Singular laser beams nanofocusing with dielectric nanostructures: theoretical investigation

Abstract

We show that near-field nanoscale focusing is feasible using not only metallic but also dielectric sharp-edged structures. Based on vector Rayleigh–Sommerfeld integrals, we prove that the effect of an enhancement of the longitudinal electromagnetic field component occurs in the vicinity of the incident field phase discontinuities due to abrupt jumps in the optical element’s microrelief. Using a finite-element method, we simulate diffraction of the electromagnetic field on sharp edges of metallic and high-contrast dielectric structures. The resulting focal spot size is shown to be near directly proportional to the structure tip’s curvature radius. We propose a focusing arrangement that contains a radiation collector in the form of a conic axicon and a nanofocuser in the form of a nanosphere on the axicon’s apex. Using the finite-element method, we demonstrate that the focal spot size is approximately linearly proportional to the nanosphere radius. Thus, the proposed setup enables light to be confined to a thin focal spot with the size λ/373 when the silicon nanosphere radius is 2 nm.