Near-field spin structures in nanoscale scattering systems have attracted considerable attention because they are closely related to the local polarization state, energy-flow topology, and spin-orbit interaction of structured optical fields. In chiral scattering systems, electromagnetic cross-coupling modifies the relative phases and coupling among different field components and affects the spatial distribution of spin angular momentum in the particle near field. In this work, the modulation of near-field transverse spin angular momentum by the chiral parameter is investigated for a chiral silicon sphere with a radius of 131 nm and a refractive index of 3.55. The incident wavelength is set to 1081 nm, at which the electric and magnetic dipole scattering coefficients satisfy the first Kerker condition, i.e. a₁=b₁. Based on the chiral-particle scattering model and the generalized Lorenz-Mie theory, analytical expressions for the spin angular momentum density and its spherical components are derived. Combined with numerical simulations, the effects of the chiral parameter on the near-field transverse spin distribution and its physical mechanism are analyzed.
The results show that a pronounced transverse spin angular momentum exists in the near field of an achiral particle under circularly polarized incidence, and its azimuthal component remains unchanged when the incident helicity is reversed. By introducing chirality, the transverse spin does not exhibit enhancement or suppression but shows distinct distribution features for opposite signs of the chiral parameter. These differences are mainly reflected in the asymmetric variations of the transverse-spin intensity, spatial extent, and local deflection structure around the particle surface. Moreover, the transverse spin remains unchanged when the signs of the incident helicity and the chiral parameter are simultaneously reversed, indicating that the near-field transverse spin in a chiral scattering system is determined by the polarization handedness and the sign of the chiral parameter.
To clarify the underlying mechanism, the spatial distribution of the circular polarization degree and the interference of the electric-dipole and magnetic-dipole responses with the chirality parameter, and the three-dimensional distribution of the total spin angular momentum density are further analyzed. The regions with significant transverse-spin variation are found to correspond closely to the regions where the circular polarization degree is strongly modified, which shows that the chirality parameter affects the transverse spin through changes in the local polarization structure and the associated spin-orbit interaction in the near field. Meanwhile, the interference between electric and magnetic dipole responses evolves asymmetrically with the chirality parameter, which further explains the different modulation behaviors for opposite signs of the chiral parameter. This effect is also reflected in the three-dimensional redistribution of the total spin angular momentum density around the particle. Our results demonstrate that the chirality parameter can reshape not only the distribution of the near-field transverse spin, but also the angular-momentum structure in the scattering near field and may be useful for localized chiral-response detection and near-field optical manipulation in chiral nanostructures.