Spin-Orbit Interactions of Light in the Near Field of Metal Nanoparticles

Light carries both spin and momentum. Spin-orbit interactions of light come into play at the subwavelength scale of nano-optics and nano-photonics, where they determine the behavior of light [1,2]. These phenomena, in which the spin affects and controls the spatial degrees of freedom of light, are attracting rapidly growing interest [3,4]. Here we investigate spin-orbit interactions in the near field of metal nanostructures supporting localized surface plasmons. These systems can confine light to very small dimensions below the diffraction limit, leading to a striking near-field enhancement. In contrast to the propagating evanescent waves of surface plasmon-polariton modes, the electromagnetic near-field of localized surface resonances does not exhibit a definite position-independent momentum or polarization. All the calculations have been carried out beyond the quasistatic approximation, using the Mie theory implemented within the T-matrix formalism [5]. We find that the spin of the incident light can control the rotation direction of the canonical momentum. Our results represent an interesting example of spin-orbit interactions of light for complex evanescent fields. Investigation of optical forces and torques around nanoparticles and nanostructures is interesting for experimental studies and applications, since the huge light concentration around metal nanoparticles can give rise to very strong optical forces and torques, even with moderate illumination. The present study can be extended to more complex nanostructures, considering for example metal nano-dimers were very high field-amplification effects in the dimer gap can be obtained at specific wavelengths.