Topological properties of localized electric fields in plasmonic nanocavities

We have developed “optical nanoshaping” technique that can realize focusing optical fields into nanocavity beyond the diffraction limit and freely controlling the amplitude, phase, and polarization distribution of the nano-localized fields. We designed the tailored plasmonic nanocavities composed of metal multimer surrounding a nano-sized gap. The plasmonic nanocavities make it possible to confine the optical vortex fields into the gap spaces with conserving the high-order orbital and spin angular momenta. Here, we present the unique topological properties of the nano-localized plasmonic fields formed by metal multimer nanocavities. We show that the nano-multimer irradiated with circularly polarized beam generates the nano-vortex field with the half-integer total angular momenta corresponding to monopole, tripole, tetrapole, etc. which are categorized to Möbius strip states. We applied this topological plasmonic fields to chiral crystallization from achiral compounds and succeeded in achieving giant crystal enantiomeric excess of sodium chlorate and ethylenediamine sulfate. We also demonstrate that the transverse spin loop can be generated around the nanogap, and the energy flow simultaneously appears due to the spin-momentum locking effect, which results in forming the electric-field-spin skyrmion textures above and below the nanocavities. This nanoscale photonic spin skyrmion field is applicable to generation and control of magnetic skyrmion in ferromagnetic, ferrimagnetic, and antiferromagnetic materials.