No abstract

The interference between conventional multipoles (e.g., electric and magnetic dipole, electric quadrupole, etc.) is known as the cause of unidirectional backward and forward scattering of nanoparticles. However, an unconventional multipole moment, toroidal dipole moment is generally overlooked in the unidirectional scattering. In this work, we systematically investigate the unidirectional scattering in the system of plasmonic nanoparticles. It is found that the toroidal dipole moment can play a significant role in the unidirectional backward scattering. The structural tunability of the unidirectional scattering is also demonstrated. Our results can find applications in the design of nanoantennas.

The scattering properties of a dielectric cylinder wrapped by a monolayer of graphene under an external optical pump are investigated. The THz cloaking is realized due to the strong suppression of scattering at certain range of frequencies. While at some other frequencies, the scattering can be dramatically enhanced, and a THz lasing can be achieved due to the enormous amplification of the local field by the graphene plasmon. Both the work frequencies of THz lasing and cloaking can be tuned continuously by the quasi-Fermi energy of graphene. Our proposal can pave the way to alloptical manipulation of THz lasing and cloaking based on the optically pumped graphene.

Complementary metal structures manifest a remarkable scattering feature, known as Babinet's principle. Meanwhile, for surface modes confined to one or two dimensions, the relation between the modes on complementary structures has not been thoroughly studied. Here, spoof surface plasmon polaritons supported on complementary metal films are systematically investigated. The duality of electromagnetism guarantees that these surface modes on complementary metal films possess precisely the same dispersion regardless of the geometry of the grooves, which is confirmed by both numerical simulations and experimental measurements. This work may open another avenue for spoof surface plasmon polaritons in both the theoretical and practical aspects.

Plasmonic nanoparticle clusters are widely considered experimentally and numerically. In the clusters consisting of one central particle and N satellite particles, not only the magnetic modes but also the toroidal modes can exist. Here, the eigenmodes of such clusters and the corresponding excitation efficiency under the illumination of a plane wave are studied analytically by using the eigen-decomposition method. The angular dependence of the optical response of these clusters is clearly demonstrated. The behavior of excitation efficiency is dependent on both the value and the parity of N, the number of satellite particles. Our results may provide a guide for the selective excitation of plasmonic modes in the plasmonic nanoparticle clusters.