Wavelength-independent light couplers in the form of planar arrays of pyramids have been proposed for use on multiple-quantum-well infrared photodetectors. Wavelength-independent light couplers are needed for focal-plane arrays of QWIPs designed to operate in multiple and/or broad wavelength bands. In the proposed pyramid light couplers, wavelength independence would be achieved by sizing and shaping the pyramids to exploit reflections and refractions that, to a first approximation, depend on geometry only.
Because of a quantum selection rule related to polarizations, a quantum-well infrared photodetector (QWIP) does not absorb light incident normal to the planes that make the quantum-well layers: The rule arises because the quantum wells can absorb only light polarized perpendicularly to the planes that bound the quantum-well layers, whereas normally incident light is polarized parallel to these planes.
Most commonly, a QWIP device is fabricated so that the planes that bound the quantum-well layers are parallel to the broad outermost faces of the device; therefore, by virtue of this quantum mechanical selection rule, light incident normal to these faces is not detected. A light coupler is needed to redirect incident light so that it traverses the QWIP layers in a direction that includes at least some vector component parallel to the planes that make the quantum wells. In other words, the function of a light coupler on a QWIP is to redirect normally incident light to oblique incidence.
Heretofore, the only wavelength-independent light couplers for QWIPs have been corrugated ones - parallel ridges and valleys. A corrugated light coupler can redirect normally incident light in a direction with vector component perpendicular, but not parallel, to the ridge lines. On the other hand, a pyramidal light coupler could redirect normally incident light along directions with vector along both mutually perpendicular axes in a plane parallel to the quantum-well surfaces; as a result, light should be coupled more efficiently by a pyramidal than by a corrugated coupler.
Corrugated light couplers are fabricated by wet chemical etching that is selective with respect to crystallographic planes. Consequently, a corrugated light coupler can be oriented only parallel to one crystallographic plane; it is not possible to fabricate two crossed, superimposed corrugated light couplers by wet chemical etching to obtain a pyramidal light coupler.
The proposed pyramidal light couplers would be fabricated by poly(methy methacrylate)- (PMMA)-pattern-transfer techniques: In preparation for fabricating an array of pyramids on a GaAs-based QWIP, PMMA would be spun over the surface of the GaAs epitaxial material. A pattern corresponding to the array of pyramids would be formed in the PMMA by electron-beam lithography and development of the electron-beam-exposed PMMA. Finally, the pattern would be transferred to the epitaxial GaAs by reactive-ion etching in a plasma generated by electron cyclotron resonance.
This work was done by Sarath Gunapala, Sumith Bandara, and John K. Liu of Caltech for NASA's Jet Propulsion Laboratory.
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