Quantum cascade detectors (QCDs) are atypical mid-infrared (MIR) detectors that challenge the long-held perceptions of the photonics world about MIR detectors. A room-temperature detector with high specific detectivity, extremely low noise, a speed well within the optical communication domain, and a small package is generally not associated with an MIR detector QCDs however, have achieved this feat. Hamamatsu Photonics is the first company to launch room-temperature operating QCDs to the market [1].
Principle of Quantum Cascade Detectors
Quantum cascade detectors (QCDs) share design similarities with quantum cascade lasers (QCLs) in the sense that both use intraband transitions in semiconductor superlattice structures [2] . In QCDs, absorption of MIR photons leads to intraband transitions, and the resultant photoelectrons cascade down the quantum levels created by the superlattice structure. This design does not require external voltage bias. In other words, QCDs do not need a power source for operation. However, the more significant advantages of this feature are room temperature operation and low noise characteristics. This low noise characteristic boosts the specific detectivity of QCDs to greater than 1 × 109 cm √ Hz /W. The design of QCDs also allows them to operate at high speeds, reaching 20 GHz (3 dB cut-off) [3] . The wavelength sensitivity of QCDs can be tuned by the design of the superlattice structure, as with the output wavelength tuning of QCLs.
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