Interrogating Quantum Nonlocal Effects in Nanoplasmonics through Electron-Beam Spectroscopy

TL;DR

This paper introduces a method to extract quantum surface response parameters of metals from electron spectroscopy data, revealing significant quantum nonlocal effects on optical spectra at nanometer scales, aiding the design of plasmonic devices.

Contribution

It presents a scheme to retrieve quantum surface response parameters from EELS and CL measurements, enabling practical inclusion of nonlocal quantum effects in nanoplasmonics modeling.

Findings

Quantum nonlocal effects cause spectral shifts and damping in EELS and CL spectra.

The scheme allows probing quantum effects at nanometer length scales.

Results facilitate rigorous modeling of quantum effects in plasmonic nanostructures.

Abstract

A rigorous account of quantum nonlocal effects is paramount for understanding the optical response of metal nanostructures and for designing plasmonic devices at the nanoscale. Here, we present a scheme for retrieving the quantum surface response of metals, encapsulated in the Feibelman $d$-parameters, from electron energy-loss spectroscopy (EELS) and cathodoluminescence (CL) measurements. We theoretically demonstrate that quantum nonlocal effects have a dramatic impact on EELS and CL spectra, in the guise of spectral shifts and nonlocal damping, when either the system size or the inverse wave vector in extended structures approach the nanometer scale. Our concept capitalizes on the unparalleled ability of free-electrons to supply deeply subwavelength near-fields and, thus, probe the optical response of metals at length scales in which quantum-mechanical effects are apparent. These results pave the way for a widespread use of the $d$-parameter formalism, thereby facilitating a rigorous yet practical inclusion of nonclassical effects in nanoplasmonics.