Simple nanoantenna separates colors of light

Ashley N. Paddock, ashley.paddock@photonics.com

A new nanoantenna directs red and blue colors in opposite directions and could lead to optical nanosensors that can detect very low concentrations of gases or biomolecules.

Historically, a structure that is smaller than the wavelength of visible light (390 to 770 nm) should not be able to scatter light, but that is precisely what the new nanoantenna from Chalmers University does. Researchers there found that the trick is to build an antenna with an asymmetric material composition, creating optical phase shifts.

The antenna comprises two nanoparticles — one silver, one gold — situated about 20 nm apart on a glass surface. The researchers showed that the antenna scattered visible light so that red and blue colors traveled in opposite directions, creating an optical phase shift. The phase shift arose because nanoparticles of gold and silver have different optical properties — in particular, different plasmon resonances, said Chalmers researcher Timur Shegai. The plasmonic resonance difference means that the free electrons of the nanoparticles can oscillate strongly in pace with the light frequency, which then affects the light propagation even though the antenna is small, Shegai explained.

Gold and silver nanoparticles have differing optical properties, allowing a new nanoantenna to act as a router for red and blue light. Image courtesy of Timur Shegai, Chalmers University.

Controlling light by using asymmetric material composition — such as silver and gold, or copper and aluminum — is a completely new technique. The researchers have shown that the antennas can be fabricated densely over large areas using cheap colloidal lithography. Their findings appeared in the Sept. 20 issue of Nature Communications (doi: 10.1038/ncomms1490).

The rapidly growing research field of nanoplasmonics, which concerns controlling how visible light behaves at the nanoscale using a variety of metal nanostructures, can be applied in many areas, said Mikael Käll, a professor in the research group at Chalmers. One example is optical sensors, where plasmons could be used to build sensors that are sensitive enough to detect much lower concentrations of toxins or signaling substances than currently is possible.

"Another potential application could be directing emission of single quantum sources such as quantum dots, dye molecules or nitrogen vacancies in diamonds coupled to the nanoantenna," Shegai said. "The concept is universal, and so potential applications in sensing are numerous."