Four-Wave Mixing Paves Way for Optical Computing

Researchers have reduced the distance over which light can interact by 10,000 fold. Their work could advance optical computing by bringing optical processing into the range of electrical transistors, which are currently used to power personal computers.

These are nanofocusing and optical mode properties of the organic hybrid gap plasmon waveguide on the silicon platform used for degenerate four-wave mixing. The insets depict the scaled electromagnetic mode distributions for a wide metal gap of 500 nm and a narrow metal gap of 25 nm, along with the chemical formula for MEH-PPV. Courtesy of Nielsen et al., 2017/Imperial College London.

Researchers at Imperial College London squeezed light into a channel only 25 nm wide using an integrated plasmonic gap waveguide that confined light within a nonlinear organic polymer. The gap waveguide intensified light by nanofocusing it to a mode cross-section of a few tens of nanometers, thus generating a nonlinear response so strong that efficient four-wave mixing (FWM) accumulated over wavelength-scale distances.

They used a polymer material with a high nonlinear coefficient that they embedded within the plasmonic cavity and used a metal channel to focus the light inside the polymer. The plasmonic cavity focused the light down to the nanoscale, providing an intense electromagnetic field that induced the nonlinear process of FWM in the polymer.

While light can carry a higher density of information and is faster and more efficient than conventional electronics, it is traditionally not very good at processing information. By performing efficien FWM over micrometer-scale interaction lengths at telecommunications wavelengths on silicon, the researchers found that photons interacted more strongly over a short distance, changing the property of the light.

This is a fabricated structure of W=25 nm and L=2µm with grating couplers and 30° tapers. The images were taken by scanning electron microscopy. Courtesy of Nielsen et al., 2017/Imperial College London.

“Because light does not easily interact with itself, information sent using light must be converted into an electronic signal, and then back into light, said researcher Michael Nielsen. "Our technology allows processing to be achieved purely with light.”

Along with paving the way toward optical computing, the research could resolve a longstanding problem in nonlinear optics. Since interacting light beams with different colors pass through a nonlinear optical material at different speeds, they can become ‘out of step’ and the desired effect can be lost.

With the Imperial team’s technique, the light travels such a short distance that it does not have time to become out of step. This could allow nonlinear optical devices to be more versatile, opening it to a regime of relaxed phase matching, with the possibility of compact, broadband and efficient frequency mixing integrated with silicon photonics.

“This research has ticked one of the boxes needed for optical computing,” said Neilsen.

The research was published in Science (doi: 10.1126/science.aao1467).