Reactive optical matter: light-induced motility in electrodynamically asymmetric nanoscale scatterers

From Newton’s third law, which is known as the principle of actio et reactio1, we expect the forces between interacting particles to be equal and opposite for closed systems. Otherwise, “nonreciprocal” forces can arise.2 This has been shown theoretically in the interaction between dissimilar optically trapped particles that are mediated by an external field.3 As a result, despite the incident external field not having a transverse component of momentum, the particle pair experiences a force in a direction that is transverse to the light propagation direction.3,4 In this letter, we directly measure the net nonreciprocal forces in electrodynamically interacting asymmetric nanoparticle dimers and nanoparticle structures that are illuminated by plane waves and confined to pseudo one-dimensional geometries. We show via electrodynamic theory and simulations that interparticle interactions cause asymmetric scattering from heterodimers. Therefore, the putative nonreciprocal forces are actually a consequence of momentum conservation. Our study demonstrates that asymmetric scatterers exhibit directed motion due to the breakdown of mirror symmetry in their electrodynamic interactions with external fields.