Symmetry breaking-induced N-body electrodynamic forces in optical matter systems

Breaking symmetry can give rise to non-reciprocal forces–unequal and opposite forces–typically observed in active matter systems involving asymmetric 2-body interactions. So far, there are few examples of N-body non-reciprocal forces induced by symmetry breaking. Here we show, through experiment, numerical simulation, and theoretical analysis, that N-body non-reciprocal forces emerge in optical matter systems comprised of three or more electrodynamically interacting (nano)particles when spatial symmetries are broken. The requisite symmetry breaking is realized in experiment by trapping Ag nanoparticles in a curved geometry using an optical ring trap. The ordered ring of nanoparticles is observed to rotate collectively in a direction governed by the handedness of the trapping beam's circular polarization. This force, distinct from spin-to-orbit angular momentum conversion, depends strongly on particle number and inter-particle separations. These N-body non-reciprocal interactions induced by symmetry breaking are general and should arise in other "coherently illuminated" active matter systems.