  We have demonstrated optical excitation and detection of collective
precessional dynamics in arrays of coupled Ni80Fe20 (permalloy) nanoelements
with systematically varying areal density by an all-optical time-resolved Kerr
microscope. We have applied this technique to precisely determine three
different collective regimes in these arrays. At very high areal density, a
single uniform collective mode is observed where the edge modes of the
constituent elements are suppressed. At intermediate areal densities, three
nonuniform collective modes appear and at very low areal density, we observe
noncollective dynamics and only the centre and edge modes of the constituent
elements appear.
