Quantum-Dot-Based Camera Images Two Colors

Michael A. Greenwood

Infrared imaging is a familiar tool in applications as diverse as firefighting and military patrols. But because most focal plane arrays currently in use for such imaging can capture only a single color, the information provided is limited.

A single focal plane array that images multiple colors is considered the next step in IR technology. It could provide spectral information in multiple bands to better identify objects and discriminate between them, as well as offer improved temperature sensitivity.

A quantum-dot-based camera that operates at 60 K and with a reverse bias of 1.15 V captured this image of a person holding a lighter and a cup of ice. The image was collected with a long-wave IR filter. Courtesy of Sanjay Krishna.

Researchers from the University of New Mexico in Albuquerque and from QmagiQ LLC in Nashua, N.H., have developed a quantum-dot-based imaging system that captures two colors in a single array.

They created the array with indium arsenide quantum dots embedded in indium gallium arsenide quantum wells, a design known as dots in a well, or DWELL, detectors. A 320 × 256-pixel single focal plane array was created with the DWELL design. It had one indium bump per pixel to provide electrical contact.

Researcher Sanjay Krishna said the design offers several benefits besides its multicolor capability. It can be tuned to any wavelength in the mid- and long-wave infrared by adjusting the thickness of the well structure, and the responses can be tuned easily by adjusting the voltage bias.

The investigators hybridized the quantum dot focal plane array to a readout circuit from Indigo Systems Inc. of Santa Barbara, Calif., and outfitted it with either an f/2 or an f/2.3 IR lens from Janos Technology Inc. of Keene, N.H. They tested the camera at 77 K at a nominal bias voltage ranging from 0.5 to 1.0 V.

The multicolor responses ranged from the midwave infrared region (3 to 5 μm) to the long-wave infrared (8 to 12 μm). The researchers also observed a very long wavelength response (~24 μm) that was attributed to transitions between two bound states in the quantum dots. Krishna said that the DWELL approach eventually should be able to achieve at least four colors. Possible applications could include medical diagnostics.

During experiments, the researchers noticed that detection was better in the long wave than in the midwave. Possible explanations include that the long wave had more photon flux from room temperature emission and could not be measured accurately in a single pixel at the low bias applied to the camera, resulting in an overestimate of the responsivity.

The technique also resulted in low quantum efficiency. The team is looking at approaches to overcome this, including shape engineering of quantum dots and the creation of low-strain designs with many more DWELL stacks.

Applied Physics Letters, Aug. 20, 2007, Vol. 91, 081120.