Thermal · photothermal heating
In an array each particle is heated by its own absorption and by the heat from all the others. For many particles under a wide beam the collective part dominates, so the temperature is much higher than for one particle.
The temperature across the laser spot for an array with a chosen lattice and period, with the particles' optical response included.
Optically it sets the lattice resonances; thermally, denser arrays put more heat into the same area.
Collective heating of a nanoparticle array under light, in one click.