Effective-medium properties of dense plasmonic balls

The electromagnetic behavior of dense plasmonic balls under plane wave illumination is investigated. High-precision T-matrix numerical calculations are used to compute the response of such balls for varying radii and volume fractions of gold nanoninclusions, up to 44%. The average scattered field is accurately reproduced by that of a spatially dispersive homogeneous sphere characterized by three effective-medium parameters: an electric permittivity $\varepsilon_{\text {eff }}$, a magnetic permeability $\mu_{\text {eff }}$ and a wave vector $k_{\mathrm{L}}$, associated to a longitudinal mode. Our results show that artificial magnetism can arise in the presence of strong interparticle couplings even when the inclusions exhibit no magnetic dipole. We also reveal the importance of the longitudinal mode to accurately reproduce the fields internal to the ball.