Recent progress in short wavelength quantum cascade lasers
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The 3-4μm wavelength range is of key technological importance for a wide range of applications. As a consequence of the strong fundamental C-H stretch mode that occurs at around 3.3μm, the detection of many important hydrocarbon species has maximum sensitivity (potentially ~ parts per trillion) in this range. This leads to many important applications in various areas, for example, clinical diagnostics, process monitoring and remote detection of oil and gas deposits. This short wavelength IR region is also of interest for several defence and security applications as well as free-space “last mile” telecommunications. The unsuitability of the “conventional” InGaAs/AlInAs materials system for high performance QCL sources in this short wavelength range has led to significant interest in new QCL materials with very high conduction band offsets (ΔEc) based on III-V antimonides such as InAs/AlSb/InAs (ΔEc ~ 2 eV) and InGaAs/AlAs(Sb)/InP (ΔEc ≤ 1.6 eV). Very recently, significant progress has been made in the development of these “short wavelength” QCLs operating in the close vicinity of λ ~ 3 μm. In our work, we have focussed on the InGaAs/AlAsSb system, which provides both a very high ΔEc that can comfortably accommodate the high energy intersubband transitions required, and lattice-matched compatibility with InP. This latter feature is extremely important as it greatly simplifies optical waveguide development and facilitates advanced device fabrication. Recent highlights of our research on InGaAs/AlAs(Sb) QCLs have included strain compensated QCL growth, the shortest wavelength room temperature InP-based QCL operation at around 3.1μm and novel high performance structures at λ ~ 4.1 μm with selectively incorporated AlAs barriers in the active regions.
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