Optical coupling in quantum well infrared photodetector by diffraction grating

Because of the isotropic energy band structure of the Gamma electrons in a GaAs/AlGaAs quantum well infrared photodetector (QWIP), normal incident radiation absorption is not possible so that the optical grating and its optimization become key requirements for such QWIPs. In this work we study the optical grating structure on top of the QWIP based on the Kirchhoff formula. It has been shown that in general, optimal optical coupling is obtained when the wavelength of the incoming radiation is comparable to the geometric sizes of the grating structure and the photodetector. For our GaAs/AlGaAs QWIP with a wavelength response around 8.0 mm, a thin uniform reflective metal film is enough to reach optimal optical coupling. The theoretical conclusions are confirmed by the fabrication of the photoconductive QWIPs of a 1 X 128 focal plane array which works at 80 K. (C) 1998 American Institute of Physics. [S0021-8979(98)00322-3]