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PHYSICS NEWS UPDATE A digest of physics news items by Phillip F. Schewe, American Institute of Physics Number 177 May 3, 1994 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, 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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