Nanoparticles with diameters of hundreds of nanometres can scatter visible light in preferential directions. This occurs because the size of the nanoparticle is of the same order of magnitude as the wavelength of the light and the electric field can interact with an electric dipole that forms inside the nanoparticle to produce an antenna effect. This effect is usually small, but 30 years ago it was theoretically predicted that in certain cases the backward direction of the scattered light could be suppressed and all the light redistributed forward. The scatterer could then act as a nanoscale light source.
Two independent research teams have now experimentally demonstrated directional scattering of visible light from single nanoparticles. The key to the success of both experiments lies in the choice of a high-refractive-index material for the nanoparticle. In such materials the electric and magnetic dipoles are simultaneously excited and their mutual interference greatly reduces backward scattering.
Yuan Hsing Fu, Arseniy Kuznetsov and colleagues at the A*Star Institute in Singapore and the Australian National University in Canberra used silicon nanoparticles to show that a forward-to-backward scattering ratio of more than six can be achieved, and that by changing the particle size and shape the effect can embrace a large part of the visible spectrum. Alternatively, Lukas Novotny and colleagues at the University of Rochester, ETH Zürich and the Universidad Autónoma de Madrid used GaAs nanoparticles to show that for a specific wavelength in the red region of the visible spectrum the backward component can be completely suppressed.