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.

InAs/InAsSb Strained-Layer Superlattice Mid-Wavelength Infrared Detector for High-Temperature Operation.

This paper reports an InAs/InAsSb strained-layer superlattice (SLS) mid-wavelength infrared detector and a focal plane array particularly suited for high-temperature operation. Utilizing the nBn architecture, the detector structure was grown by molecular beam epitaxy and consists of a 5.5 µm thick n-type SLS as the infrared-absorbing element. Through detailed characterization, it was found that the detector exhibits a cut-off wavelength of 5.5 um, a peak external quantum efficiency (without anti-reflection coating) of 56%, and a dark current of 3.4 × 10-4 A/cm2, which is a factor of 9 times Rule 07, at 160 K temperature. It was also found that the quantum efficiency increases with temperature and reaches ~56% at 140 K, which is probably due to the diffusion length being shorter than the absorber thickness at temperatures below 140 K. A 320 × 256 focal plane array was also fabricated and tested, revealing noise equivalent temperature difference of ~10 mK at 80 K with f/2.3 optics and 3 ms integration time. The overall performance indicates that these SLS detectors have the potential to reach the performance comparable to InSb detectors at temperatures higher than 80 K, enabling high-temperature operation.

Simultaneous detection of ultraviolet and infrared radiation in a single GaN/GaAlN heterojunction.

Results are presented for a dual-band detector that simultaneously detects UV radiation in the 250-360 nm and IR radiation in the 5-14 microm regions with near zero spectral cross talk. In this detector having separate UV- and IR-active regions with three contacts (one common contact for both regions) allows the separation of the UV and IR generated photocurrent components, identifying the relative strength of each component. This will be an important development in UV-IR dual-band applications such as fire-flame detection, solar astronomy, and military sensing, eliminating the difficulties of employing several individual detectors with separate electronics-cooling mechanisms.

Dual-band pixelless upconversion imaging devices.

We have proposed a type of mid-infrared (MIR) and far-infrared (FIR) dual-band imaging device, which employs the photon frequency upconversion concept in a GaN/AlGaN MIR and FIR dual-band detector integrated with a GaN/AlGaN violet light emitting diode. On the basis of the photoresponse of single-period GaN/AlGaN dual-band detectors, we present the detailed optimization of multiperiod GaN emitter/AlGaN barrier detectors and their applications to dual-band pixelless upconversion imaging. Satisfying images have been received through the analysis of the modulation transfer function and the upconversion efficiency in the GaN/AlGaN dual-band pixelless upconverters, which exhibit good image resolution, high quantum efficiency, and negligible cross talk.