Multipole excitation of localized plasmon resonance in asymmetrically coated core–shell nanoparticles using optical vortices

Plasmonic interactions between an asymmetrically coated core–shell (ACCS) nanoparticle and an optical vortex produce a novel engagement of the spin angular momentum (SAM) and the orbital angular momentum (OAM) of the input. Simulations based on a discrete dipole approximation (DDA) indicate that the SAM and the OAM of the incident beam determine the modal order of resonance, correctly identifying the peak wavelength, and both the direction and magnitude of optical torque exerted upon the excited, localized plasmon resonance in the ACCS particle. These simulations also indicate higher-order resonances, including hexapole and octupole modes, and a zero-order resonance (expressible as a monopole mode), can be excited by judicious selection of the SAM and OAM. A detailed symmetry analysis shows how the multipoles associated with eigenmode excitations connect to the radiation multipoles at the heart of the multipole expansion. It is also shown how additional, distorted resonance modes due to the asymmetricity of the structure are also exhibited. These specific plasmonic characteristics, which cannot be realized by plane wave excitation, become possible through the ACCS asymmetry engaging with the distinct optical vortex nature of the excitation.