Spatial Nonlocality in the Optical Response of Metal Nanoparticles

Spatial nonlocality is known to play an important role in nano-optics when small nanometer-sized structures are involved, but few efforts have been made to assess nonlocal effects in a rigorous way. We present two different approaches to account for nonlocality in metal nanoparticles: (i) the nonretarded specular reflection model and (ii) the retarded hydrodynamical model. Excellent agreement with available experiments is obtained from our parameter-free simulations, which lead to dramatic differences with respect to local theory. Both models predict sizable plasmon blue shifts and broadenings in individual metal nanoparticles, nanoshells, particle dimers, and Yagi-Uda antennas. An analysis of plasmon resonances for varying particle size and spacing allows us to separate nonlocal and retardation effects within the hydrodynamical model. We find a wide range of geometrical parameters for which nonlocal effects coexist with significant retardation. This study is particularly relevant for broad, active areas involving applications of local field enhancement to biosensing and nonlinear optics in plasmonics.