Approaches to low-cost infrared sensing.

The Air Force Research Laboratory's Sensors Directorate has multiple missions, including the development of next generation infrared sensors. These sensors reflect advancements in both academic and research communities, as well as requirements flow-down from operators. There has been a multitude of developments over the past decade in each community. However, there has also been consilience that low-cost infrared sensing will be necessary for the Air Force. This paradigm stands in contrast to the current generation of high performance infrared sensors, i.e., cryogenically cooled, hybridized HgCdTe, InSb, and III/V strained layer superlattices. The Sensors Directorate currently has a multi-pronged approach to low-cost infrared sensing to meet this paradigm shift, including research in silicides, SiGeSn, and lead salts. Each of these approaches highlights our integration of materials, devices, and characterization.

Microconical silicon mid-IR concentrators: spectral, angular and polarization response.

It is widely discussed in the literature that a problem of reduction of thermal noise of mid-wave and long-wave infrared (MWIR and LWIR) cameras and focal plane arrays (FPAs) can be solved by using light-concentrating structures. The idea is to reduce the area and, consequently, the thermal noise of photodetectors, while still providing a good collection of photons on photodetector mesas that can help to increase the operating temperature of FPAs. It is shown that this approach can be realized using microconical Si light concentrators with (111) oriented sidewalls, which can be mass-produced by anisotropic wet etching of Si (100) wafers. The design is performed by numerical modeling in a mesoscale regime when the microcones are sufficiently large (several MWIR wavelengths) to resonantly trap photons, but still too small to apply geometrical optics or other simplified approaches. Three methods of integration Si microcone arrays with the focal plane arrays are proposed and studied: (i) inverted microcones fabricated in a Si slab, which can be heterogeneously integrated with the front illuminated FPA photodetectors made from high quantum efficiency materials to provide resonant power enhancement factors (PEF) up to 10 with angle-of-view (AOV) up to 10°; (ii) inverted microcones, which can be monolithically integrated with metal-Si Schottky barrier photodetectors to provide resonant PEFs up to 25 and AOVs up to 30° for both polarizations of incident plane waves; and iii) regular microcones, which can be monolithically integrated with near-surface photodetectors to provide a non-resonant power concentration on compact photodetectors with large AOVs. It is demonstrated that inverted microcones allow the realization of multispectral imaging with ∼100 nm bands and large AOVs for both polarizations. In contrast, the regular microcones operate similar to single-pass optical components (such as dielectric microspheres), producing sharply focused photonic nanojets.