This book is a sequel of The Physics of Quantum Well Infrared Photodetectors (1997), which covered the basic physics of QWIPs. In the intervening 27 years, QWIP properties pertinent to infrared detection are much better understood, and QWIP technology has become a mainstream, widely deployed infrared technology. The main progress is the ability to know the QWIP absorption quantum efficiency quantitatively through rigorous electromagnetic modeling. The lack of theoretical prediction has impeded QWIP development for a long time. Generally, an arbitrary choice of detector structures yields substantial variations of absorption properties, and QWIP was regarded as a low quantum efficiency detector. With the advent of electromagnetic modeling, quantum efficiency of any detector geometry can be known exactly and be optimized to attain a large satisfactory value. Consequently, all properties of QWIPs are predictable, not unlike prevailing silicon devices. This unique characteristic enables QWI