Novel Diffraction Pattern for Optical Coupling in QWIP
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Mat. Res. Soc. Symp. Proc. Vol. 607 0 2000 Materials Research Society
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Fig. I Schematic representation of a QWIP pixel cross-section
(2 steps)
"Al (1 step)
Fig. 3 Elliptical curve diffraction pattern formed with single etching step and having wider features.
Fig 2 Pseudo random grating having square
unit-cells formed by two etching steps
xo, yo+a tical curve #2 tical curve #1 b2 b,,
•
a2
Fig. 4 Section of elliptical curve pattern
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Moreover, it requires two etching steps. If the grating pattern is not fabricated accurately, the performance of QWIP is lowered. Therefore, it has been thought that at X,= 8.5 Pam wavelength, it is essential to use periodic grating as they have broader features than the pseudo-random grating [4]. NOVEL OPTICAL COUPLING PATTERN Our intention was to develop a diffraction pattern for optical coupling that was easier to fabricate but had better performance than the pseudo-random grating for X.--8.5 pmu.We tried several different patterns by theoretical simulation of optical coupling based on calculation of diffracted field [5]. Then reached a novel pattern that is defined by elliptical curves and is formed in a single etching step as schematically depicted in Fig. 3. Its design was optimized to minimize radiation escaping out of the pixels while maximizing random diffraction and keeping feature size broad. The pattern is mathematically represented by two elliptical curves with below mentioned conditions for region shown in Fig. 4. Points (x, y, z) in the region defined by x0 < x < xo+C and yo < Y•-< yo+C, will have z = zo-d (i.e., etched), when yo < y < yo+bl and x, < x < x2, 2 2 2 yo+b 2and xo < x _ X2; elsewhere z= zo (i.e., not etched); where x, = [a, b1 - a1 or yo+ bi
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