Noble metal nanoparticles support surface plasmon resonances, collective oscillations of surface electrons that provide means to manipulate light at the nanoscale. Plasmonic nanomaterials are promising components in emerging technologies for improved photovoltaic1
devices and they have long been known for their ability to generate strong optical near-fields4
, enabling a family of surface-enhanced molecular spectroscopies that include surface-enhanced Raman scattering5
and enhanced fluorescence6
. Nanoplasmonic structures have also been used to control the polarization and propagation direction of light emitted by individual quantum sources7
, to generate strong optical non-linearities11
and as a basis for negative-index meta-materials for optical frequencies12
. The latter application, as well as directional optical antennas9
and Fano resonant structures19
, typically requires coherent oscillations of several dipolar plasmonic oscillators with specific phase retardations. For example, artificial magnetic dipoles used to induce negative refraction can be based on antisymmetric plasmon modes in dimers, that is, two coupled dipoles oscillating with an internal phase shift of 180°13
, whereas directional emission from nanostructures typically use spatial phase retardation between nearby dipolar antenna elements. However, in all the cases, the phase optimization process involves both spectral tuning of the plasmon resonance and spatial tuning of the device geometry, which requires very precise nanofabrication and puts constraints on possible outcomes.
In this work we explore a novel degree of freedom for manipulating the phase by introducing an asymmetric material composition24
. As a proof of principle, we experimentally realize a planar nano-optical antenna in the form of a bimetallic particle dimer and show that it exhibits prominent colour routing properties; that is, it is able to sort light of different colours into different scattering directions. The additive phase accumulation provided through the bimetallic composition makes it possible to achieve this result for an antenna that is deep subwavelength in size, something that is difficult or impossible in previously reported colour routing schemes, including those based on plasmonic particle on a film systems27
metal grating structures29
or plasmonic holography32
. Moreover, we show that dense assemblies of bimetallic colour routers can be fabricated over large areas using cheap colloidal lithography. This may facilitate applications in, for example, optical sensor development, spectroscopy and photonics.