Michael D. Wheeler
A team of researchers has designed a quantum cascade laser using one of the most common low-cost semiconductor compounds in the world: gallium arsenide (GaAs). The advance could prove important in sensing applications requiring mid-IR wavelengths such as pollution monitoring and the detection of illegal drugs.
The research group, led by Carlo Sirtori at Central Laboratories of Thomson-CSF, in conjunction with researchers at the University of Neuchâtel and EPFL in Lausanne, Switzerland, designed quantum wells using GaAs and aluminum gallium arsenide (AlGaAs). They sandwiched wafers of the two materials together to fabricate each well.
Until now, scientists had been limited to only one material -- AlInAs/GaInAs/InP -- raising questions over whether the success of the design was tied to a specific material. By creating an operable laser emitting at 9.4 µm using GaAs, Sirtori finally may have silenced the doubters.
Federico Capasso, who pioneered the quantum well design, said GaAs may soon bring these lasers into the commercial sector. "The GaAs quantum cascade laser from Thomson is the new kid on the block, but it looks promising. ... There already is a good quantum well detector technology covering the mid-IR. When you have a detector in the same material as the laser, you can put them on the same chip."
Besides integration, the new material offers other advantages. Sirtori said GaAs is easier to grow than the material Capasso used in the original devices. By what Sirtori calls a "miracle of nature," the crystal structure of GaAs matches perfectly with that of AlGaAs, allowing the team to vary the amount of aluminum in the second compound. The amount of aluminum affects the depth of each well, enabling more freedom in design and tunability.
"We used the 'king' of materials in gallium arsenide. It covers about 90 percent of all semiconductor lasers," said Sirtori. "That's really the important point. It's as if someone had been using gold and all of a sudden could make the same device in silver or iron."
While the team's results mark a breakthrough in quantum cascade laser research, Sirtori said it has a long way to go. The device emits at 70 mW -- higher than most conventional mid-IR semiconductor lasers, but below the 700 mW of Bell Labs' lasers at the same wavelength. Besides increasing the pulse power, the next challenge will be to push the operating temperature higher.