Published online 4 August 2011 | Nature | doi:10.1038/news.2011.459

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No turning back for light

'Optical diode' could help make commercial photonic chips a reality.

one-way lightA diode that allows light to pass only one way is key to the commercial development of optical chips.Science/AAAS

A one-way system for light rays could allow optical computer chips to overtake their standard electronic counterparts. The new device should eventually help to improve the speed of data processing and ease Internet traffic.

Optical, or photonic, chips use light rather than an electrical current to carry information. State-of-the-art optical chips already transfer data at rates of around 10 gigabits per second — more than 100 times faster than the best electronic chips, says Liang Feng, an electrical engineer at the California Institute of Technology in Pasadena.

"That's the noticeable difference between a Google search you carry out today taking a few seconds to load, and a search being done in the future in less than a blink," he says.

For more than a decade, engineers have been working to make commercially viable optical chips, but to do so they need to come up with the optical equivalent of the electronic diode. This allows current to pass in only one direction, preventing back-scattered current from interfering with other components and the forward signal.

Such 'optical diodes' have been created in the past, but they either use materials that are incompatible with silicon or rely on magnetic fields to block backward light1. "Unfortunately, you can't stick something magnetic near your computer or it will disrupt it," says Feng.

Guiding the ray

Feng and his colleagues have now created a silicon waveguide — a slab with a rectangular cross-section measuring 200 nanometres thick and 800 nanometres wide — that channels light in only one direction. Standard waveguides allow waves to pass through in both directions, but Feng's team realized that adding extra layers of materials with different reflective and refractive properties, at specific points along the tunnel, could break this symmetry.

"It has been known for a long time that adding layers to the sides of waveguides can affect forward and backward motion, but it was tricky to calculate the particular structure that would manipulate the light just as we needed," says Feng.

Using calculations and computer simulations, the team hit on the right materials and pattern for a waveguide that would allow a forward-moving light wave to progress symmetrically — so that its peaks and troughs remain parallel — while disrupting the backward wave in such a way that its successive peaks and troughs deviate from the parallel. The solution involved adding a number of sinusoidal-shaped bumps of silicon, 40 nanometres thick, along one side of the waveguide, and similar bumps, made of a layer of germanium sandwiched with chrome, on the other side.

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The team monitored the passage of light through the waveguide using a near-field scanning optical microscope and confirmed that a narrow beam of light successfully passes through the waveguide forwards, but that the wave's symmetry breaks down when travelling backwards2. The next step is to incorporate the waveguide into a device that filters out the asymmetric light. "We hope to have this completed soon," says Feng.

Nasser Peyghambarian, an optical scientist at the University of Arizona in Tuscon, says that the work is an "important step for building optical chips". But he adds that it may be another 15 years before a full range of optical components, including laser sources and optical amplifiers, are ready to be integrated together: "Only then can we talk about using photonic chips in real commercial products." 

  • References

    1. Wang, Z., Chong, Y., Joannopoulos, J. D. & Soljačić, M. Nature 461, 772-775 (2009). | Article | ISI | ChemPort |
    2. Feng, L. et al. Science 333, 729-733 (2011). | Article | ChemPort |
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