IBM has developed a silicon chip that can
slow light down on demand. Although researchers have been able to
slow light dramatically using gases of cold atoms for some years,
repeating the effect in a device fabbed using standard silicon
processes is a first.
The company said the work is an important
step towards creating small, integrable photonic components for
things like optical delay lines, optical buffers, and eventually an
optical memory. At the moment the majority of what might be called
photonic ICs employ elements that interchange signals between the
optical and electronic domains, introducing complexity and reducing
operational speeds.
The IBM device consists of a strip
waveguide joined to a photonic crystal waveguide on which two
metallic contacts are deposited. By applying a voltage across these
contacts the refractive index of the photonic crystal waveguide
could be altered, changing the speed of the light passing through
it.
This is a significant departure from
previously demonstrated techniques for varying the speed of a light
signal, which used external lasers and low temperatures and
pressures, and were effective over only a narrow range of
wavelengths.
The results showed that light could be
slowed to less than 1/300th of its velocity in a vacuum. Using 2mW
of localised heating - delivered through the metal contacts - to
change the refractive index of the photonic crystal waveguide, the
speed could be varied with a response time of 100ns.
The details of the experiment are rather
fiendish, not least the development of structures required to
reliably measure the speed of the signal.