Optica 4, 1498–1502 (2017)

Mid-infrared image sensor chips are useful for sensing gases that have absorption bands in the 3–5 μm spectral window such as methane, carbon dioxide, carbon monoxide and nitrous oxide. However, present designs are expensive as a photodetector array made from HgCdTe or InSb needs to be flip-chip bonded to silicon read-out electronics. Now, scientists in the UK claim to have made the first monolithic mid-IR imaging array based on on-chip integration of InSb photodetectors with GaAs read-out circuitry. Arrays that operate at room temperature and support video-rate imaging have been fabricated and tested with a variety of mid-IR sources including a glow bar and a quantum cascade laser emitting light at 4.57 μm. At this wavelength, a responsivity of 5,000 V W–1 and a specific detectivity of 6.12 × 107 cm Hz1/2 W–1 are obtained. While these devices are limited in their fill factor of ~14% and array size of up to 8 × 8 pixels, the team says that devices with values as large as 60% and 64 × 64 pixels, respectively, should be possible.