Angular-momentum-controlled multipolar scattering for geometry-sensitive spectral tuning.

Structured electromagnetic beams provide a unique degree of freedom for controlling light-matter interactions through their phase, polarization, and angular momentum. The influence of the total angular momentum projection, which combines the orbital and spin (helicity) contributions of the incident field, on multipolar scattering by nonspherical dielectric particles remains largely unexplored. We systematically investigate the scattering response of four canonical dielectric particle geometries illuminated by right-circularly polarized beams carrying total angular momentum projections m = 1 and m = 2 in the microwave regime. To enable rigorous full-wave simulations of structured-wave scattering by arbitrarily shaped particles, we employ an antenna-based structured-field excitation scheme together with exact Cartesian multipole decomposition. We show that the m = 1 beam simultaneously excites dipolar and quadrupolar resonances, whereas the m = 2 beam couples predominantly to quadrupolar resonances, yielding a quadrupole-dominated scattering response with the quadrupole purity exceeding 99% at the magnetic-quadrupole resonances of the sphere, cylinder, and cube and remaining above 90% at all quadrupole-dominated resonances. This mode-selective excitation remains robust against variations in particle geometry, aspect ratio, and alignment imperfections. At the same time, the quadrupole resonance frequencies exhibit pronounced sensitivity to geometric parameters, enabling precise spectral tuning. These findings establish a symmetry-driven mechanism for controlling multipolar scattering with structured electromagnetic fields and demonstrate that the proposed full-wave approach provides a versatile computational framework for investigating structured-wave interactions with complex dielectric resonators. The results highlight the potential of angular-momentum-controlled beams for mode-selective excitation and geometry-sensitive spectral tuning.