Dec. 1, 1998 MURRAY HILL, N.J. -- Scientists at Bell Labs, the research and development arm of Lucent Technologies, have demonstrated the first semiconductor laser that can simultaneously emit light at multiple widely separated wavelengths.
The experimental new light source does the work of three conventional semiconductor lasers, each with a hundred times less power.
It emits light in the invisible region of the spectrum, where most gases and vapors leave telltale light-absorption fingerprints, and could find important applications in areas such as pollution and environmental monitoring, industrial process control, and combustion and medical diagnostics. It is not a communications laser.
Federico Capasso, head of the Semiconductor Physics Research Department and a member of the team, said, "The key breakthrough in this development was the design and synthesis of a new laser material consisting of tens of layers of elements, each 5 to 20 atoms thick.
"By precisely tailoring the layers' thickness we have been able to create an artificial material in which electrons have energy levels designed to emit light at several pre-selected wavelengths."
Under operating conditions, an electric current is injected into the device and electrons cascade through 25 stages of the new material, emitting laser photons at two or three wavelengths in each stage. This cascade scheme boosts the power of the laser to high levels, and the laser emits a peak power of 100 milliwatts at wavelengths of 6.6 microns and 8.0 microns.
A further increase of the current generates an additional laser wavelength at 7.3 microns. Three different semiconductor lasers with a hundred times less power would normally be required to emit these widely different wavelengths.
The three wavelengths can be further tuned by changing the temperature of the laser.
The laser is fundamentally different from conventional semiconductor lasers, which are used in fiber-optic communications and compact disc players. It is based on a revolutionary approach pioneered by Capasso and his group, using quantum engineering of the electronic energy levels in materials to produce quantum-cascade lasers, in which the wavelength is entirely determined by the thickness of the active layers rather than by the chemical composition of the material. In this way, a huge wavelength region can be covered using the same material, and multiple wavelengths can be simultaneously emitted by a single laser.
The new laser material consists of regions of a semiconductor alloy, aluminum indium arsenide, four atomic layers thick, alternated with another alloy, gallium indium arsenide, 18 atomic layers thick. One atomic layer equals 10 billionths of an inch.
It was grown by molecular beam epitaxy (MBE), a crystal-growth technology developed in the 1960s by Alfred Cho, Director of the Bell Labs Semiconductor Research Lab, which involves "spray painting" atoms to build new materials one atomic layer at a time.
The Bell Labs team includes Alessandro Tredicucci, Claire Gmachl, Federico Capasso, Albert Hutchinson, Deborah Sivco and Alfred Y. Cho. Their work will be described in the November 26 issue of the journal Nature.
The new device is the latest in a long line of laser innovations from Bell Labs, where 40 years ago Arthur Schawlow and Charles Townes described the concept and design for the laser -- one of the century's greatest inventions. Semiconductor lasers are now the most widely used and versatile class of lasers.
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