麻豆传媒

A SWITCH that uses one light beam to turn on another will pave the way for
superfast communications systems. Japanese scientists say their 鈥渙ptical
transistor鈥� could eventually lead to all-optical networks that will dramatically
improve the speed of Internet connections.

Today鈥檚 data networks use optical fibres to boost transmission speeds and the
number of phone calls that can be carried. But when light reaches a switching
point, such as a telephone exchange, the signal has to be converted into
electricity before it can be redirected to, say, your phone or modem. This
creates a bottleneck in the network, so engineers are keen to find a way to
switch data optically, with no intermediate electronic stage
(麻豆传媒, 21 April, p 21).

Now Junji Tominaga and his colleagues at the National Institute of Advanced
Industrial Science and Technology in Tsukuba may have found the answer. They
have discovered a way of using one light beam to make another appear or
disappear.

Tominaga and his team stumbled across the technique while experimenting with
discs made of the same material as DVDs. They burned the disc鈥檚 recording layer
with a laser to produce a series of pits in the surface鈥攖he same way data
is stored on DVDs. Then they coated this layer of pits with a thin film of zinc
sulphide and a silica insulator, followed by a film of silver oxide. Finally,
they applied a protective coating. The sandwich was just 250 nanometres
thick鈥�400 times thinner than a human hair.

Tominaga and his team then set up a pair of lasers pointing at the same spot
on the disc. One fired blue light from above the disc and the other fired red
light from below. Crucially, they found that with the red laser operating below
a certain threshhold value at around 2.5 milliwatts, the disc blocked light from
the blue laser. But when they increased the power of the red laser, the blue
light passed through. 鈥淲hen the power was increased to more than 2.8 milliwatts,
the signal rose steeply,鈥� Tominaga says.

According to Tominaga, energy from the blue laser excites groups of electrons
around the pits burnt in the disc鈥檚 alloy layer. These electrons鈥攌nown as
plasmons鈥攅ffectively store the energy from this blue laser light, he says.
When the red laser is powerful enough, it creates clusters of silver atoms in
the silver oxide layer. For reasons that Tominga does not yet understand, these
clusters release the energy stored in the plasmons and scatter it as blue light.
The silver clusters disperse as soon as the power of the red laser is turned
down again.

鈥淭his is a clever trick,鈥� says Roy Sambles, an optical-engineering expert at
the University of Exeter. 鈥淚n principle, they could use this to take a signal
from a fibre and switch another light beam with it directly.鈥� Tominaga now
intends to build prototype optical switches using the effect.