Figure 1. Comparison of the relative phase shift between the two resonances in a bimetallic Ag–Au dimer and its monometallic counterparts.

Full lines show relative phase shift, while dashed lines show scattering cross-sections versus wavelength. Inset in (a) shows the model system, consisting of a pair of dipoles oriented parallel to each other and separated by a distance L=120 nm. The particles were modelled using the coupled dipole approximation34 and size-corrected dipole polarizabilities for oblate spheroids (a=b=50(65) nm and c=10 nm) using tabulated metal permittivities41. In the monometallic cases, shown in panels (a,b), the phase drops only at wavelengths in between the plasmon resonances of the dipoles, whereas in the bimetallic dimer, (c), the phase first drops and then changes sign due to a mismatch in the permittivity between the two particles. Red and blue arrows indicate the orientation of the dipole moments induced on the particles. The green arrow indicates inversion of the dipole due to interband transitions. Circular insets in (a–c) show radiation patterns (angular resolved scattering diagrams) calculated at the antisymmetric mode frequencies (labelled inside each of the circles) and assuming particle pairs supported on an air–glass interface. The contrast varies from zero intensity dark regions to maximum intensity bright regions. Radial coordinate of the radiation pattern scales as numerical aperture NA=nsinθ and tangential as angle ϕ, with angles θ and ϕ having standard spherical coordinate meaning.