update.322 (fwd)

[Jim Choate <ravage@EINSTEIN.ssz.com>]

Forwarded message:
>From physnews@aip.org Tue May 20 14:20:29 1997
Date: Tue, 20 May 97 09:14:36 EDT
From: physnews@aip.org (AIP listserver)
Message-Id: <9705201314.AA25712@aip.org>
To: physnews-mailing@aip.org
Subject: update.322


PHYSICS NEWS UPDATE                         
The American Institute of Physics Bulletin of Physics News
Number 322 May 20, 1997   by Phillip F. Schewe and Ben Stein

IN QUANTUM CASCADE LASERS electrons are put in a
barrel, as it were, and sent over a series of waterfalls.  Instead of
recombining with holes to create photons, as in conventional
semiconductor lasers (one injected electron resulting in one
photon), electrons in a QC laser pass through a succession of
closely coordinated quantum wells---each well consisting of a
sandwich of semiconductor layers---unloading energy as they go,
in the form of photons (one electron creating 25 photons, one for
each stage in the stack). QC lasers are unique in that the output
light wavelength is determined not by semiconductor chemistry
(the type of materials used) but by the thickness and spacing of
the layers (sometimes only a few atoms thick). Cascade lasers,
first developed in 1994 by Federico Capasso and Jerome Faist at
Bell Labs, can operate in the mid infrared wavelength region (4-
12 microns).  This technologically important range is currently
being served primarily by low-power lasers which can only work
at low temperatures. By contrast, Bell Labs' new QC laser can
not only operate at room temperature with high output power (60
mW, with even higher power evident in recent experiments) but
can also be tuned to a single wavelength through the use of
gratings within the laser.  These features will allow scientists in
the field to carry out remote chemical sensing (of, say, pollutants
present at parts-per-billion levels)  by selectively exciting, and
detecting, specific chemical species. (Jerome Faist et al., Applied
Physics Letters, 19 May 1997; and a talk at this week's
Conference on Lasers and Electro-Optics in Baltimore.)

ZERO-DIMENSIONAL METALS are studied by physicists at
Harvard.  In general, reducing the dimensionality of an object
makes its quantum nature more manifest.  In a semiconductor,
for example, confining mobile electrons to a plane (2D) or a wire
(1D) or a dot (0D) enforces an ever sharper limit on the allowed
energies, and this can be exploited in producing compact and
highly controllable electronic devices.  The Harvard scientists
(contact Dan Ralph, now at Cornell, ralph@msc.cornell.edu)
have succeeded in attaching leads to 10-nm-sized metal particles;
this allows them to apply a gate voltage which turns the tiny
particle into a transistor. Unlike semiconductor dots, the metal
nanoparticle can be made magnetic or superconducting, allowing
forces inside the sample to be analyzed. Indeed, with this speck
of aluminum, the discrete quantum-mechanical spectrum of
electrons in a metal have been measured more accurately than
ever before. One can watch the electron spectrum even as
magnetic fields break up the superconducting state. (D.C. Ralph
et al., upcoming in Physical Review Letters, 26 May 1997.)

GAMMA RAY BURSTERS HAVE REVEALED
THEMSELVES at optical and radio wavelengths.  Astronomers
have sought to establish whether these once-only gamma sources
were near at hand (in the halo of our galaxy) or resident in distant
galaxies.  On May 8 the BeppoSAX satellite spotted a new GRB
at gamma wavelengths.  Alert Caltech scientists soon viewed the
same object with the Palomar and Keck optical telescopes,
establishing from the redshift that the object was some billions of
light years away.  Meanwhile, the Very Large Array radio
telescope has also glimpsed the object.  As of last week the
visible signal was decreasing and the radio signal increasing in
intensity.  (Press releases from Caltech (May 14) and VLA (May
15).)