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Room temp QCL stretches to 3.1 microns

13 Feb 2009

Short wavelength QCL research gets a boost as a UK team reports a room temperature device emitting at 3.1 microns.

Researchers at the University of Sheffield, UK, have published details of what they believe is the shortest wavelength InP-based room temperature quantum cascade laser (QCL). Based on the strain compensated InGaAs/AlAsSb material system, the device emits 8 mW of optical power in the pulsed regime at wavelength of 3.1µm. (Applied Physics Letters 94 031106)

Until recently, the prospects of developing high performance QCLs emitting between 3 and 4 microns seemed limited. Such short wavelengths are below the region that can be readily accessed using the conventional material system of InGaAs/AlInAs on InP.

However, since attention has turned to heterostructure systems incorporating III-V antimonides (InGaAs/AlAsSb on InP and InAs/AlSb on InAs), the situation has been transformed.

"Our present InGaAs/AlAsSb QCLs are the shortest wavelength room temperature QCLs grown on an InP substrate," Dmitry Revin, a researcher in John Cockburn's group at Sheffield told optics.org. "This is significant as InP-based devices have much better compatibility with existing optoelectronic device growth and fabrication technology."

"Our laser brings us one step closer to the development of high performance short wavelength QCLs," he added. "Our main aim was to show that short wavelength QCLs grown on InP substrates and operating at high temperatures were feasible."

According to Revin, one of the key challenges has been perfecting the MBE growth. "AlAsSb is hard to grow because it tends to separate naturally into AlAs and AlSb," he explained. "After establishing reliable growth for AlAsSb lattice matched to InP, the next step was to develop the growth of strain compensated InGaAs/AlAsSb which is crucial, we believe, for the realization of QCLs emitting close to 3µm."

With a pulsed emitter in place, the team is now targeting a continuous wave QCL. "We obtained encouraging results when AlAsSb layers were partly substituted by AlAs layers," commented Revin. "Optimizing the growth of these complex structures as well as improving the laser design should result in a 3µm source that operates in continuous mode at room temperature."

One particular goal is a singlemode QCL operating at 3.35µm at room temperature. Such a source would be ideal for a range of spectroscopy and sensing applications, as ethane gas has a strong absorption line at this wavelength. Monitoring the ethane content in exhaled breath has potential for early detection of disease, for example, but it can also be used for remote detection of oil and gas deposits.

Although Revin and his colleagues have no immediate plans to commercialise their work, they would welcome any interest from industrial companies to help move these devices from the laboratory to the real world.

Jacqueline Hewett is editor of Optics & Laser Europe magazine.

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