Text 664, 96 rader
Skriven 2005-12-03 14:52:57 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 756
===============
PHYSICS NEWS UPDATE                                                            

The American Institute of Physics Bulletin of Physics News
Number 756   November 30, 2005  by Phillip F. Schewe, Ben Stein

FIRST NEGATIVE-INDEX REFRACTION IN THE NEAR INFRARED.  One of the
hottest subjects in optical science right now is the study of
materials---sometimes "meta-materials" consisting of arrays of tiny
metal rings and rods---characterized by a negative index of
refraction.  First in the microwave portion of the electromagnetic
spectrum, and later at shorter wavelengths, negative- index research
has sought to characterize and exploit a process in which a light
beam, passing from air into the special medium, refracts not toward
but away from the perpendicular to the medium's surface.  Such
negative index materials (NIMs) might lead to novel lensing,
antennas, waveguides, and filtering applications. NIMs operating in
the optical range promise to create entirely new prospects for
controlling and manipulating light, optical sensing, and nanoscale
imaging and photolithography, and thus enable entirely new device
applications.  A Purdue group observes negative-index behavior all
the way into the near-infrared spectral region, around a wavelength
of 1.5 microns, exactly where fiber-optic telecommunications is
carried out. Valdimir Shalaev and his colleagues achieve negative
refraction in a material consisting of tiny gold rods residing in a
dielectric matrix.  The material, with a refractive index of -0.3,
was too "lossy" (too much of the light was absorbed) to exhibit
"perfect lensing," a type of refraction in which a cone of light
falling on a flat-panel sample of negative-index material could be
focused to a point.  However, Shalaev (shalaev@ecn.purdue.edu,
765-494-9855) believes this problem can be overcome.  Furthermore,
he is confident his lab will be able to extend his negative-index
results into the visible-light part of the spectrum.  (Shalaev et
al., Optics Letters, 15 December 2005)

MEASURING HIGHER-LEVEL QED.  A new experiment at Livermore National
Lab has made the best measurement yet of a complicated correction to
the simplest quantum description of how atoms behave.  Livermore
researchers did this by measuring the Lamb shift, a subtle shifting
of quantum energy levels, including a first measurement of
"two-loop" contributions, in a plasma of highly charged uranium
ions.
    Quantum electrodynamics (QED), the modern theory of the
electromagnetic force (the development of which earned Richard
Feynman, Sin-Itiro Tomonaga, and Julian Schwinger a Nobel Prize in
1965), was an improvement over early quantum mechanics since it took
into account that electrons inside atoms don't merely interact with
the nucleus; they also interact with the vacuum.  To be more
precise, in determining various quantum energy levels available to
the electron in a simple hydrogen atom QED accounts for occurrences
in which an electron will spontaneously emit, and shortly thereafter
reabsorb, a photon.  If one portrayed such an event in graphlike
form, on a Feynman diagram, the photon would be depicted as a
squiggly line leaving and rejoining the line depicting the electron
moving through time.  Conversely, a photon can spontaneously
rematerialize as an electron-positron pair of virtual particles,
providing that these particles very quickly recombine into a
photon.  These are examples of "single-loop" processes since the
Feynman diagram features a loop where the virtual particle or
particles pass into and back out of existence.  One can imagine
additional, higher-order processes, depicted by loops within loops,
which play a lesser but still considerable role in the overall sense
of what an electron or photon "is."  Such hidden processes (hidden
in the sense that they cannot be directly observable in the lab) can
be probed, however, by looking at the alteration, or Lamb shift, in
the spectrum of light emitted by atoms.   For hydrogen atoms,
containing but a single electron and a proton for a nucleus, the
Lamb shift can be measured to an accuracy of a few parts in a
million, and theoretical and experimental values agree very well.
One would like also to measure the Lamb shift (and hence test basic
QED precepts) for other elements.  One would like also to measure
separately the contribution of higher-order contributions to the
Lamb shift.  In hydrogen, two-loop and other higher contributions
play a very small role in the Lamb shift.  Furthermore, uncertainty
in the size of the proton limits any effort to measure two-loop
effects.
    This is not true for a uranium atom in which nearly all the
electrons have been stripped away.  With a much larger nucleus, the
proton-size issue is much reduced, and the electric fields holding
electrons inside the atom are a million times stronger (10^17 V/m)
than in hydrogen.  Thus, QED can be tested under extreme
conditions.  The Livermore physicists (contact Peter Peiersdorfer,
beiersdorfer@llnl.gov) study U atoms that have been stripped of all
but three electrons.  These lithiumlike U ions, held in a trap, are
then carefully observed to search for the Lamb discrepancy from
simple quantum predictions as to the frequencies of light emitted by
the excited ions (for a background article on the Livermore
electron-beam ion trap, or EBIT, see Physics Today, Oct 1994).  In H
atoms, the two-loop corrections constitute only a few parts per
million, but inU atoms they contribute about one third of one
percent of the Lamb shift. In this way, the Livermore team has
measured this higher-level QED term for the first time, with an
accuracy of about one part in ten (or ten percent).  (Beiersdorfer
et al., Physical Review Letters, 2 December 2005)

---
 * Origin: Big Bang (1:106/2000.7)