Position-dependent plasmonic chirality of particles in tightly focused light field

Plasmonic chirality garners considerable attention owing to its distinctive optical characteristics and wide-ranging applications. Understanding the optical activity of single particles holds paramount significance for both theoretical frameworks and experimental measurements of chirality. When probing the optical response of chiral particles, high numerical aperture (NA) objectives, adept at concentrating light into exceedingly small regions, are frequently employed. With escalating NA, the incident light no longer satisfies the paraxial approximation, engendering electric field components perpendicular to the focal plane. However, this electric field component is usually neglected in research. Here, we utilize angular spectrum theory to explore the optical response and circular dichroism of single chiral nanoparticles subjected to tightly focused circularly polarized light. Remarkably, we find that the electric field component perpendicular to the focal plane exhibits a pronounced influence on chirality, potentially inducing a reversed circular dichroism response when the particle deviates from the optical axis. Simultaneously, variations in the position of particles within the light field yield significant discrepancies in higher-order modes, particularly electric quadrupole moments. Our findings hold profound implications for the measurement and analysis of chiral particles, offering invaluable guidance for the future investigations of chirality.