  The self-assembly of metallic nanoparticles is a promising route to
metasurfaces with unique properties for many optical applications, such as
surface-enhanced spectroscopy, light manipulation, and sensing. We present an
in-depth theoretical study of the optical properties of mono- and bilayers
assembled from gold and silver nanoparticles. With finite-difference
time-domain simulations, we predict the occurrence of two plasmon modes, a
bright and a dark mode, which exhibit symmetric and antisymmetric dipole
configurations between the layers, respectively. The dark mode resonance energy
depends sensitively on the size of the particles and the interparticle gaps.
Hotspots with a nearfield intensity enhancement of up to 3000 are expected,
which, together with the fact that the dark mode is roughly four times narrower
than the bright mode, reveals how promising these materials are for
spectroscopy purposes.
