By combining plasmonics and optical microresonators, researchers at the University of Illinois at Urbana-Champaign (Urbana, IL) and Iowa State University (Ames, IA) have created a new optical amplifier design that operates in the visible (563–675 nm) and can potentially route narrowband optical power on a chip.1 A gain medium tethered to a Raman-line-producing whispering-gallery-mode (WGM) resonator, a protein, and a plasmonic surface, the device can amplify one or a few Raman lines produced by the resonator.
The researchers, led by J. Gary Eden, a professor of electrical and computer engineering (ECE) at Illinois and ECE associate professor Logan Liu, found that plasmonic nanostructures can serve as a bridge between photonics and nanoelectronics, to combine the size of nanoelectronics and the speed of dielectric photonics.
At the heart of the amplifier is a microsphere made of polystyrene or glass that is approximately 10 μm in diameter. When activated by an intense beam of light, the WGM resonator formed by the sphere internally generates a narrowband Raman line. Molecules tethered to the surface of the sphere by a protein amplify the Raman signal, and in concert with the nanostructured plasmonic surface in contact to the sphere, the amplifier produces visible (red or green) light having a bandwidth that matches the internally generated signal.
Conventional comparably-sized optical oscillators and amplifiers have typically produced light based on the buildup of a field from the spontaneous-emission background; however, this limits the temporal coherence of the output, lengthens the time required for the optical field to grow from the noise, and often is responsible for complex, multiline spectra.
"In our design, we use Raman assisted injection-seeded locking to overcome the above problems," says Manas Ranjan Gartia, lead author of the article. "In addition to the spectral control afforded by injection-locking, the effective Q of the amplifier can be specified by the bandwidth of the injected Raman signal." This characteristic contrasts with previous WGM-based lasers and amplifiers for which the Q is determined solely by the WGM resonator.
Potential medical uses
"Their potential applications in medicine are exciting because the amplifiers are pumped by light that is able to pass through human skin," says Eden. "For this reason, these microsphere-based amplifiers are able to transmit signals from cells and buried biomedical sensors to electrical and optical networks outside the body."
1. Manas Ranjan Gartia et al., Scientific Reports (2014); doi: 10.1038/srep06168