AT&T Bell Laboratories has been describing a fundamentally new kind of laser, which it calls a quantum cascade laser, in which the wavelength of light emitted is determined by a staircase-like structure created by laying down steps of thin sandwiches of semiconductor material. As a negative voltage is applied, electrons spill down the staircase, causing each step to lase and emit light as the electrons bounce down. By contrast, a conventional semiconductor laser generates light as both positively- and negatively-charged particles are injected into a band of semiconductor material and the two types of particles combine, and the wavelength emitted is dependent on the semiconductor used. With the new laser, which operates in the infra-red range of the spectrum, the wavelength is determined by the detailed structure of the staircase and a wide range of the infra-red spectrum can be covered simply by varying the thickness of the steps. At each step, the electrons make a quantum jump between well-defined energy levels. The emitted photons are reflected back and forth between built-in mirrors, stimulating other quantum jumps and the emission of other photons until the amplified pulse escapes the laser cavity. In the quantum cascade laser, the multilayer material, including the quantum-well active regions, consists of alternated nanometre-thick Aluminium Indium Arsenide and Gallium-Indium-Arsenide layers grown on an Indium-Phosphide substrate. Developed at the Quantum Phenomena & Device Research Department at Bell Labs in Murray Hill, New Jersey, the quantum cascade laser, whose operation has also been likened to an electronic waterfall, is seen as having applications in free-space point-to-point communications systems, industrial process control, spectroscopy, environmental monitoring of air quality, and analysis of light absorption properties of materials.
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