Multipole decomposition for interactions between structured optical fields and meta-atoms.

Interactions between structured optical fields (SOFs) and meta-atoms have been intensively studied, and stimulated by recent advancements on the generation of SOFs and on the synthesis of exotic meta-atoms. Multipole expansion is an efficient and accurate theoretical framework for studying such problems. In this work, explicit expressions of SOFs and their beam-shape coefficients are provided, and their properties are also briefly discussed; the considered SOFs include Laguerre-Gaussian (LG) beams, tightly-focused LG beams, Bessel beams, and cylindrical vector beams. Using the multipole expansion, selective excitations of multipolar resonances of a sphere is discussed. In addition, angular momentum dichroisms of a chiral sphere and an anisotropically chiral meta-atom are calculated to demonstrate selective excitation of multipoles with the desired order, parity, and orientation using engineered SOFs with angular momentum.