Optical skyrmions and tunable fine spin structures in deep-subwavelength scale at metal/graded index material interfaces

A skyrmion is a topological quasiparticle that has been studied widely in nuclear physics, condensed matter physics, cosmology, and optics. Previously, the optical skyrmions in the surface plasmon polaritons platform were not tunable because the dielectric properties of the material were fixed. In the study, we introduce the graded refractive index materials into the near-field optical system and systematically investigate the propagation properties, dispersion relations, and spin-orbit decomposition of the surface waves at the metal/graded refractive index materials interface. Our theoretical results exhibit that the topological spin skyrmions can be formed in the system and the dimensions of optical skyrmions can be tuned by varying the central permittivity and exponent of the graded refractive index materials. Additionally, the spin fine structure, in which the spin state varies sharply from the ‘up’ state to the ‘down’ state, can be also controlled by adjusting the materials properties of the graded refractive index materials. The minimal full width of the spin fine structure is 0.254λ, which has the potential for achieving the displacement metrology with a sensitivity of 2.54 × 10 −7 λ theoretically. Our findings provide an extra degree of freedom to control the formation and scale of fine spin structures in optical skyrmions and open an avenue for next-generation pico-photonics.