Fano Resonance in Asymmetric Plasmonic Nanostructure: Separation of Sub-10 nm Enantiomers

Separating enantiomers is vital in chemical syntheses, life sciences, and physics. However, the usual chemical processes are inefficient. Recently, plasmonic nanostructures have drawn considerable attention for manipulating nanoparticles; however, only a few approaches are proposed to discriminate between entities that differ in terms of their handedness. This is because the chiral polarizability is much smaller than the electric polarizability, and therefore the non-chiral gradient force dominates over the chiral gradient force. This limit means that the enantioselective sorting of chiral nanoparticles is a formidable challenge. A plasmonic nanostructure consisting of a disc-double split ring resonator exhibiting a dipole–octupole (DO) Fano resonance (FR) is designed and fabricated. It is theoretically demonstrated that such a DO-FR can markedly enhance the chiral gradient force on the paired enantiomers. The coaxial channel of the resonator possessing high chirality density gradients around the DO-FR is derived. This provides an enhanced chiral gradient force that dominates over the non-chiral gradient forces on sub-10 nm chiral nanoparticles. Enantiomeric pairs can thus experience distinct potential wells in terms of signs. This proposed structure may advance the techniques of enantiopurification and enantioseparation, bringing a new perspective to state-of-the-art all-optical enantiopure synthesis.