Number 181 (Story #2), June 3, 1994 by Phillip F. Schewe and Ben Stein IN A QUANTUM CASCADE (QC) LASER electrons emit light in a succession of steps as they traverse numerous thin quantum wells stacked together by epitaxial techniques. The wavelength of the output laser light depends on the relative thickness and spacings of the layers rather than on the specific semiconductor composition of the device. Federico Capasso and colleagues at AT&T Bell Labs, who developed the 500-layer QC laser, believe that the ability to tailor the output wavelength will make the laser useful for a variety of applications, such as detecting levels of atmospheric pollution and for inter-chip telecommunications. (Jerome Faist et al., Science, 22 April 1994.)