Tridirectional diffraction gratings have been proposed to provide optical coupling to quantum-well infrared photodetectors (QWIPs) in focal-plane arrays. The tridirectional gratings would be improved versions of the bidirectional (rectangular-pattern) gratings described in "Cross-Grating Coupling for Focal-Plane Arrays of QWIPS" (NPO-19657), NASA Tech Briefs, Vol. 22, No. 1 (January 1998), page 6a. The tridirectional gratings would comprise metal patches or holes arranged in a pattern of closely packed regular hexagonal cells (see figure), with dimensions chosen to optimize diffraction patterns to maximize coupling within narrow spectral bands of interest.
A brief review of the problem of coupling light into a QWIP is prerequisite to an explanation of the improvement expected to accrue from the use of the tridirectional gratings. The QWIP light-coupling problem results from three considerations: (1) the direction through the thicknesses of the quantum wells is perpendicular to the focal plane; (2) 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 (3) 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.
By diffracting light so that at least some of it propagates within the quantum wells at angles other than perpendicular to the focal plane, one changes the plane of polarization so that the through-the-thickness component of polarization is increased. Thus, coupling is increased. The problem in designing a grating (whether linear, bidirectional, or tridirectional) for coupling is to choose the dimensions of the grating to maximize the first-order diffraction of optical power to suitable angles away from the perpendicular. Because a tridirectional grating would exhibit the same periodicity along three directions in the focal plane (instead of only two directions for a corresponding rectangular-pattern cross grating or one direction for a corresponding linear grating), the tridirectional grating would diffract more light to higher angles and thus give rise to increased coupling.
One might be tempted to extend the tridirectional-grating concept to gratings with octagonal or even more complex unit cells in the hope of exploiting periodicity along a greater number of directions. However, this approach would be unlikely to yield any improvement because of a fundamental geometric limitation: Any packing of regular octagons or regular higher polygons in a regular pattern would necessarily create empty interstitial areas.
This work was done by Sumith V. Bandara, Sarath Gunapala, Daniel Wilson, and John K. Liu of Caltech for NASA's Jet Propulsion Laboratory.
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Refer to NPO-20703
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Tridirectional Gratings as Improved Couplers for QWIP's
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