A structure that is smaller than the wavelength of visible light (390-770 nanometers) should not really be able to split white light in colors. But that is exactly what the new nanoantenna does. The trick employed by the Chalmers researchers is to build an antenna with an asymmetric material composition, thus automatically generating optical phase shifts between the antenna components.
Employing asymmetric material composition, such as silver and gold, to direct and control light on a nanoscale is completely new. The researchers have shown that the antennas can be fabricated densely over large areas using cheap colloidal lithography – showing its commercial potential.
The antenna consists of two nanodisks about 20 nanometers apart on a glass surface, one of silver and one of gold. Experiments evidently show that the antenna scatters visible light so that red and blue colors are directed in opposite directions.
This could open up a variety of new applications such as – highly sensitive optical sensors or efficient single-photon sources. Chemical species absorbing on either of the antenna elements could modulate how the nanoantenna directs light and thus allow for tracking of these entities. For example, biomolecules or molecules of various gases could be detected in this way. Moreover, single-photon sources, such as quantum dots or dye molecules, could be coupled to and directed by the nanoantenna. This is a more classical antenna-type application – in the sense that it is similar to radio-frequency high-directivity TV antennas of the so-called Yagi-Uda type. These directional antennas could work both in transmission and reception modes while being subwavelength in size.
In addition, the material asymmetry approach implemented in this work is expected to be universal with respect to light directing and so should work not just for gold and silver but essentially for any pair of nanoparticles that supports plasmon resonances. Scientists now have a whole new parameter – asymmetric material composition – to explore in order to control the light on a nanoscale.