Nonlocal formalism for nanoplasmonics: phenomenological and semi-classical considerations

The plasmon response of metallic nanostructures is anticipated to exhibit nonlocal dynamics of the electron gas when exploring the true nanoscale. We extend the local-response approximation (based on Ohm's law) to account for a general short-range nonlocal response of the homogeneous electron gas. Without specifying further details of the underlying physical mechanism we show how this leads to a Laplacian correction term in the electromagnetic wave equation. Within the hydrodynamic model we demonstrate this explicitly and we identify the characteristic nonlocal range to be vF/omega where vF is the Fermi velocity and omega is the optical angular frequency. For noble metals this gives significant corrections when characteristic device dimensions approach ~1-10 nm, whereas at more macroscopic length scales plasmonic phenomena are well accounted for by the local Drude response.