Nanoscale Structures Act as Photonic Antennas

Daniel S. Burgess

Researchers at the University of Massachusetts Amherst and at Oak Ridge National Laboratory in Tennessee have observed far-field modulated fluorescence lifetimes indicative of dipole-dipole coupling in semiconductor polymer nanostructures. The phenomenon has potential applications in the development of components for scalable photonic information systems.

Michael D. Barnes, an associate professor of chemistry at the university, said that the investigators dubbed the structures "nantennae" because of their scale -- on the order of 10 nm -- and because they interact over distances of microns in a manner similar to that of transmitter/receiver pairs in conventional phased antenna arrays.

The scientists fabricated the nantennae from solution using ink-jet printing to deposit single molecules of cyano-substituted polyphenylenevinylene on a glass substrate, all standing nearly straight up from the surface. Previous work had indicated that the emission of individual structures takes the form of a classical dipole pattern.

To investigate the interaction of the nantennae, they excited one element of a pair using 100-ps pulses of 514-nm light from an argon-ion laser and measured its fluorescence decay rate using a photon-counting avalanche photodiode and time-to-digital converter. Studying more than 200 pairs separated by 200 nm to 2.5 µm, they observed a clear dependence of the decay rate on the presence of a second structure, reproducible to within ±5 percent. The effect, they noted, is the result of a modification in the vacuum field at the position of one dipole near another, coherently radiating dipole.

Potentially, the nantennae may be constructed so that they radiate light together as a phased array, and the researchers are investigating means of fabricating well-defined patterned arrays so that they may study complicated, collective interactions.

"Once we are able to place the particles exactly where we want them, we can begin to study the effects of number, symmetry, particle spacing, etc., to tune the optical properties of this 2-D system," Barnes explained.

He said that the group also is collaborating with synthetic chemists at the university to create alternative polymer architectures, noting that such "smart design" work might enable further control of the nantennae's luminescent properties.