Optical Torque Modulation of Cs₂AgBiBr₆ Perovskite-Coated Gold Nanospheres by Vector Bessel Beams

Based on generalized Lorenz-Mie theory (GLMT) and the Maxwell stress tensor (MST) method, this study investigates the modulation mechanism of the axial optical torque N_z exerted on Cs₂AgBiBr₆ (CABB) perovskite-coated gold nanospheres under vector Bessel-beam illumination. The results show that the CABB shell reconstructs the torque-resonance channels of the coated particle by modifying both the dispersive dielectric environment around the gold core and the core-shell interfacial response. As the shell thickness increases, the dominant response undergoes a continuous redshift. The polarization state, half-cone angle α₀, and order l of the incident vector Bessel beam serve as external optical-field degrees of freedom that regulate the incident angular-momentum channels, thereby enabling coordinated control over the torque peak magnitude, spectral line shape, and torque direction. Analyses of the near-field distributions, Poynting-vector distributions, and Mie-order decomposition reveal that the strong torque response arises from selective coupling between the intrinsic Mie channels of the core-shell particle and the vectorial structure of the incident light, rather than simply from local field-intensity enhancement. This study provides a theoretical basis for tunable N_z responses in perovskite-plasmonic hybrid nanostructures and for structured-light-driven rotational manipulation at the nanoscale.