Breaking the symmetry of forward-backward light emission with localized and collective magnetoelectric resonances in arrays of pyramid-shaped aluminum nanoparticles.

We propose aluminum nanopyramids (ANPs) as magnetoelectric optical antennas to tailor the forward versus backward luminescence spectrum. We present light extinction and emission experiments for an ANP array wherein magnetoelectric localized resonances couple to in-plane diffracted orders. This coupling leads to spectrally sharp collective resonances. Luminescent molecules drive both localized and collective resonances, and we experimentally demonstrate an unconventional forward versus backward luminescence spectrum. Through analytical calculations, we show that the magnetic, magnetoelectric, and quadrupolar moments of ANPs­which lie at the origin of the observed effects­are enhanced by their tapering and height. Full-wave simulations show that localized and delocalized magnetic surface waves, with an excitation strength depending on the plane wave direction, direct the forward versus backward emitted intensity.