In the June 12 issue of Applied Physics Letters, scientists at the Institut de Physique at Université de Neuchâtel in Switzerland describe the development of quantum cascade detectors designed to operate at 5.3 and 9 μm. Such detectors, which require no applied bias voltage to operate and, therefore, display no dark current, may find a place in small-pixel large-area focal plane arrays for various sensing and imaging applications. Fabricated on InP by molecular beam epitaxy, the 5.3- and 9-μm detectors feature 30 and 50 layers of InGaAs quantum wells and InAlAs barriers, respectively, sandwiched between N-doped InGaAs contact layers. The 5.3-μm device displays a peak responsivity of 3.2 mA/W and detectivity of 2 × 108 cm · Hz1/2/W in the background-limited IR photodetector regime, and the 9-μm device displays a responsivity of 9 mA/W and a detectivity of 3 × 109 cm · Hz1/2/W. Optimizing the thickness of the barriers and the concentration of the dopants, the researchers suggest, will enable them to reduce the number of layers in the detectors and thereby boost their responsivity.