"We are excited about the many potential applications for NASA's QWIP technology," said Dr. Murzy Jhabvala, chief engineer of NASA Goddard Space Flight Center's Instrument Technology Center.

The development effort was led by the Instrument Systems and Technology Center at NASA Goddard, Greenbelt, Md. The Army Research Laboratory (ARL), Adelphi, Md., was instrumental in the design and fabrication of the QWIP array and the Rockwell Science Center, Camarillo, Calif., provided the silicon readout and hybridization.

Engineers at NASA's Jet Propulsion Laboratory (JPL), Pasadena, Calif., also participated in the project. The new array was fabricated in Goddard's Detector Development Laboratory and tested at both Goddard and the ARL.

Infrared light is invisible to the human eye, but some types are generated by and perceived as heat. A conventional infrared detector has a number of cells (pixels) that interact with an incoming particle of infrared light (an infrared photon) and convert it to an electric current that can be measured and recorded.

They are similar in principle to the detectors that convert visible light in a digital camera. The more pixels that can be placed on a detector of a given size, the greater the resolution, and NASA's latest QWIP array is a significant advance over earlier 300,000-pixel QWIP arrays, previously the largest available.

NASA's new QWIP detector is a Gallium Arsenide (GaAs) semiconductor chip with 60 to 100 layers of detector material on top. Each layer is extremely thin, about 500 atoms thick, and the layers are designed to act as quantum wells.

Quantum wells employ the bizarre physics of the microscopic world, called quantum mechanics, to trap electrons, the fundamental particles that carry electric current, so that only light with a specific energy can release them.

If light with the correct energy hits one of the quantum wells in the array, the freed electron flows through a separate chip above the array, called the silicon readout, where it is recorded. A computer uses this information to create an image of the infrared source.

Quantum wells can be designed to detect light with different energy levels by varying the composition and thickness of the detector material layers. Thus, a detector using quantum well technology can be made to sense light (in this case, infrared) with a wide range of energy levels. This is called a broadband detector.

"The advantages of GaAs QWIP technology over other infrared detector technologies is the relative ease of fabrication which translates to low production costs and high yield, the ability to spectrally tune the infrared response of the detector over a broad portion of the infrared region (3-18 microns), the very high pixel-to-pixel uniformity and the almost non-existent low frequency (1/f) noise," said Jhabvala.

This work was conceived for, and funded by NASA Goddard. The team has recently been selected to develop a broadband (8-14 micrometers) one million-pixel QWIP array-based imaging system as part of the Advanced Component Technology (ACT) development for NASA;s Earth Science Technology Office (ESTO).

The initial development of a prototype narrowband one million-pixel QWIP array is a critical first step that significantly contributes to the feasibility of building a broadband far-infrared QWIP camera system under the ESTO program.

"The spectral response of the prototype array was between 8.4 and 9.0 micrometers and achieved background limited performance at an operating temperature of 76 Kelvin (minus 197 degrees Celsius or minus 323 degrees Fahrenheit). Numerous imaging experiments (f/2 lens) were performed at the ARL and we are continuing to improve the detector fabrication processes and the detector performance," said Jhabvala.

There are many Earth-observing applications as well as potential commercial applications for QWIP detector arrays including: studying the troposphere and stratosphere temperatures and identifying trace chemicals:
• measuring cloud layer emissivities, droplet/particle size, composition, and height:
• SO2 and aerosol emissions from volcanic eruptions:
• CO2 absorption:
• ocean/river thermal gradients and pollution:
• coastal erosion:
• tree canopy energy balance measurements:
• tracking dust particles from remote areas of the world:
• analyzing radiometers and other scientific equipment used in obtaining "ground truthing" and atmospheric data acquisition:
• ground based astronomy:
• temperature profiling:
• medical instrumentation:
• location of unwanted vegetation encroachment:
• monitoring crop health:
• monitoring deforestation of tropical rain forests:
• locating power line transformer failures in remote areas:
• monitoring pollution and effluents from industrial operations, such as paper mills, mining operations, and power plants:
• searching for thermal leaks:
• possible earthquake detection (under review by the US Geological Survey), and not least of all:
• locating new sources of spring water for bottling.

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