A quantum well infrared photodetector which transform long wavelength infrared radiation into short wavelength or visible radiation are proposed and considered. The quantum well infrared photodetectors under concideration utilize the intraband absorption of infrared radiation in a quantum well and interband emission in either classical or quantum well in the collector.

Schematic band diagram view of the device of this kind based on a heterostructure with two n+ and p+ narrow gap regions and N+ and P+ wide gap contacts (emitter and collector regions) is seen in the figure. The absorption spectrum is defined by the energy separation in the quantum well and spectrum of emitted radiation is defined by the energy gap of narrow gap material of the collector well.     It is shown that the intensity of output short-wavelength radiation can significantly exceed the intensity of input long-wavelength rediation, so that the optical gain can be much more than unity. The mechanism of the optical gain is connected with the injection of extra electrons from the emitter caused by the photoexcitation of the electrons from the emitter quantum well (see figure).