Pointing Light Toward Faster Optical Communications

A device that focuses light into a point just a few nanometer across could lead to more efficient optical devices and higher-resolution imaging systems.

California Institute of Technology engineers have produced light beams smaller than the wavelength of the light itself but that carry the same signals. This could help increase the bandwidth of optical communications by transmitting data through a narrower beam of light and could pave the way for smaller optical devices that require less power.

Focusing light to such minute scales, however, is inherently difficult because of the diffraction limit. Once you reach sizes smaller than the wavelength of light, it is physically impossible to focus the light any further.

Engineers at Caltech have created a device (illustrated here) that can focus light into a point just a few nanometers across — an achievement they say may lead to next-generation applications in computing, communications and imaging. Courtesy of Young-Hee Lee.

The researchers built a 2-nm-long rectangular waveguide with a tapered point at one end that gets around the natural limit of the size of light beams by focusing not just the light, but also the coupled electron oscillations, called surface plasmon polaritons (SPPs). The SPPs travel through the waveguide — composed of amorphous silicon dioxide and covered in a thin layer of gold — and are focused as they go through the pointy end. Because the SPPs are directly coupled with the light, they carry the same information and properties and serve as a proxy signal.

Previous nanofocusing devices were much more inefficient, typically focusing only a few percent of the incident photons into a narrow line, with the majority absorbed and scattered as they traveled through the devices.

The new waveguide can focus light in three dimensions, producing a point a few nanometers across and using half the light. Focusing the light into a slightly bigger spot, 14 × 80 nm, boosts the efficiency to 70 percent.

The device is built on a semiconductor chip with standard nanofabrication techniques, making it easy to integrate with existing technology, said electrical engineering assistant professor Hyuck Choo, co-leader of the project and co-author of a paper published in Nature Photonics (doi: 10.1038/nphoton.2012.277).

In creating high-resolution imaging devices, the waveguide could focus very small beams of light onto biological cells containing fluorescent proteins. Dyeing molecules within the cells could help map them at very high resolutions.

A scanning electron microscope image of the nanofocusing device. Courtesy of Caltech/Hyuck Choo and Myung-Ki Kim.

And because light can travel in the reverse direction through the device, it also could be used to create a high-resolution microscope.

The instrument also could lead to computer hard drives that hold more memory via heat-assisted magnetic recording or very narrow-band lasers tiny enough to heat small magnets individually, making it possible for hard drives to pack more magnets and, therefore, more memory. Such a nanofocusing device could bump disc storage from 1 TB to 50 TB per sq. in., Choo said.

“Our new device is based on fundamental research, but we hope it’s a good building block for many potentially revolutionary engineering applications,” said Myung-Ki Kim, a postdoctoral scholar and project co-leader and co-author of the paper.

The next step is to optimize the design and to begin building imaging instruments and sensors, Choo said.

For more information, visit: www.caltech.edu