Quantum cascade lasers: 25 years after the first demonstration

This year marks the 25th anniversary of the first demonstration of quantum cascade lasers by Faist, et al. QCLs have proven to be unique sources of mid wave infrared (MWIR) and long wave infrared (LWIR) radiation, capable of operating in continuous wave mode at room temperature. QCLs are the only solid state sources for generating laser radiation at these wavelengths, converting electrical power directly into optical energy. The only other sources that meet the criterion of direct conversion of electrical power into optical power are the molecular gas lasers, such as carbon dioxide and carbon monoxide, which were first demonstrated fifty five years ago. Both of these systems have been commercially developed to produce high powers necessary for many industrial, defense and medical applications. However, both the CO2 and the CO laser operate on multiple discrete infrared transitions of the respective molecules and thus are only discretely tunable over limited wavelength ranges, from ~9.0 μm to ~11.5 μm for the CO2 laser (using normal and isotopic CO2) and from ~5.0 μm to ~7.0 μm for the CO laser. On the other hand, QCLs are able to cover a spectral range from ~3.5 μm to terahertz, albeit, with much lower power. In spite of the lower powers available from QCLs, there are a very broad range of applications that have made QCLs attractive for continued research and development.