Cavity-induced SAM to OAM conversion for sub-wavelength focused chiral field generation

We show how spin-to-orbital angular-momentum (SAM to OAM) conversion allows the generation of focused chiral fields and demonstrate numerically the utility of these fields for tweezing and separating chiral objects. The proposed setup consists of a micron-size Helmholtz hemisphere resonator fed at the pole by a circularly polarized Gaussian laser field in the visible range. The fields formed at the equator plane are shown to possess an intrinsic orbital angular momentum component with respect to the axis of the Helmholtz resonator. When brought in proximity to a metal plate, diffraction of the fields embodying OAM results in the formation of a sizable, nanoscale focused chirality density. This sub-wavelength chiral field formation is a result of an interplay between resonator cavity modes and diffraction. No plasmonic losses are involved, and the fields can be generated at tunable frequencies by varying the radius of the resonator. We demonstrate that the formed fields can move and separate radially chiral objects such as molecules on surfaces. The in-situ motion of the chiral molecules is illustrated using analytical and fully numerical simulations. The generated fields are tunable by the parameters of the input fields (such as frequency and polarization) as well as by the radius of the hemisphere. Our results demonstrate an effective tool for chiral tweezing and enantiomer separation on the nanoscale under a realistic setting and with moderate input laser intensity.