Solid-State Single-Photon Counter

Avalanche photodiode used to detect single photons if cooled to optimum temperature and overbiased beyond breakdown voltage. Offers two to three times sensitvity of photomultiplier tubes commonly used for photon detection. When cooled to reduce number of thermal carriers, avalanche photodiode reverse-biased beyond normal breakdown voltage to give it extremely-high internal gain of 10 to seventh power to 10 to eigth power. Advantages of solid-state devices to applications requiring low-level light detection, optical communication, astronomy, remote sensing, optical metrology, and optical signal processing.