A proposed focal-plane array (FPA) of quantum-well infrared photodetectors (QWIPs) would exhibit peak response at two wavelengths. Heretofore, single-focal-plane arrays of photodetectors optimized for two or more wavelengths have not been available. This FPA is intended to be a prototype of multispectral imaging devices for a variety of scientific, industrial, and military infrared instruments.
In the proposed device, the two-wavelength-peak response would be achieved by use of a multiple-alternating-quantum-well structure in AlxGa1-xAs photodetectors (see Figure 1). This structure would comprise 50 periods of alternating quantum wells. The energy depth and geometric thickness of the first well in each period would be optimized to obtain peak response at a wavelength of 8.5 µm; the corresponding parameters of the second well in each period would be optimized for peak response at a wavelength of 10.5 µm. The quantum wells would be separated by AlxGa1-xAs barriers 500Å thick.
To prevent additional quantum reflections in the continuum transport band at the locations of the barriers, all the barriers would have to be of the same energy height and thus fabricated with the same proportion (x) of aluminum. The wells for 8.5-µm peak response would be made of GaAs. The wells for 10.5-µm response, being of a different energy depth, would have to be made with a different proportion (y) of aluminum.
The left-hand part of Figure 2 shows the calculated spectral responsivity of the proposed QWIP. To increase quantum efficiency and wavelength selectivity, the FPA would also include cross diffraction gratings like those described in the companion article, "Cross-Grating Coupling for Focal-Plane Arrays of QWIPs" (NPO-19657). These gratings would be optimized for the wavelengths of 8.5 and 10.5 µm and would be positioned in alternation over successive columns. As a result, all the photodetectors in one column would exhibit peak response at one of these wavelengths and all the photodetectors in the next column would exhibit peak response at the other wavelength, as shown to the right in Figure 2. Thus, the FPA would detect two column-interlaced images of the same scene in different wavelength bands. The outputs from the photodetectors in alternate columns could be fed to two video monitors to display both wavelength images simultaneously.
This work was done by Sarath D. Gunapala, Sumith V. Bandara, True L. Lin, and Jin S. Park of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Electronic Components and Circuits category, or circle no. 138 on the TSP Order Card in this issue to receive a copy by mail ($5 charge).
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