(December 14, 2015) An
optical device at nanoscale which allows light to pass in only one direction
has been developed at TU Wien (Vienna). It consists of alkali atoms which are
coupled to ultrathin glass fibres.
If light is able to propagate from left to right, the
opposite direction is usually allowed as well. A beam of light can normally be
sent back to its point of origin, just by reflecting it on a mirror.
Researchers at TU Wien have developed a new device for breaking this rule. Just
like in an electrical diode, which allows current to pass only in one
direction, this glass fibre-based device transmits light only in one direction.
The one-way-rule holds even if the pulse of light that passes through the fibre
consists of only a few photons. Such a one-way-street for light can now be used
for optical chips and may thus become important for optical signal processing.
Optical Signal
Processing Instead of Electronics
Elements which allow light to pass in only one direction are
called “optical isolators”. “In principle, such components have been around for
a long time”, says Arno Rauschenbeutel, from the Vienna Center for Quantum
Science and Technology at the Atominstitut at TU Wien. “Most optical isolators,
however, are based on the Faraday effect: A strong magnetic field is applied to
a transparent material between two crossed polarization filters. The direction
of the magnetic field then determines the direction in which light is allowed
to pass.”
For technical reasons, devices using the Faraday effect
cannot be constructed at the nanoscale – an unfortunate fact, because this
would have many interesting applications. “Today, researchers seek to build
optical integrated circuits, similar to their electronic counterparts”, says
Rauschenbeutel. Other methods for breaking this symmetry only work at very high
intensities. But in nanotechnology, an ultimate goal is to work with extremely
faint light signals, which may even consist of individual photons.