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Unique waveguide provides pinpoint control of light

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A tunnel-like device that channels light effectively into a point just a few nanometers across may lead to next-generation applications in computing, communications and imaging.

Beams of light are fantastic carriers of data. They can carry more information more efficiently than the traditional electricity-copper wire route used for much of the modern era. Fiber optics has led the way in this regard, transmitting everything from high-definition television and the Internet to energy.

For all of its strengths, light has a weakness when it comes to transmitting information: The smaller its beam gets (for carrying more data), the less physical control one has over it. An ideal optical system would be able to image light as a point. However, because of the diffraction limit, based on the wavelength of the light and the f-stop of the optical system’s aperture, a built-in limit exists as to how focused a light beam can be. The result is a large energy loss.

Dr. Hyuck Choo and his team at the California Institute of Technology believe they have found a way to get past this natural roadblock using a specially designed waveguide that translates the incident light into coupled electron oscillations – called surface plasmon polaritons (SPPs) – that can be focused past the diffraction limit.

The new waveguide design is a 3-D rectangular box that tapers to a point at one end. The device, made of amorphous silicon dioxide, is about 2 µm long and is coated with a thin layer of gold. This composition allows the incident light coming into the waveguide to interact with the electrons at the interface between the silicon dioxide and gold, creating SPPs. These SPPs are then ferried through the waveguide and focused by the tapered end.

A scanning electron microscope image of the 3-D plasmonic waveguide.

The waveguide was manufactured on a semiconductor chip with standard nanofabrication techniques, allowing it to be integrated into existing technology fairly easily. Previous attempts at on-chip manufacturing resulted in 2-D waveguides that could focus only a small percentage of the incident light in a narrow line.

With the new design, light can be focused in three dimensions and can convert half of the light that enters the waveguide, according to Choo.

The improvement is all in the geometry, Choo told Photonics Spectra. “It’s the 3-D geometry. The idea of using tapered plasmonic structures has existed for several years. While a few theoretical articles discuss 3-D structures, most experimental work demonstrated and analyzed 2-D structures because it’s not easy to implement engineered 3-D structures on a chip. So we took a few extra (and creative) steps and demonstrated the structure.”

“We hope it’s a good building block for many potentially revolutionary engineering applications,” said Myung-Ki Kim, a postdoctoral scholar and co-lead author of the paper, which was published in Nature Photonics (doi: 10.1038/nphoton.2012.277). These applications include high-resolution biological imaging and data storage technology.

Choo suspects that, provided their design is the best approach, the technology will be implemented for research purposes within two to three years and be available for consumer technology shortly thereafter.

The research was funded by DARPA’s Science and Technology Surface-Enhanced Raman Spectroscopy program, the US Department of Energy, and the Division of Engineering and Applied Science at Caltech.

Photonics Spectra
Mar 2013
3-D plasmonic waveguide3-D waveguideAmericasBasic ScienceCaliforniaCalifornia Institute of TechnologyCommunicationsDARPADOEHyuck ChooindustrialMicroscopyMyung Ki-ChimNature PhotonicsResearch & TechnologySPPssurface plasmon polarizationsTech PulseUS. Department of Energywaveguides

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