Narrow Results

The Inexpensive Chemical Agent Detection System (ICADS) consists of a network of affordable line-of-sight sensors, each designed to detect chemical threats passing between two points with high sensitivity and a low false-alarm rate. Each leg of the ICADS system is composed of two devices, a broadband IR transmitter, and a receiver containing a long-wave-IR spectrometer. The spectrometer continually measures the spectrum of the radiation emitted by the transmitter, which is separated from the receiver by up to several hundred meters, forming a line of protection. A chemical vapor or aerosol plume with sufficient long-wave-IR absorption causes a characteristic change in the spectrum of light collected by the receiver as the plume crosses the protected line, signaling a threat. Background measurements were conducted to determine background-limited performance. Additionally, a sensor composed of a long-wave-IR fixed-grating spectrometer and a hot-filament transmitter was designed and built. Measurements of the signal-to-noise ratio (SNR) and resolution agree with our analytical model and meet sensor requirements.

The background photon shot noise, induced by the signal itself and all other in-band surroundings, provides the fundamental infrared imaging sensitivity limit (background limited performance or BLIP). Optical diffraction constrains resolution angle to values greater than the ratio of the detection wavelength to the optics diameter. Imaging technology tries to attain BLIP at spatial frequencies approaching the diffraction limit while minimizing system size, weight, power, and life-cycle cost.

To minimize costs, detector array technology must raise the detector operating temperature while keeping dark currents and other noise mechanisms below background shot noise. This requires reducing material and process defects. Recently HgCdTe has seen the MWIR-LWIR device dark currents to be limited only by fundamental mechanisms over a wide range of temperatures and wavelengths. Novel materials systems (e.g. Type-II superlattices) in theory have even lower fundamental limits than HgCdTe, but as yet lag in demonstrated performance. At least for now HgCdTe provides a convenient well-characterized benchmark and gives insight into further improvement possibilities

Infrared imaging technology must also exploit advanced processing and packaging techniques to reduce detector pitch to a few wavelengths of detected light. Mid- and long-wave infrared imaging arrays with ~15μm detector dimensions are now approaching this goal.

We report time-resolved photoluminescence measurements on a set of long-wave infrared InAs/GaSb type II superlattice absorber samples with various widths as a function of temperature and excitation density. Careful analysis of the photoluminescence data determines the minority carrier lifetime and background carrier density as a function of temperature, and provides information on the acceptor energy and density in each sample. Results indicate that carrier lifetime is dominated by Shockley-Read-Hall recombination with a lifetime of ~30 ns at 77 K for all samples. Below 40 K, background carriers are observed to freeze-out in conjunction with increased contributions from radiative recombination. An acceptor energy level of ~20 meV above the valance band is also determined for all samples. Variations of carrier lifetime between each sample do not strongly correlate with absorber width, indicating that barrier recombination is not the dominant factor limiting the carrier lifetime in our samples.

The Inexpensive Chemical Agent Detection System (ICADS) consists of a network of affordable line-of-sight sensors, each designed to detect chemical threats passing between two points with high sensitivity and a low false-alarm rate. Each leg of the ICADS system is composed of two devices, a broadband IR transmitter, and a receiver containing a long-wave-IR spectrometer. The spectrometer continually measures the spectrum of the radiation emitted by the transmitter, which is separated from the receiver by up to several hundred meters, forming a line of protection. A chemical vapor or aerosol plume with sufficient long-wave-IR absorption causes a characteristic change in the spectrum of light collected by the receiver as the plume crosses the protected line, signaling a threat. Background measurements were conducted to determine background-limited performance. Additionally, a sensor composed of a long-wave-IR fixed-grating spectrometer and a hot-filament transmitter was designed and built. Measurements of the signal-to-noise ratio (SNR) and resolution agree with our analytical model and meet sensor requirements.