The science and technology of Quantum Cascade Lasers

Quantum Cascade Lasers (QCLs) are revolutionary light sources due to their radical conceptual departure from solid-state lasers and in particular conventional semiconductor lasers and the fact that they are the first lasers in which the wavelength can be designed to cover almost the entire infrared spectrum from a few micron to hundreds of micron wavelength, with the additional advantage of unprecedented tuning range. As such they are having a major influence on spectroscopy, trace gas analysis and chemical sensing impacting a wide range of applications, which has led to full scale commercial effort by ~ 17 companies. From a physical point of view QCLs have an intrinsic line width smaller than the Schawlow-Townes limit usually applied to all lasers. They are uniquely suited for studying coherent effects due to the large Rabi frequency which, along with spatial hole burning become a key factor in dictating the dynamics of the laser. Recently we have in fact observed for the first time unambiguously the elusive Risken-Nummedal-Haken-Graham instability predicted 40 years ago for ring lasers.QCLs, because they are based on ultrafast tunneling and phonon-limited optical transitions, belong to the class of lasers which have an extremely fast gain recovery, in the range of a few ps. This has major implication for modelocking forbidding conventional mechanisms. We have recently circumvented these difficulties by achieving ps modelocking by external gain modulation of cavity sections. The talk will conclude with the applications of plasmonics to wave-front engineering of QCLs which has enabled the realization of highly collimated sources, multi-beam lasers and laser with controlled near field for sub-wavelength imaging and controlled polarization