High-numerical-aperture focusing of radially polarized doughnut beams with a parabolic mirror and a flat diffractive lens

TLDR

Radially polarized doughnut beams enable very small focal spots (~0.26 λ²) with high‑NA aplanatic microscope objectives. The study proposes two simple alternatives for focusing such beams: a parabolic mirror and a flat diffractive lens. These alternatives employ a parabolic mirror and a flat diffractive lens to achieve high‑NA focusing. The method reduces spot area by up to 1.76× at NA = 1 compared with the aplanatic system. Citation: Dorn et al., Phys.

Abstract

Recently, Dorn [Phys. Rev. Lett.91, 233901 (2003)] demonstrated the significance of radially polarized doughnut beams in obtaining very small focal spots (with an area of ∼0.26λ2) with high-numerical-aperture (NA) aplanatic microscope objectives. We propose two simple alternative ways to focus such radially polarized beams: a parabolic mirror and a flat diffractive lens. Because of their large apodization factor for a high NA, a significant further reduction in spot area (up to a factor of 1.76 at a NA of 1) compared with the aplanatic system can be achieved.