Plasmonically Enhanced Chiral Optical Fields and Forces in Achiral Split Ring Resonators

The two enantiomers (mirror images) of a biomolecule can show drastically different behaviors, requiring the development of sensitive approaches for their identification and separation. Plasmonic nanostructures have shown promise for enhancing the sensitivities of chiral spectroscopies, but the generation of chiral near fields with a specific handedness in the spatial domain surrounding the plasmonic structures remains a challenge. Here we demonstrate that achiral bianisotropic structures, which couple the electric and magnetic fields, can achieve high enhancements of optical chirality in an extended spatial region. Magneto-electric coupling in such structures facilitates electrically excited magnetic resonances in the near IR and optical regimes, which in turn can result in highly enhanced optical chirality in an extended region of space. We apply this concept to achiral double split ring resonators (DSRRs) and demonstrate their potential in generating enhanced chiral fields and forces. Also, the behavior of optical chirality density gradient and chirality flux in such structures is examined, and it is shown that plasmonically generated chiral forces may pave the way to a new class of chiral biosensors.