PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 825 May 23, 2007 by Phillip F. Schewe, Ben Stein
www.aip.org/pnu
SON ET LUMIERE, which in French means *sound and light,* is the
name
for popular nighttime outdoor shows in which pictures are projected
on the walls of famous buildings (in France and other countries)
accompanied by stories played out over loudspeakers. Now scientists
hope to make a miniature sound-and-light show in fibers with the
intention of producing not entertainment but ultrasensitive optical
switches or the means of transporting bits in future all-optical
computers. The new scheme being developed by scientists at
Ben-Gurion University and Tel Aviv University uses sound waves to
help slow light nearly to a halt under conditions (ordinary
materials at room temperature) more practicable than for most other
slow-light experiments.
Richard Tasgal ([EMAIL PROTECTED]) and his colleagues use as their
medium a so-called Bragg grating fiber; the UV-sensitive core of a
fiber is exposed through a mask to ultraviolet light. This
treatment changes the germanium-doped silica fiber core in periodic
way along its length so that the index of refraction varies
periodically. Light sent into such a fiber, and encountering a
regularly changing index of refraction, will reflect multiply, not
just at the ends but all along the fiber. A fiber with this
condition is sometimes referred to as a distributed mirror. If,
furthermore, the light beam is intense and the fiber material
possesses a nonlinear response to light, the net effect of light
wavelets propagating in the forward and backward direction can be a
light pulse traveling at speeds much less than the speed of light in
vacuum.
Here*s where the sound part comes in. Very intense light will cause
a slight bunching in the density of the fiber and this can create
sound waves. This process is enhanced when the light pulse is
traveling close to the speed of sound (around 5 km/s) in the
material, and recent work has shown this could be achieved. But the
enhancement can work both ways. A passing sound wave alters very
slightly the material*s index of refraction and this in turn can
result in a shortening and slowing of a passing light pulse-in this
case referred to as opto-acoustic solitons. Tasgal says that he and
his colleagues are the first to recognize the potential of sound
waves in slowing and processing light pulses in this way. The first
great difficulty in implementing the whole scheme is to getting
light pulses to enter and stay in a Bragg fiber in the first place
since the fiber looks at first just like one long mirror. This might
be achieved by gradually increasing the strength of the grating
along the fiber. (Tasgal, Band, Malomed, Physical Review Letters,
upcoming article)
