From far-infrared detectors to THz QCLs.

On this special occasion as we celebrate the 30th anniversary of quantum cascade lasers (QCLs), the author would like to reflect his own journey from the previous research on far-infrared superconducting devices to the development of THz QCLs. The initial phase of this journey spanned over 12 years (1990-2002) till we finally reached the lasing threshold, and it involved three generations of graduate students (Jurgen Smet, Bin Xu, and Ben Williams). The journey was not totally smooth, but full of joy from learning new things that are quite different from the author's previous background to the eventual witness of laser operation of the very first THz QCL developed in the author's group. This article is mostly a memoir of the author's own process in the development of THz QCLs with many unpublished episodes, and it is by no means intended as a review article of the overall field. As such, work from other groups working in the field will not be cited thoroughly.