Enantiomer separation is a critical step in many chemical syntheses, particularly for pharmaceuticals, but prevailing chemical methods remain inefficient. Here, we introduce an optical technique to sort chiral specimens using coaxial plasmonic apertures. These apertures are composed of a deeply subwavelength dielectric channel embedded in silver (or gold) and can stably trap sub-20-nm dielectric specimens. Using both full-field simulations and analytic calculations, we first show that selective trapping of enantiomers can be achieved with circularly polarized illumination and proper index-matching of the immersed liquid with the particles being trapped. Opposite enantiomers experience distinct trapping forces in both sign and magnitude: one is trapped in a deep potential well while the other is repelled with a potential barrier. These potentials maintain opposite signs across a range of chiral polarizabilities and enantiomer-aperture separations. We also demonstrate how atomic force microscopy can be used to directly probe the near field optical forces from our coaxial nano-aperture. Our measurement reveals the spatial distribution of the optical near-field forces on a nanometer-sized dielectric specimen. To directly visualize the enantio-selective optical forces, we pattern silicon AFM-probes with chiral patterns. Our near-field force mapping indicates a differentiable force in the piconewton range on the chiral probes, exerted by our coaxial aperture with circularly polarized illumination. Our theoretical and experimental demonstrations indicate that the interaction of chiral light and chiral specimens can be mediated by achiral plasmonic apertures, providing a possible route toward all-optical enantiopure syntheses.
Recent ideas involving plasmonic metamaterials have been put forward to enhance the overall bandwidth of operation of
quarter-wave plates for circular polarization detection. The proposed metamaterial geometries are inherently complex to
realize and difficult to scale beyond the near-infrared frequencies. Here, we show how proper stacks of lithographically
printed plasmonic metasurfaces with simple patterns may provide large extinction ratios for the detection of circular
polarization, combined with broadband operation and simplicity of design and realization. In this paper, we will discuss
some physical insights into the modeling of these structures, fundamental advantages and some limitations of this