Abstract
Optical forces allow manipulation of small particles and control of nanophotonic structures with light beams. While some techniques rely on structured light to move particles using field intensity gradients, acting locally, other optical forces can ‘push' particles on a wide area of illumination but only in the direction of light propagation. Here we show that spin–orbit coupling, when the spin of the incident circularly polarized light is converted into lateral electromagnetic momentum, leads to a lateral optical force acting on particles placed above a substrate, associated with a recoil mechanical force. This counterintuitive force acts in a direction in which the illumination has neither a field gradient nor propagation. The force direction is switchable with the polarization of uniform, plane wave illumination, and its magnitude is comparable to other optical forces.
Highlights
Optical forces allow manipulation of small particles and control of nanophotonic structures with light beams
We have shown that the scattering of a centrosymmetric, nonchiral particle over a substrate can be accompanied by a novel lateral force under polarized illumination, even if neither the particle nor the surface break the left–right symmetry
It is worth noting that small helicity-dependent lateral forces acting on particles, similar to those described above, can arise from the transverse optical momentum and spin that exist in evanescent waves[29] or in interfering polarized plane waves[31]
Summary
Optical forces allow manipulation of small particles and control of nanophotonic structures with light beams. The possibility of a net force that acts simultaneously on several particles at different locations within a wide area (not requiring focusing of a light beam onto the individual objects), and, in addition, is directed laterally (parallel to the substrate and perpendicular to the illumination direction), would enable the mass movement, arrangement and sorting of particles on a substrate or waveguide in a simple way Such a net total force acting on a particle perpendicular to the direction of plane wave illumination is different from the well-known torque forces exerted on particles illuminated by circular polarization due to the transfer of angular momentum[22,23,24,25,26]. Analytically and numerically, the presence of such counterintuitive lateral force, which does not exist for linearly polarized dipolar scattering, and which switches its direction for circularly polarized dipoles of opposite handedness, providing opportunity to control it uniquely with the polarization of the illuminating light
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