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On a battlefield, knowledge is power. Imaging systems monitor sites for weapons and personnel, or acquire targets for attack. Capturing clear, high-resolution images in the lab poses little challenge, but battlefield conditions are seldom so kind—dust, fog, camouflage, and cover of night act to obscure targets. The demands of stealth and portability mandate the lightest possible systems even as performance demands rise. Defense research labs are prepared to meet the challenge, however, with the development of novel detector architectures that promise to increase imaging capabilities across all wavelength bands.
A simple lens produces a curved focal surface while today’s focal-plane arrays (FPAs) are flat. With enough lenses, nearly any image wavefront can be planarized, but at the cost of considerable size and weight, as anyone who has ever seen a semiconductor lithography objective knows. Developing high-resolution, wide field-of-view imagers that are compact and lightweight enough to be used by soldiers and in unmanned aerial vehicles presents an ever-increasing challenge. Often, the results are only well-corrected for a narrow field-of-view. The use of aspheric optics can mitigate those effects to some degree, but the trade-off is added cost.
Instead of designing a complex lens system to match the wavefront to the detector, scientists at the Defense Advanced Research Projects Agency (DARPA) have decided to go at the problem of another way by designing a detector to match the wavefront. The Hemispheric Array Detector for Imaging (HARDI) Program focuses on developing a curved FPA that will match the focal surface produced by a simple convex lens. Such a detector could enable low-aberration, wide field-of-view imagers for use in unmanned vehicle and perimeter defense applications.
The curved focal plane approach is nothing new. Previous groups have reported FPAs with radii of curvature as small as a few meters. The goal for the HARDI program is far more ambitious—to develop a FPA with radii of curvature on the order of 1 cm and an operating wavelength of 400 nm to 1.9 μm. The materials currently used over that wavelength range, including silicon and mercury cadmium telluride (MCT), involve brittle, planar substrates that are incompatible with such tight curvatures, meaning that engineers have in some ways replaced one challenge with another. “Although the program will leverage existing technologies for readout integrated chips (ROICs), a new approach is required for materials, focal plane array design, and fabrication,” says HARDI program manager Devanand Shenoy.
It’s a difficult challenge but nanotechnology comes to the rescue. In one project associated with the HARDI program, researchers from the University of California, Santa Barbara and collaborators have blended small-bandgap semiconductor polymers with fullerene derivatives to yield polymer photodetectors with spectral response over the wave band from 300 nm to 1,450 nm. The devices consist of a combination of the small-band-gap conjugated polymer, poly (5,7-bis (4-decanyl-2-thienyl)-thieno (3, 4-b) diathiazole-thiophene-2,5) (PDDTT), blended with the fullerene derivative, (6,6)-phenyl-C61-butyric acid methyl ester (PC60BM), and spin cast onto a substrate. “We literally mix the materials together in a common solvent and cast from solution,” says UCSB physics professor Alan Heeger. “The phase-separated nanostructure forms spontaneously as the solvent evaporates.”