Center for Quantum Devices graduate student Edward Huang holds a lighter and a narrow-band filter centered at 11.3 µm. The flame can only be seen when imaged with the band-pass detectors sensitive up to 13 µm (right) but not in the ones with shorter detection wavelength up to 9.5 µm (left). Image: Northwestern University
Recent breakthroughs have enabled scientists from the Northwestern University’s Center for Quantum Devices
to build cameras that can see more than one optical waveband or “color” in the
dark. The semiconducting material used in the cameras—called type-II
superlattices—can be tuned to absorb a wide range of infrared wavelengths, and
now, a number of distinct infrared bands at the same time.
The idea of capturing light simultaneously at different
wavelengths isn’t new. Digital cameras in the visible spectrum are commonly
equipped with detectors that sense red, green, and blue light to replicate a
vast majority of colors perceived by the human eye. Multi-color detection in
the infrared spectrum, however, offers unique functionalities beyond color
representation. The resonant frequencies of compounds can often be found in
this spectral range, which means that chemical spectroscopy can be relayed in
images real time.
“When coupled with image-processing algorithms performed on
multiple wavebands, the amount of information rendered in a particular scene is
tremendous,” says Manijeh Razeghi, Walter P. Murphy Professor in Electrical
Engineering and Computer Science at the McCormick School of Engineering and
director of the Center for Quantum Devices.
Razeghi’s group engineered the detection energies on the
cameras to be extremely narrow, close to one-tenth of an electron volt, in what
is known as the long-wave infrared window. Creating the cameras was difficult,
however, because the light-absorbing layers are prone to parasitic effects.
Furthermore, the detectors were designed to be stacked one on top of another,
which provided spatially coincident pixel registration but added significantly
to the growth and fabrication challenges. Nevertheless, a dual-band long-wave
infrared 320-by-256 sized type-II superlattice camera was demonstrated for the
first time in the world, the results of which were published in Optics Letters.
Such infrared photon cameras based on another material
called HgCdTe were used in disaster relief in March 2011 when a catastrophic
tsunami damaged Japans’ nuclear reactors. These cameras provided accurate
temperature information about the reactors from unmanned aerial vehicles,
providing officials the information they needed to orchestrate cooling efforts
and prevent nuclear meltdown.
HgCdTe, however, is considered to be an expensive technology
in the long-wave infrared due to its poor spectral uniformity and therefore
yield—areas in which type-II superlattices may prove more efficient.
“Type-II superlattices can be grown uniformly even at very
long-wavelengths because its energy gap is determined by the alternating InAs
and GaSb quantum well thicknesses, rather than its composition as is the case
with HgCdTe,” Razeghi says. The high-resolution multi-band type-II superlattice
camera also offered very impressive performances, requiring only 0.5 msec to
capture a frame with temperature sensitivities as good as 0.015 C. “The high
performance, multifunctionality, and low cost offered by type-II superlattices
truly make it an attractive infrared technology,” she adds.