Number 359 (Story #2), February 18, 1998 by Phillip F. Schewe and Ben Stein|
RECORD HIGH POWERS FROM QUANTUM CASCADE LASERS, devices which produce technologically important light in the mid-infrared range, were described by Federico Capasso of Bell Labs (908-582-7737) at the AAAS meeting in Philadelphia last week. In the Bell Labs design, an electron travels through a sandwich of ultrathin semiconductor layers (grouped into 25 "active regions") and drops to a successively lower energy every time it enters an active region, releasing a photon with each energy drop. (Update 322) Unlike other semiconductor-based lasers, in which an electron combines with a positively charged "hole" to release but a single photon (with a color that is determined by the semiconductor's chemical makeup), Capasso said that their latest, higher-power-output designs enable virtually all electrons injected into the sandwich (above and beyond the electric current needed to initiate the laser process) to contribute their maximum of 25 photons apiece (with colors that can be easily tailored by modifying the layer thicknesses). At temperatures of 80 K, quantum cascade lasers can now produce continuous streams of 5.2-micron-wavelength light at powers of 200 milliwatts, whereas other kinds of semiconductor-based lasers can at best provide only 1-2 milliwatts in this range (J. Faist et al., IEEE J. Quantum Electronics, February 1998). At room temperature the Bell Labs group produced 50-ns pulses of 8.2-micron light with powers of 170 milliwatts. Since this mid-infrared light transmits excellently through clean air but scatters from or gets absorbed by greenhouse gases and other pollutants, researchers at Pacific Northwest National Laboratories (Jim Kelly and Steve Sharpe, 509-375-2699) are testing these lasers with the objective of developing systems that can detect pollutants at concentrations of 10 parts per billion or lower.