Landau Damping and Limit to Field Confinement and Enhancement in Plasmonic Dimers

Plasmonic dimers and other similarly shaped plasmonic nanoantennas are capable of achieving large field enhancements inside a narrow gap where surface plasmon polaritons (SPPs) are excited. As the electric field concentration increases, two primary nonlocal effects emerge: an increase in energy dissipation and an expansion of the region in SPP mode (diffusion). While phenomenological theories of nonlocality exist, fundamentally nonlocality is very well-described by Landau damping, i.e. direct excitation of electron-hole pairs in the metal by the highly confined electric field of SPPs. This work verifies and extends our original, simple, self-consistent model by (1) calculating the effect of Landau damping on the field enhancement, effective volume, and line width of the SPP mode in the plasmonic dimer, and (2) demonstrating with extensive numerical simulations that major changes of SPP properties occur in the dimers with gaps as large as 1-2 nm, where they cannot be caused by the electron tunneling. Landau damping presents the most practically relevant limit to the achievable degree of plasmonic enhancement.