Powell, David Anthony2020-04-212020-04-212331-7019http://hdl.handle.net/1885/203337The modes of silicon meta-atoms are investigated, motivated by their use as building blocks of Huygens metasurfaces. A model based on these modes is presented, giving a clear physical explanation of all features in the extinction spectrum. Counterintuitively, this model can show negative contributions to extinction, which are shown to arise from the interference between nonorthogonal modes. The direct and interference contributions to extinction are determined, showing that conservation of energy is preserved. The Huygens condition of matched electric-and magnetic-dipole moments leads to strong forward scattering and suppressed backscattering. It is shown that higher-order modes with appropriate symmetry generalize this condition, leading to multiple bands of directional scattering. The presented results are obtained using a robust approach to find the modes of nanophotonic scatterers, commonly referred to as quasinormal modes. By utilizing an integral formulation of Maxwell ' s equations, this work avoids the problem of normalizing diverging far fields, which other approaches require. The model and presented results are implemented in open-source code.application/pdfen-AU© 2017 American Physical Society201710.1103/PhysRevApplied.7.0340062019-11-25