Emergence of non-reciprocal forces in optically bound colloids

Optical binding is a powerful tool for producing complex inter-particle forces in colloidal systems, mediated by an intense light field. This force is reciprocal for weakly scattering or symmetric configurations of particles. Experiments with large numbers of wavelength-sized particles, however, demonstrate different behavior: they spontaneously assemble clusters which exhibit driven motion. We will show these driving forces arise because the optical binding force is not pairwise: the contributions from many particle scattering gives rise to non-reciprocal forces. As a result, optically bound colloids behave like a novel active matter system where the driving forces depend on the specific configuration of particles.