Uncooled GaAs arsenide emits midinfrared

June 1, 2001
In results presented at the Conference on Laser and Electro-Optics (May 6-11, Baltimore, MD), a team of scientists from Thales (Orsay, France) describe the room-temperature operation of a gallium arsenide/aluminum gallium arsenide (GaAs/AlGaAs) quantum cascade (QC) laser for the first time.
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In results presented at the Conference on Laser and Electro-Optics (May 6-11, Baltimore, MD), a team of scientists from Thales (Orsay, France) describe the room-temperature operation of a gallium arsenide/aluminum gallium arsenide (GaAs/AlGaAs) quantum cascade (QC) laser for the first time. The QC laser is a compact and robust source of midinfrared radiation, with its output wavelength chosen by proper engineering of the laser's multilayer structure. Earlier GaAs-based QC lasers—also developed at Thales—were strictly limited to cryogenic temperatures (other QC lasers are based on indium phosphide). The great advantage of GaAs is that it is an established material in the laser-diode industry, and thus makers of QC lasers based on GaAs can benefit from years of expertise in the area of mass manufacture.

The Thales laser contains a GaAs/Al0.45Ga0.55As heterostructure of a three-well active-region design (an older design was based on (GaAs/Al0.33Ga0.67As). Previously, the temperature dependence of GaAs-based QC lasers was improved in two ways. First, if the lasing wavelength was made longer, the lasing transition could be moved deeper into the quantum wells. But in addition to providing less flexibility in choosing wavelength, this technique raised threshold currents. Second, the band offset could be increased by using a GaAs/AlAs heterostructure, but at the cost of introducing a sudden switching off at a critical voltage corresponding to a breaking of the quantum-mechanical alignment of the structure, limiting the operating range of the device.

The new laser reaches room-temperature (300 K) operation by a third route: increasing the Al concentration of the barrier material from 33% to 45% (the limit of the degeneracy of three important quantum states). The researchers estimate that the higher Al concentration increases the band offset by 95 meV.

At cryogenic temperatures, the new laser shows threshold current densities, slope efficiencies, and peak powers similar to the older design: a 2-mm x 30-µm device emitting 100-ns 9-µm pulses at 1 kHz produces a threshold current density of 4 kA/cm2, a slope efficiency of 300 mW/A, and a peak power of 1.3 W. At room temperature, however, the new laser (pulsed at 44-ns duration and 10 kHz) still produces appreciable power (see figure, p.18), while the old design peters out at just above 200 K. The highest operating temperature for the new Thales laser is 308 K; at 233 K (a temperature accessible by a standard Peltier thermoelectric cooler) the laser emits 600 mW. The lasing spectrum at 300 K consists of several longitudinal modes.

The researchers say their laser can be made to emit anywhere in the 1 to 10-µm range. The more-common indium phosphide based QC lasers are already being sold commercially, helped by their robust room-temperature operation. Now that the ubiquitous and inexpensive GaAs has been shown to be a viable base for a room-temperature QC laser, the availability of GaAs-based QC lasers on the marketaided by the fast technological pace in the QC-laser fieldcannot be far off.

About the Author

John Wallace | Senior Technical Editor (1998-2022)

John Wallace was with Laser Focus World for nearly 25 years, retiring in late June 2022. He obtained a bachelor's degree in mechanical engineering and physics at Rutgers University and a master's in optical engineering at the University of Rochester. Before becoming an editor, John worked as an engineer at RCA, Exxon, Eastman Kodak, and GCA Corporation.

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