Broadband spin-controlled focusing via logarithmic-spiral nanoslits of varying width

Abstract

This work presents analytical, numerical and experimental demonstrations of light diffracted through a logarithmic spiral (LS) nanoslit, which forms a type of switchable and focus-tunable structure. Owing to a strong dependence on the incident photon spin, the proposed LS-nanoslit converges incoming light of opposite handedness (to that of the LS-nanoslit) into a confined subwavelength spot, while it shapes light with similar chirality into a donut-like intensity profile. Benefitting from the varying width of the LS-nanoslit, different incident wavelengths interfere constructively at different positions, i.e., the focal length shifts from 7.5 μm (at λ = 632.8 nm) to 10 μm (at λ = 488 nm), which opens up new opportunities for tuning and spatially separating broadband light at the micrometer scale. A logarithmic spiral (LS) nanoslit with switchable focusing functionalities has been experimentally demonstrated as a promising candidate to identify the handedness of incident circular polarization. Strongly depending on spin of incident photons, this LS-nanoslit converges the incoming light with opposite handedness (with respect to the handedness of the LS-slit) into a confined subwavelength spot, while the light with identical chirality is structured into a donut-shape profile at the focal plane.