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.
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-20521
This Brief includes a Technical Support Package (TSP).
Pyramid-Shape Light Coupler for Multi-Quantum-Well Photodetectors
(reference NPO20521) is currently available for download from the TSP library.
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