Scientists from Ghent University and imec announce today that they
demonstrated interaction between light and sound in a nanoscale area. Their
findings 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, the field of 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 tiny structures known as silicon photonic wires, which are roughly
a hundred times narrower than a typical human hair. These nanowires carry
optical signals from one point to another at the speed of light. They are
fabricated with the same technological toolset as electronic circuitry.
Fundamentally, the wires work only because light moves slower in the silicon
core than in the surrounding air and glass. Thus, the light is trapped
inside the wire by the phenomenon of total internal reflection. Simply
confining light is one thing, but manipulating it is another. The issue is
that one light beam cannot easily change the properties of another. This is
where light-matter interaction comes into the picture: it allows some
photons to control other photons.
Publishing in Nature Photonics [1], researchers from the Photonics Research
Group of Ghent University and imec report on a peculiar type of light-matter
interaction. They managed to confine not only light but also sound to the
silicon nanowires. The sound oscillates ten billion times per second: far
more rapid than human ears can hear. They realized that 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. Thus, the
scientists sculpted the environment of the core to make sure any vibrational
wave trying to escape it would actually bounce back. Doing so, they confined
both light and sound to the same nanoscale waveguide core – a world’s first
observation.
Both light (left) and sound (right) are trapped in a nanoscale silicon core.
Trapped in that incredibly small area, the light and vibrations strongly
influence each other: light generates sound and sound shifts the color of
light, a process known as stimulated Brillouin scattering. The scientists
exploited this interaction to amplify specific colors of light. They
anticipate this demonstration to open up new ways to manipulate optical
information. For instance, light pulses could be converted into sonic pulses
and back into light – thereby implementing much-needed delay lines. Further,
the researchers expect that similar techniques can be applied to even
smaller entities such as viruses and DNA. These particles have unique
acoustic vibrations that may be used to probe their global structure.
[1] R. Van Laer, B. Kuyken, D. Van Thourhout and R. Baets. Interaction
between light and highly confined hypersound in a silicon photonic nanowire.
Nature Photonics (2015) https://dx.doi.org/10.1038/nphoton.2015.11
The article also made it to the cover, as well as
the cover image of the March 2015 issue of Nature
Photonics.
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