Reactive optical matter: Light-induced motion

Newton's third law dictates that forces between interacting particles are equal and opposite for closed systems. In a non-equilibrium environment, the third law can be defied, giving rise to "nonreciprocal" forces. Theoretically, this was shown when dissimilar, optically trapped particles were mediated by an external field. In a recent study, Yuval Yifat and colleagues measured the net nonreciprocal forces in electrodynamically interacting, asymmetric nanoparticle dimers and nanoparticle aggregates. In the experiments, the nanoparticle structures were confined to pseudo one-dimensional geometries and illuminated by plane waves. The observed motion was due to the conservation of total momentum for particles and fields with broken mirror symmetry (represented by a changed direction of motion). The results are now published on Light: Science & Applications.