Nonreciprocity in optically bound colloidal ensembles

Interactions mediated by the exchange of scattered waves generically are not constrained by Newton’s third law. Nonreciprocal wave-mediated interactions give rise to center-of-mass forces and torques that can propel clusters of scatterers. Such nonreciprocity has been observed in the emergent activity of millimeter-scale particles levitated in sound waves. Here, we demonstrate an analogous form of emergent activity in colloidal particles illuminated by optical traps. Our system consists of a heterogeneous dispersion of colloidal spheres all moving in the optical force field of a holographically projected optical ring trap. The extended trap localizes the spheres on a one-dimensional manifold and serves as a reservoir of energy that the spheres tap into to fuel their motion. We present the theory of emergent activity powered by light together with initial experimental observations.