PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 735  June 29, 2005
by Phillip F. Schewe, 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{at}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{at}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. 
(Jones et al., Physical Review Letters, 20 May 2005, Cover Figure article;
http://jilawww.colorado.edu/YeLabs/ )

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