Monthly updates add over 2,000 abstracts each year and incorporate numerous physical science topics within such disciplines as the geophysical, chemical, engineering, astrophysical, and meteorological sciences. Designed for undergraduate college and high school students, Science and Technology Digest provides easy access to the latest scientific developments. The abstracts present important facts, ideas, results, and conclusions in concise, inclusive summaries of original articles. For maximum comprehension, scientific terms and new or difficult concepts are explained within the abstracts.

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TI:	Title Quantum Cascade Lasers
AU:	Author Capasso, Federico; Gmachi, Claire; Sivco, Deborah L.; Cho, Alfred Y.
SO:	Source Physics World, June 1999, vol. 12, no 6, pp. 27-33
AB:	Abstract A new type of laser, the quantum cascade laser, was developed recently by researchers at Bell Laboratories in Murray Hill, New Jersey. Based on a fundamentally different principle than the traditional diode lasers, quantum cascade lasers are much more powerful than diode lasers emitting at the same wavelength, because each injected photon creates many photons, rather than one as in diode lasers. A quantum cascade laser works like an electric waterfall--electrons cascade down a series of identical energy steps built into the material during crystal growth and emit a photon at each step. The photons are emitted as the electron jumps from a discrete energy level high in the conduction band to a lower discrete level in the same conduction band. One of the keys in the development of quantum cascade lasers was the invention of the thin film growth technique called molecular beam epitaxy (MBE). MBE is a high-vacuum deposition technique in which beams of various elements to be deposited are evaporated and directed onto a crystalline substrate. The substrate is kept at a high temperature, which allows the atoms hitting the surface to replicate the underlying crystal structure and orientation. MBE is used to grow layers with precision at the atomic scale, and it allows for growth of artificial materials with specified electronic and optical properties. Known as band-structure engineering, this technique has led to a new generation of artificially structured materials and semiconductor devices. The quantum cascade laser is a demonstration of the combined power of MBE and band-structure engineering. Quantum cascade lasers built thus far are based on aluminum indium arsenide/gallium indium arsenide and cover a wavelength range of 3.4-17 micrometers. The ability to tune quantum cascade lasers to different wavelengths offers the possibility of building lasers that emit light of a specific wavelength for a specific application. By increasing the temperature at which the laser operates, it is possible to increase its wavelength. Thus, it is possible to use these lasers for environmental sensing and pollution monitoring. Researchers have been able to detect traces of nitrous oxide in air with a sensitivity of 250 parts per billion at a 1 hertz bandwidth. Other applications include industrial process controls, high resolution spectroscopy, medical diagnosis, and applications in the automotive industry and the military.
PY:	Publication Year 1999
DE:	Descriptors United States; Older Adults; Age Segregated Housing; Nursing Homes; Assisted Living Facilities; Projection; Technology; Home Care; Nonprofit Organizations
AN:	Accession Number 99/00-5947

AB = Abstract	PY = Publication Year
AN = Accession Number	SO = Source
AU = Author	TI = Title
DE = Descriptor