Cross-diffraction gratings have been proposed to provide optical coupling to quantum-well infrared photodetectors (QWIPs) in focal-plane arrays (FPAs). The cross-grating coupling scheme is intended to enhance optical coupling (and thus increase quantum efficiency) within narrow spectral bands; this would make it possible to construct improved wavelength-selective infrared-imaging instruments for a variety of scientific, industrial, and military applications.
The structure of a QWIP FPA poses an optical-coupling problem because of a confluence of three considerations:
- The direction through the thicknesses of the quantum wells is perpendicular to the focal plane;
- Quantum selection rules allow the detection of only that part of the incident light that is electrically polarized along the direction through the thicknesses of the quantum wells and thus perpendicular to the focal plane; and
- The light to be detected is incident along directions approximately perpendicular to the focal plane, and thus only a small fraction of it is electrically polarized along the thicknesses of the quantum wells.
Heretofore, the coupling problem has been solved by use of random reflectors, which provide reasonable coupling efficiency over a broad spectral band. Like random reflectors, crossed diffraction gratings would effect the desired conversion of polarization via scattering; however, the inherent periodicity of a diffraction grating could be exploited to enhance conversion at a desired wavelength. The design of a cross grating for this purpose involves the following mathematical model (see Figure 1): The grating is considered to be a perfectly electrically conductive slab with a rectangular array of holes through its thickness, and each hole is regarded as a waveguidelike cavity. The task is to choose the dimensions of the grating to maximize the power diffracted, at the desired wavelength, from a normally incident beam into waveguide modes with through-the-thickness polarization.
In a preliminary assessment of the utility of the cross-grating concept, the absorption quantum efficiency as a function of wavelength was computed for a QWIP FPA of 30-by-30-µm pixels with a cross grating optimized for a wavelength of 8.4 µm and light incident through f/2.3 optics. The result of this computation was compared with the measured quantum efficiency vs. wavelength of the same QWIP with a 45°-polished-edge reflector instead of a cross grating. As illustrated in Figure 2, the computation predicted that the use of the cross grating would increase the quantum efficiency at the wavelength of 8.4 µm wavelength and would reduce the width of the spectral pass band.
This work was done by Sarath D. Gunapala and Sumith V. Bandara 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. 137 on the TSP Order Card in this issue to receive a copy by mail ($5 charge).
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