18 February 2015

Ghent University and Imec demonstrate interaction between light and sound in nanoscale waveguide

Researchers at Ghent University and nanoelectronics R&D center Imec of Leuven, Belgium have demonstrated interaction between light and sound in a nanoscale area using a silicon photonic nanowire (R. Van Laer et al, Nature Photonics (2015); http://dx.doi.org/10.1038/nphoton.2015.11). Their findings are said to elucidate the physics of light-matter coupling at these scales, and pave the way for enhanced signal processing on mass-producible silicon photonic chips.

In the last decade, silicon photonics has gained increasing attention as a key driver of lab-on-a-chip biosensors and of faster-than-electronics communication between computer chips. The technology builds on silicon photonic nanowires carrying optical signals from one point to another at the speed of light. They are fabricated with the same toolset as microelectronic circuits.

Fundamentally, the wires work only because light moves more slowly in the silicon core than in the surrounding air and glass, trapping light inside the wire by total internal reflection. But manipulating the light is not easy, because one light beam cannot easily change the properties of another. However, light-matter interaction can allow some photons to control other photons.

Picture: Both light (left) and sound (right) are trapped in a nanoscale silicon core.

Researchers from the Photonics Research Group of Ghent University and Imec have reported a type of light-matter interaction where they managed to confine not only light but also sound to the silicon nanowires. Oscillating 10 billion times per second, the sound cannot be trapped in the wire by total internal reflection. Unlike light, sound moves faster in the silicon core than in the surrounding air and glass. So, the researchers sculpted the environment of the core to make sure that any vibrational wave trying to escape it would actually bounce back. They hence confined both light and sound to the same nanoscale waveguide core, which is claimed to be a world's first observation.

Trapped in that small area, the light and vibrations strongly influence each other: light generates sound and sound shifts the color of light, via the process of stimulated Brillouin scattering. The researchers exploited this interaction to amplify specific colors of light. They anticipate that this demonstration will open up new ways to manipulate optical information. For example, light pulses could be converted into sonic pulses and back into light, implementing much-needed delay lines. The researchers also expect that similar techniques could be applied to even smaller entities such as viruses and DNA, which have unique acoustic vibrations that may be used to probe their global structure.

Tags: Nanophotonics silicon photonics

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