The realisation of high performance quantum cascade laser (QCL) sources at the short wavelength end of the 3-5 micron atmospheric transmission window is of major interest for a wide range of technological applications. Many of these are potentially of great significance for healthcare, security and the environment. However, conventional QCL materials systems such as InGaAs-AlInAs are fundamentally unsuitable for such short wavelength devices, as they do not have sufficiently deep quantum wells to support the high energy intersubband transitions required. Consequently, in recent years, attention has turned to alternative QCL materials systems based on III-V antimonides. At Sheffield we have established considerable expertise in the InGaAs-AlAsSb materials system. In addition to very deep quantum wells (~1.6 eV), this system provides lattice matched compatibility with InP-based waveguide and device fabrication technology. In this project we will develop short wavelength InGaAs-AlAsSb QCLs that will redefine the state of the art for semiconductor lasers in the 3-4 micron region, and provide unprecedented levels of performance and functionality for trace gas sensing and countermeasures applications. We will also exploit the potential of such deep QW devices for new developments in intersubband non-linear optics, in particular the demonstration of QCL operation at telecommunications wavelengths via intracavity second harmonic generation.