Two tiny emitters, each about the size of an atom, can together do what neither can do alone. We're talking about light scattering — when light hits particles and bounces off in all directions. Usually, a pair of particles scatters light only slightly more than a single one. But in new calculations, a regime has been found where the pair's response becomes enormous, as if two weak flashlights suddenly shone brighter than a spotlight.
The secret is that the particles must be driven out of equilibrium. In an ordinary, calm system, a rule applies: the energy that particles absorb cannot exceed what they give off. It's like a scale that always stays balanced. But if you pump energy into the particles from outside (say, with a laser) the scale tips. Then exact resonances appear, at which scattering is sharply enhanced. In equilibrium, this is impossible; there, resonances are only approximate and the enhancement is limited.
Especially interesting is the case where one dipole is electric and the other is magnetic. The magnetic response of a single particle is usually weak, but in a pair it can be amplified. The authors showed that even metal nanoparticles described by the Drude model cannot, in equilibrium, exceed the optimal response of a single dipole. But out of equilibrium, they can. This opens the way to creating nanoantennas that capture and redirect light with unprecedented efficiency.
Resonant scattering of light by a pair of particles resembles the effect when two tuning forks tuned in unison make each other sound louder than each would alone.