Sharp bending and power distribution of a focused radially polarized beam by using silicon nanoparticle dimers

Control and manipulation of radiation direction and directivity is highly desirable in future integrated optical circuits. Here, we investigate theoretically and numerically the scattering properties of a silicon nanosphere dimer illuminated by a focused radially polarized beam. As compared with single silicon nanospheres, a scattering peak with a significantly enhanced intensity and a dramatically reduced linewidth was observed in the scattering spectrum of the silicon nanosphere dimer. Relying on the multipole expansion method, it was revealed that the radiation at the scattering peak originates mainly from the total electric dipole and the magnetic quadrupole excited in the silicon nanosphere dimer. It was found that the radiation direction of the silicon dimer is parallel to its axis, implying a sharp (90°) bending of the radially polarized beam. In addition, the radiation directivity is significantly improved as compared with single silicon nanospheres because of the interference between the total electric dipole and magnetic quadrupole modes. For a homodimer composed of two identical silicon nanospheres, the scattering light is equally distributed in the two radiation directions. In comparison, the incident light is preferentially scattered to the small Si nanosphere for a heterodimer composed of two silicon nanospheres with different diameters. As a result, a unidirectional lateral scattering can be realized by using a single silicon nanosphere displaced appropriately from the focal point. Our findings are helpful for understanding the mode hybridization in silicon nanosphere dimers illuminated by a focused radially polarized beam and useful for designing photonic devices capable of manipulating the radiation direction and directivity of structured light.