Anomalous Lateral Optical Force as a Manifestation of the Optical Transverse Spin

Interacting with an evanescent wave, an achiral particle can experience a lateral optical force (LOF) in a direction which is perpendicular to that of the illuminating photon momentum while the system apparently has the left-right symmetry. Here, using the Cartesian multipole expansion theory, a general relationship between LOF and spin and momentum is established. Most importantly, it is demonstrated that an anomalous LOF exerted on an isotropic achiral particle of arbitrary size and composition is proportional to the optical transverse spin in the incident evanescent wave that carries pure transverse spin associated with spin-momentum locking. Manifesting itself as a mechanical consequence of the Berry phase effect, this anomalous LOF suggests a direct detectability of the transverse spin of light, as well as mimicking the transverse shift of light in the spin Hall effect of photons.