However, at the deep nanoscale, the electromagnetic response of electron systems is significantly impacted by quantum surface-response at material interfaces, which is challenging to probe using standard optical techniques. Here, we show how ultra-confined acoustic graphene plasmons (AGPs) in graphene--dielectric--metal structures can be used to probe the quantum surface-response functions of nearby metals, here encoded through the so-called Feibelman $d$-parameters. Based on our theoretical formalism, we introduce a concrete proposal for experimentally inferring the low-frequency quantum response of metals from quantum shifts of the AGPs' dispersion, and demonstrate that the high field confinement of AGPs can resolve intrinsically quantum mechanical electronic length-scales with subnanometer resolution. Our findings reveal a promising scheme to probe the quantum response of metals, and further suggest the utilization of AGPs as plasmon rulers with \r{a}ngstr\"{o}m-scale accuracy.