QUANTUM-CASCADE LASERS
Gallium arsenide-based QC laser emits at about 9.4 µm
Roland Roux
The world`s first unipolar-injection or quantum-cascade (QC) laser in a gallium arsenide-based heterostructure has been demonstrated by Carlo Sirtori, Peter Kruck, Stephano Barbieri, and colleagues at Central Laboratories of Thomson-CSF (Orsay, France). The project is a collaboration with the University of Neuchatel and the Ecole Polytechnique Federale de Lausanne (Switzerland), as well as part of a Brite/EURAM project for the development of unipolar semiconductor lasers at the European level. Sirtori presented the team`s preliminary research results at CLEO/Europe-EQEC `98 (Glasgow, Scotland).
Quantum-cascade lasers operate like an electronic waterfall. When an electric current flows through the laser, electrons cascade down an energy staircase. At each step, they emit an infrared photon by making a quantum jump between well-defined energy levels. The emitted photons reflect back and forth between built-in mirrors, stimulating other quantum jumps, as well as the emission of other photons. This amplification process boosts output power.
Since their birth almost five years ago, QC lasers have been fabricated with the same hetero-structural material--aluminum indium arsenide/gallium indium arsenide/indium phosphide (AlInAs/GaInAs/InP). The experiment at the Central Laboratories proved that QC-laser principles are not bound to one material. More specifically, scientists demonstrated QC lasing with GaAs/AlGaAs (see micrograph). The cost implications could be dramatic because GaAs is the most-common and lowest-cost of the III-V compound semiconductors.
The GaAs-based QC laser currently reaches threshold at cryogenic temperatures and operates in the range of 150 K in pulse mode. The emission wavelength is near 9.4 µm. The French research team is now working to push the laser operating temperature to 300 K and to open up new avenues in laser technology by exploiting GaAs processing combined with quantum engineering of materials.
Scientists can, for example, easily tailor the QC laser`s spectral range of emission to 8-15 µm by varying the thickness of the laser active region with the same combination of materials. Technology within this spectral range has implications for applications such as pollution monitoring, production-line quality control and diagnostics, and environmentally safe manufacturing--many hazardous and toxic chemicals have optical absorption "fingerprints" at these wavelengths. Other potential applications for mid-infrared technology include detection of illegal drugs and explosives, as well as military-countermeasure and medical-testing applications.