  Light can exert radiation pressure on any object it encounters and that
resulting optical force can be used to manipulate particles. It is commonly
assumed that light should move a particle forward and indeed an incident plane
wave with a photon momentum hk can only push any particle, independent of its
properties, in the direction of k. Here we demonstrate using full-wave
simulations that an anomalous lateral force can be induced in a direction
perpendicular to that of the incident photon momentum if a chiral particle is
placed above a substrate that does not break any left-right symmetry.
Analytical theory shows that the lateral force emerges from the coupling
between structural chirality (the handedness of the chiral particle) and the
light reflected from the substrate surface. Such coupling induces a sideway
force that pushes chiral particles with opposite handedness in opposite
directions.
