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Patterned mid-IR source lowers cost of gas detection

28 Jun 2006

Vehicle exhaust monitoring and fire rescue operations could benefit from an affordable, surface-structured PbSe chip that operates at 25 degC and emits more than 2 mW in the mid-infrared.

Scientists in Germany have developed a mid-infrared (4-5 µm) source offering up to 2 mW of continuous wave output that could dramatically reduce the cost of trace gas monitoring apparatus. According to the team, surface patterning of the device's optically-pumped lead selenide (PbSe) layer gives a six-fold increase in light extraction efficiency compared with untreated chips. (J. Appl. Phys. 99 114506)

"The patterned surface allows more photons to be outcoupled from the semiconductor film," Jens W. Tomm of the Max-Born-Institute for Nonlinear Optics and Short Pulse Spectroscopy, Berlin, told Optics.org. "Thus the intrinsically poor external quantum efficiency of a high-index-material can be improved."

The patterning concept is broadband and, according to Tomm, can be implemented using relatively simple and economical processing. In fact, the team employs a combination of standard optical lithography and wet chemical etching to create periodic square, circular or hexagonal structures from 3 to 25 µm in diameter within the chip's PbSe active layer.

The 1-2 µm thick semiconducting film is grown on a BaF2 substrate, which is mounted on a diamond heat spreader using infrared transparent adhesive. When pumped with a standard laser diode array emitting at 940 nm, the device generates up to 2 mW of continuous wave output at mid-infrared wavelengths and has a slope efficiency of 0.6 mW/A.

Tomm and his colleagues, which include researchers at the Fraunhofer Institute for Physical Measurement Techniques, Freiburg, are confident that they can extract more light from the source over the coming months. What's more, they believe that the device could potentially cost as little as EURO 100.

James Tyrrell is News Editor on Optics.org and Opto & Laser Europe magazine.

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