Selective Enhancement of Optical Chirality and Spin Angular Momentum in Plasmonic Near-Field

Abstract: The interaction between circularly polarized (CP) light and matter is governed by two fundamental quantities: spin angular momentum (SAM) and optical chirality (OC). While these quantities are inseparable in free space, they can be selectively enhanced in plasmonic near-field regions through appropriately designed structures. We demonstrate that the excitation of circular plasmonic nanostructures with CP light enables selective or simultaneous enhancement of SAM and OC through the excitation of rotating plasmon modes. Electromagnetic field analysis reveals that SAM enhancement originates from transverse SAM induced by unidirectional evanescent waves, whereas OC enhancement is governed by the interference between the plasmonic electric field and incident magnetic field. The finite element method simulations confirm that circular dichroism signals arising from these enhanced near fields clearly depend on the SAM and OC of the local fields, underscoring the importance of structural design in the detection and enhancement of optically active phenomena at the nanoscale.