Transverse Chiral Optical Forces by Chiral Surface Plasmon Polaritons

Recently, the concepts of transverse spin angular momentum and Belinfante spin momentum of evanescent waves have drawn considerable attention. Here, we investigate these novel physical properties of electromagnetic fields in the context of chiral surface plasmon polaritons. We demonstrate, both analytically and numerically, that chiral surface plasmon polaritons possess transverse spin angular momentum and Belinfante momentum with rich and nontrivial characteristics. We also show that the transverse spin angular momentum of chiral surface plasmon polaritons leads to the emergence of transverse optical forces in opposite directions for chiral objects of different handedness. The magnitude of this chiral transverse optical force on a chiral particle is comparable to the magnitude of the achiral optical forces on the particle, namely, the gradient force that arises from the intensity gradient and the scattering force that is the result of the linear momentum transfer of the photon to the particle. This finding may pave the way for the realization of optical separation of chiral biomolecules.