Waveguide enables 3D focusing of light

A team of engineers from the California Institure of Technology (Caltech) have developed new kind of waveguide that gets around diffraction limit which usually hinders the focusing of light into tinier spaces to squeeze more data through optical fibres and increase bandwidth.

The waveguide, which is described in a recent issue of the journal Nature Photonics, is made of amorphous silicon dioxide—similar to common glass -and is covered in a thin layer of gold. Just under two microns long, the device is a rectangular box that tapers to a point at one end.

As light is sent through the waveguide, the photons interact with electrons at the interface between the gold and the silicon dioxide. Those electrons oscillate, and the oscillations propagate along the device as waves—similarly to how vibrations of air molecules travel as sound waves. Because the electron oscillations are directly coupled with the light, they carry the same information and properties—and they therefore serve as a proxy for the light.

Instead of focusing the light alone—which is impossible due to the diffraction limit—the new device focuses these coupled electron oscillations, called surface plasmon polaritons (SPPs). The SPPs travel through the waveguide and are focused as they go through the pointy end.

Because the new device is built on a semiconductor chip with standard nanofabrication techniques, says assistant professor of electrical engineering Hyuck Choo, it is easy integrate with today's technology.

Previous on-chip nanofocusing devices were only able to focus light into a narrow line. They also were inefficient, typically focusing only a few per cent of the incident photons, with the majority absorbed and scattered as they traveled through the devices.

With the new device, light can ultimately be focused in three dimensions, producing a point a few nanometers across, and using half of the light that's sent through, Choo says. The key feature behind the device's focusing ability and efficiency, he says, is its unique design and shape.

The device can lead to computer hard drives that hold more memory via heat-assisted magnetic recording. With current technology, discs can't hold more than 1 terabyte per square inch. A nanofocusing device, Choo says, can bump that to 50 terabytes per square inch.

As computing becomes increasingly reliant on optics, devices that concentrate and control data-carrying light at the nanoscale will be essential—and ubiquitous, says Choo. The next step is to optimise the design and to begin building imaging instruments and sensors. The device is versatile enough that relatively simple modifications could allow it to be used for imaging, computing, or communication, he added.