Nanoscale Structures Route Light

Hank Hogan

In a development with promise for the realization of nanoscale integrated photonic systems, investigators have routed and sensed light using nanowires and nanoribbons. A team from the University of California, Berkeley, Lawrence Berkeley National Laboratory in Berkeley and NASA's Ames Research Center at Moffett Field, all in California, has demonstrated photon manipulation by structures smaller than a wavelength of light, creating single optical pulses such as might be used in communications and computation, and in manufacturing a simple optical network.

"The current work represents the first step towards integrating nanowire-based photonic elements," said Peidong Yang, a chemistry professor at the university and a member of the research team.

The scientists first synthesized SnO₂ nanoribbons and ZnO and GaN nanowires with diameters of 100 to 400 nm, shorter than the wavelength of visible light. They manipulated them to create millimeter-long waveguides; for example, by laying a GaN nanowire atop a SnO₂ nanoribbon. They focused 325-nm radiation from a Melles Griot HeCd laser onto the nanowire, exciting it. Tests showed that the waveguide carried the light without much loss. By pumping the GaN above its lasing threshold, they generated single optical pulses in the waveguide.

Because the propagation loss depends on a nanowire's cross section, each waveguide serves as a short-pass filter. Each wire can have a specific cutoff so that only light below that wavelength passes through it. This means that the waveguides can separate colors and excite narrowband fluorophores.

The researchers assembled four nanoribbons into a rectangular grid and demonstrated the routing of light with a high degree of directionality, which was expected. The waveguides were immersed in water, where they continued to function because their refractive index is around 2, much higher than that of water. Combined with the ability to interact with fluorophores, this should enable chemical and biological sensing.

"We can easily collect emission and absorption spectra from a pico- and even femtoliter liquid droplet," Yang explained.

The scientists are working to integrate nanoribbons into microfluidic devices. They also hope to build fully functional systems for chemical and biological sensing, signal processing and communication. One problem is that the current integration process is serial in nature and, thus, slow and inefficient. They are investigating the development of parallel -- and therefore more efficient -- integration methods, Yang said.