The world’s first one million-pixel Quantum Well Infrared
Photodetector (QWIP) array has been fabricated and tested by a
NASA-led team. The new detector promises to be a low-cost alternative
to conventional infrared detector technology for a wide range of
scientific and commercial applications.
“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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