Number 177, May 3, 1994 by Phillip F. Schewe and Ben Stein
A QUANTUM WIRE MICROCAVITY LASER uses quantum confinement effects in
two ways. First, the active laser medium consists of arrays of quantum
wires, practically one-dimensional semiconductor structures in which electron
energies are greatly restricted to narrow bands. Secondly, the photons
emitted by the quantum wires resonate in a microcavity (a "quantum
dot" for photons) which allows only a single light mode to operate.
Scientists at NTT Basic Research Labs in Japan (contact Arturo Chavez-Pirson,
chavez@wave.ntt.jp) have now demonstrated such a device. They grow quantum
wires in the crevices of a stepped gallium-arsenide substrate. This "fractional-layer
superlattice" provides stacks of 6nm x 8nm x 1cm quantum wires. The
wires emit light in all directions, but inside the microcavity (itself
formed of semiconductor layers) they are forced to emit light in the direction
of the end faces. The resultant laser system converts electrical energy
into light with high efficiency. The light from a microcavity quantum-wire
laser is highly anisotropic, unlike that from quantum-well lasers, in which
the active medium consists of two-dimensional semiconductor layers. This
means that laser light with an electric field parallel to the wires is
much more intense than light whose electric field is perpendicular to the
wires. The NTT scientists expect that this polarization property might
be exploitable in switching the laser on and off at rates up to 100 GHz.
(A.Chavez-Pirson et al., Applied Physics Letters, 4 April 1994.)
THE DENSITY OF EXTRAGALACTIC DEUTERIUM has been measured for the first
time by scientists using the new 10-m Keck Telescope in Hawaii. In particular,
these astronomers measured the ratio of deuterium to ordinary hydrogen
in a distant gas cloud by observing the absorption of light from an even
more distant quasar as it passes through the cloud. The D/H ratio, 2.5
x 10**-4, is much higher than the ratio measured in our own galaxy. An
accurate measurement of this ratio may help to determine what fraction
of the suspected dark matter in the universe consists of baryons, particles
(such as the proton) comprised of three quarks bonded together. The Keck
observations also provide an upper limit, 13.5 K, on the temperature of
the cosmic microwave background as it would have appeared at the location
of the cloud, at a redshift of 2.9. This is consistent with a theoretical
estimate of 10.7 K. (A. Songalia et al., Nature, 14 April 1994.)
THE DETECTION OF PLANETS AROUND A PULSAR has been established with greater
certainty by the same astronomer who made a similar claim two years ago.
Alexander Wolszczan of Penn State has found new evidence for the presence
of two Earth-mass planets around pulsar PSR B1257+12. Using the giant radio
dish at Arecibo, Puerto Rico, Wolszczan has for the past three years monitored
the pulsar's faint radio signals, which are slightly modulated by the presence
of the orbiting bodies. The pattern of signals suggests orbital periods
of 67 and 98 days. Hints of a third and a fourth planet are also present
in the data. (Science, 22 April 1994.)
CCD TROUBLES FOR HUBBLE CAMERAS : The charge-coupled devices used to
electronically image stars and galaxies on the refurbished Hubble Space
Telescope have been exhibiting an output variability as large as 10%. Hubble
scientists believe this problem can be solved. (Science News, 23 April
1994.)
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