PHYSICS NEWS UPDATE A digest of physics news items prepared by Phillip F. Schewe, AIP Publ

PHYSICS NEWS UPDATE
A digest of physics news items prepared by Phillip F. Schewe, AIP
Public Information
Number 152  November 19, 1993

THE NATURE AND ORIGIN OF COSMIC RAYS seem to change
above an energy of about 10**18.5 eV.  The Fly's Eye collaboration-
--Utah, Illinois, Adelaide, Bartol---which operates in Dugway, Utah
an array of photodetectors aimed at the sky, has measured the energy
and composition of cosmic rays since 1981 (contact Pierre Sokolosky,
801-581-5398).  The apparatus looks for the fluorescence of
atmospheric nitrogen caused by airshowers set up by the incoming
cosmic rays.  The energy spectrum of the rays falls off steeply at a
certain rate up to an energy of about 10**17.5 eV.  Until that point,
the flux is dominated by heavy nuclei.  At higher energies, above
about 10**18.5 eV, the spectrum, now dominated by protons,
flattens, indicating a different sort of cosmic ray at these higher
energies. (The data sample in this energy range consists of several
thousand events.)  Furthermore, no anisotropy in the directionality of
the highest-energy events can be detected, suggesting to the Fly's Eye
scientists that these particles do not originate in the galactic disk.
(D.J. Bird et al., Physical Review Letters, 22 Nov. 1993.)

THE DIFFUSION-CONTROLLED AGGREGATION of
nanostructures on a substrate is directed by a hierarchy of energy
barriers.  For example, adsorbed atoms on a surface need more
energy to climb from one terrace of atoms to another than to simply
move across the a terrace surface.  Moving along the edge of a
terrace requires even more energy; departing from an island of
terraces altogether requires more energy still.  Klaus Kern at the
Institute for Experimental Physics in Lausanne, Switzerland has
studied the effect of atom mobility in the nucleation and growth of
several types of atom clusters.  He finds that in this world of atom
archipelagos temperature (at least within a certain range) is destiny:
different patterns of self-organization emerge depending on such
environmental factors as temperature or the flux of new atoms onto
the substrate.  Horia Metiu of UC Santa Barbara compares this
aggregation process to the activity at a construction site: the
seemingly random motions of atoms, subject to a number of "building
codes," do eventually arrive at a condition of geometrical order,
whether in the form of one-dimensional strands or two-dimensional
islands.  (Holger Roder et al., Nature, 11 Nov. 1993.)

OPTICAL PARAMETRIC OSCILLATION (OPO) is a nonlinear
optical phenomenon (the light output is not proportional to the input
light) in which a light beam at one frequency can, by passing through
a special crystal, be split into two beams at lower frequencies.  The
whole process can be tuned---creating in effect a multicolor laser
system---by changing the refractive index of the nonlinear crystal,
which in turn can be accomplished by rotating the crystal relative to
the incident light beam.  The OPO technique may prove to have
applications in the study of quantum optics, in the activation (at
specific wavelengths) of photosensitive drugs, in the monitoring of
tiny amounts of environmental pollutants, in full-color compact
television, and in sampling a system's spectrum over very short time
intervals (time-resolved spectroscopy), an application in which the
high peak power of pulsed lasers can only make the OPO process
more efficient.  (Physics World, Oct. 1993.)