Plasmonics help ‘bulk’ silicon emit visible light

Ashley N. Rice, ashley.rice@photonics.com

A commonly used form of silicon has been made to emit broad-spectrum, visible light, but challenges remain before it can form practical electronic-photonic devices.

As engineers move ever closer to the limits of electronic circuits, light is seen as a more effective way to transmit information. The poor light emission of silicon, the base material for integrated circuits, is a major obstacle to integrating electronics and photonics on a chip.

To get bulk silicon to emit broadband light, researchers at the University of Pennsylvania wrapped silicon nanowires with a thin layer of silicon dioxide and mounted it to a glass substrate. They then added a layer of silver film, creating an omega-shaped plasmonic nanocavity.

“This research is a first step in that direction,” Ritesh Agarwal, a University of Pennsylvania associate professor, told Photonics Spectra. The work was detailed in Nature Photonics (doi: 10.1038/nphoton.2013.25).

His group used silicon nanowires coated with glass and silver to act as a plasmonic nanocavity. One of the key issues of using such highly confined optical cavities is extracting the light, which is generated in the material, Agarwal said.

“We estimate that the outcoupling efficiency of our hybrid-plasmonic silicon nanowire is on the order of one percent, which means a majority of the light remains trapped in the cavity and is eventually dissipated as heat,” he said.

To meet the challenge, Agarwal and colleagues are exploring “open” architectures such as using metal nanostructures on planar-silicon to achieve higher outcoupling efficiencies.

Ritesh Agarwal

“We are examining luminescence from larger Si nanowires (twice the thickness of those studied in Nature Photonics), which can support many more modes due to their large size and potentially more favorable values of quality factor and mode volume,” he said. “This translates to higher efficiencies.”

Better cavity designs also could minimize nonradiative losses.

“We would like to produce similar results using an electrically pumped device and, eventually, a silicon-based laser,” he said.

Previously, the investigators had conducted plasmonic cavity research, in which they wrapped a cadmium sulfide nanowire in a layer of silicon dioxide and then in a layer of silver (Photonics Spectra, November 2012, p. 22).

“Next, we need to excite silicon electrically using an on-chip power source,” Agarwal said. “The light emitted by silicon would need to be directed via optical waveguides toward optical modulators, switches, etcetera. It’s a challenging task, but not necessarily impossible, and will require a bit of a collaborative effort with other research groups.”