Focal-plane arrays of quantum-well infrared photodetectors (QWIPs) equipped with both light-coupling diffraction gratings and metal side reflectors have been proposed, and prototypes are expected to be fabricated soon. The purpose served by the metal side reflectors is to increase quantum efficiency by helping to trap light in the photosensitive material of each pixel.

Figure 1. Light Is Diffracted almost parallel to the focal plane for maximum quantum efficiency in a QWIP of typical prior design. However, in the absence of reflectors like those shown in Figure 2, much of the diffracted light is lost from the QWIP of a given pixel after a single pass through the photosensitive QWIP volume.

The reasons for using diffraction gratings were discussed in several prior NASA Tech Briefs articles. To recapitulate: In an array of QWIPs, the quantum-well layers are typically oriented parallel to the focal plane and therefore perpendicular or nearly perpendicular to the direction of incidence of infrared light. By virtue of the applicable quantum selection rules, light polarized parallel to the focal plane (as normally incident light is) cannot excite charge carriers and, hence, cannot be detected. Diffraction gratings scatter normally or nearly normally incident light into directions more nearly parallel to the focal plane, so that a significant portion of the light attains a component of polarization normal to the focal plane and, hence, can excite charge carriers. Unfortunately, light scattered in directions parallel or nearly parallel to the focal plane can escape sideways from the QWIP of a given pixel, as illustrated in Figure 1. The escaped light has made only a single pass through the interior photosensitive volume of the QWIP.

Figure 2. Reflective Layers of Gold in a QWIP array of the type now under development would make the light traverse the interior of each QWIP multiple times.

The quantum efficiency of the QWIP would be increased by trapping light so that it makes multiple passes through the photosensitive volume. As shown in Figure 2, the sides of the QWIP of each pixel would be coated with gold to reflect escaping light back into the interior.

This work was done by Sarath Gunapala, Sumith Bandara, John Liu, and David Ting of Caltech for NASA's Jet Propulsion Laboratory.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to

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Refer to NPO-30507.

This Brief includes a Technical Support Package (TSP).
Metal Side Reflectors for Trapping Light in QWIPs

(reference NPO30507) is currently available for download from the TSP library.

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