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.)