New generation of superlattice cameras add more 'color' to night vision

ScienceDaily (Oct. 20, 2011) — 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," said 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 the July 2011 issue of 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 said. The high-resolution multi-band type-II superlattice camera also offered very impressive performances, requiring only 0.5 milliseconds to capture a frame with temperature sensitivities as good as 0.015°C. "The high-performance, multi-functionality, and low cost offered by type-II superlattices truly make it an attractive infrared technology," she added.

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Edward Kwei-wei Huang, Abbas Haddadi, Guanxi Chen, Binh-Minh Nguyen, Minh-Anh Hoang, Ryan McClintock, Mark Stegall, Manijeh Razeghi. Type-II superlattice dual-band LWIR imager with M-barrier and Fabry–Perot resonance. Optics Letters, 2011; 36 (13): 2560 DOI: 10.1364/OL.36.002560

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New generation of superlattice cameras add more 'color' to night vision

ScienceDaily (Oct. 20, 2011) — 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," said 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 the July 2011 issue of 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 said. The high-resolution multi-band type-II superlattice camera also offered very impressive performances, requiring only 0.5 milliseconds to capture a frame with temperature sensitivities as good as 0.015°C. "The high-performance, multi-functionality, and low cost offered by type-II superlattices truly make it an attractive infrared technology," she added.

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The above story is reprinted from materials provided by Northwestern University.

Note: ScienceDaily reserves the right to edit materials for content and length. For further information, please contact the source cited above.

Journal Reference:

Edward Kwei-wei Huang, Abbas Haddadi, Guanxi Chen, Binh-Minh Nguyen, Minh-Anh Hoang, Ryan McClintock, Mark Stegall, Manijeh Razeghi. Type-II superlattice dual-band LWIR imager with M-barrier and Fabry–Perot resonance. Optics Letters, 2011; 36 (13): 2560 DOI: 10.1364/OL.36.002560

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here

Titanic jigsaw challenge: Piecing together a global color map of Saturn’s largest moon

ScienceDaily (Oct. 5, 2011) — An international team led by the University of Nantes has pieced together images gathered over six years by the Cassini mission to create a global mosaic of the surface of Titan.

The global maps and animations of Saturn's largest moon are being presented by Stéphane Le Mouélic at the EPSC-DPS Joint Meeting 2011 in Nantes, France.

The team has compiled all the infrared images acquired by the Visual and Infrared Mapping Spectrometer (VIMS) during Cassini's first seventy flybys of Titan. Fitting the pieces of the puzzle together is a painstaking task. The images must be corrected for differences in the illuminating conditions and each image is filtered on a pixel-by-pixel basis to screen out atmospheric distortions. Titan is veiled by a thick, opaque atmosphere composed mainly of nitrogen. It has clouds of methane and ethane and there is increasing evidence for methane rain. Only a few specific infrared wavelengths can penetrate the cloud and haze to provide a window down to Titan's surface. An exotic frozen world with many Earth-like geological features has progressively emerged from darkness.

Stéphane Le Mouélic explains: "As Cassini is orbiting Saturn and not Titan, we can observe Titan only once a month on average. The surface of Titan is therefore revealed year after year, as pieces of the puzzle are progressively put together. Deriving a final map with no seams is challenging due to the effects of the atmosphere -- clouds, mist etc. -- and due to the changing geometries of observation between each flyby."

Cassini has made 78 flybys of Titan since it arrived in orbit around Saturn in July 2004. A further 48 flybys are planned up to 2017. On flybys to date, VIMS has only had a few opportunities to observe Titan with a high spatial resolution. This means that the global map currently shows some regions of Titan in more detail than others.

"We have created the maps using low resolution images as a background with the high resolution data on top. In the few opportunities where we have VIMS imagery from the closest approach, we can show details as low as 500 metres per pixel. An example of this is from the 47th flyby, which allowed the observation of the site where the Huygens descent module landed. This observation is a key one as it might help us to bridge the gap between the ground truth provided by Huygens, and ongoing global mapping from orbit, which will continue up to 2017."

In addition to improving the spatial coverage, future mapping will allow the observation of seasonal changes in both the atmosphere and the surface. As spring comes to the northern hemispheres of Saturn and its moons, some areas are only now coming into view.

"Lakes in Titan's northern hemisphere were first discovered by the RADAR instrument in 2006, appearing as completely smooth areas. However, we had to wait up to June 2010 to obtain the first infrared images of the northern lakes, emerging progressively from the northern winter darkness," says Le Mouélic. "The infrared observations provide the additional opportunity to investigate the composition of the liquids within the lakes area. Liquid ethane has already been identified by this means."

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Europlanet Media Centre, via AlphaGalileo.

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