Tailored self-assembled nanocolloidal Huygens scatterers in the visible

International audience; Existing nanocolloidal optical resonators presenting strong magnetic resonances often suffer from multi-step low yield synthesis routes as well as a limited tunability, in particular in terms of spectral superposition of electric and magnetic resonances, which is the cornerstone for achieving Huygens scatterers. To overcome these drawbacks, we have synthesized clusters of gold nanoparticles using an emulsion-based formulation route. The fabrication technique involved the emulsification of an aqueous suspension of gold nanoparticles in an oil phase, followed by the controlled ripening of the emulsion. The structural control of the assynthesized clusters, of mean radius 120 nm and produced in large numbers, is demonstrated with microscopy and X-ray scattering techniques. Using a polarization-resolved multi-angle light scattering setup, we reach a comprehensive angular and spectroscopic determination of their optical resonant scattering in the visible wavelength range. We thus report on the clear experimental evidence of strong optical magnetic resonances and directional forward scattering patterns. The clusters behave as strong Huygens sources. Our findings crucially show that the electric and magnetic resonances, as well as the scattering patterns can be tuned by adjusting the inner cluster structure, playing with a simple parameter of the fabrication route. This experimental approach allows for the large scale production of nanoresonators with potential uses for Huygens metasurfaces.