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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).)