Generation of a Nondiffracting Superchiral Optical Needle for Circular Dichroism Imaging of Sparse Subdiffraction Objects

Chirality describes not only the structural property of three-dimensional objects, but also an intrinsic feature of electromagnetic fields. Here we report a strategy to realize a Bessel beam superchiral "needle" by focusing a twisted radially polarized beam on a planar dielectric interface. By tailoring the light spatial distribution in the pupil plane of a high numerical aperture lens, the chirality of the local field at the focus can be enhanced by 11.9-fold than that of a circular polarized beam. Through a combined interaction of chiral and achiral transitions, the dimension of the region with enhanced chiral sensitivity can be shrunk down to λ/25. This theoretical work paves the way towards a completely new label-free imaging technique using the enhanced circular dichroism for sparse subdiffraction chiral objects (e.g., individual molecules).