  Collective resonances in plasmonic nanoparticle arrays with electric dipole
moment oriented along the lattice wave propagation are theoretically
investigated. The role of electric quadrupole (EQ) and magnetic dipole (MD)
moments of gold nanoparticles in the resonant features of the arrays is
analyzed. We perform both semi-analytical calculations of coupled multipole
equations and rigorous numerical simulations varying contributions of the
electric and magnetic multipoles by changing particle size and shape (spheres
and disks). The arrays in homogeneous and non-homogeneous environments are
considered. We find that even very weak non-resonant EQ and MD moments of a
single particle are significantly enhanced in the periodic lattice at the
wavelength of collective (lattice) resonance excitation. Importantly, we show
that in the infinite arrays, the EQ and MD moments of nanoparticles are coupled
and affect each other resonant contributions. We also demonstrate that at the
lattice-resonance wavelength, the enhanced EQ and MD moments have contributions
to reflection comparable to the dipole one resulting in a significant decrease
of reflection and providing the satisfaction of the generalized Kerker
condition for reflection suppression.
