Number 735, June 29, 2005
by Phil Schewe and Ben Stein
Soliton Transistor
A transistor based not on the customary
sandwich of semiconductor layers but on a Josephson junction (itself
a sandwich consisting of two superconducting layers separated by a
thin film of insulating material) architecture, and involving not
the gated flow of electrons or holes (the empty spaces left behind
by electrons) but the controllable flow of tiny magnetic vortices,
has been built and tested by Farshid Raissi, a scientist at the
Toosi University of Technology in Tehran.
The vortices, set in
motion in the form of solitons (pulses that do not lose energy or
their shape as they travel) travel at the speed of light and
therefore are much faster than the electrons in ordinary
transistors, possibly leading, Raissi argues, to quicker switching
speeds (raissi@kntu.ac.ir). In his experimental transistor setup,
which is about 800 microns long, trains of vortex solitons, created
by applying small applied magnetic fields to the junction and set in
motion by a applying a brief current into the junction, are used to
control the flow of a separate soliton train.
Part of the reason
solitons can be used in this controllable way (and controlling
flow---turning a component on or off---is one of the hallmarks of
transistors) is the fact that solitons can be made to annihilate
with anti-solitons (solitons consisting of vortices established with
a contrary magnetic orientation). With his vortex-soliton
transistor Raissi has observed switching speeds of 8 GHz, as fast or
faster than the best existing transistors.
Raissi expects no insurmountable problems in shrinking and mass producing
his soliton device, and expects to achieve speeds of 200 GHz, which
would make this transistor architecture quite attractive for use in
supercomputers. (Applied
Physics Letters, 27 June 2005; lab website, www.ee.kntu.ac.ir
)
Ultraviolet Frequency Comb
Physicists at JILA, the joint institute
of NIST and the University of Colorado, have created a new optical
process to extend the production of coherent radiation into the
extreme ultraviolet region of the electromagnetic spectrum. This
process takes advantage of the fact that ultrafast laser pulses of
femtosecond widths, separated by nanoseconds, manifest themselves as
a superposition of light at different frequencies over a wide
spectral band.
The Fourier transform of these short pulses is long series of evenly
spaced spikes; that look like the tines of a comb (for background, see
Physics
Today, June 2000). What's new is that the JILA researchers have
pushed the coverage of the frequency comb into the extreme ultraviolet
by generating a series of high harmonics of the original, near-infrared
laser frequency comb. (A comparable result has also been achieved by
Ted Hansch's group in Munich, a result to be published elsewhere.)
In the JILA
experiment, 50-femtosecond-long pulses, spaced 10 nanoseconds apart,
are sent into a coherent storage device---an optical buildup cavity.
The cavity length is determined so that each tine of the incoming
frequency comb is matched to a respective cavity resonance mode. In
other words, the pulse train is matched exactly into the cavity such
that a pulse running around inside the cavity is reinforced by a
steady stream of incoming pulses.
After a thousand roundtrips
through the cavity, the infrared laser light becomes sufficiently
energized to directly ionize xenon atoms inside the cavity. The
quick repatriation of the xenon electrons to their home atoms is
what produces light pulses of high frequency harmonics. Coherent
high harmonic generation has been achieved with other techniques,
typically involving single, actively amplified, ultrashort laser
pulses.
The new approach demonstrated in the JILA work has
drastically improved the spectral resolution of these high harmonic
generated light sources by many orders of magnitude and will also
permit an important increase of the efficiency of the harmonic
generation process. Moreover, the buildup of intense UV happened
without the need for expensive or bulky amplifying equipment.
Optical frequency combs have led to demonstrations of optical atomic
clocks and are furthering research in extreme nonlinear optics,
precision spectroscopy, and laser pulse manipulation and control.
Jun Ye (ye@jila.colorado.edu, 303-735-3171) and his colleagues
believe that the new ultraviolet frequency comb promises to provide
an important tool for ultrahigh resolution spectroscopy and
precision measurement in that spectral domain.
It will open the door to unprecedented spectral resolution, making it
possible for scientists to study the fine structure of atoms and molecules
with coherent XUV light. (Joneset al., Physical Review Letters, 20 May 2005, Cover Figure article;
http://jilawww.colorado.edu/YeLabs/
)
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