Method could lead to smaller active/passive IR cameras

Used in civilian and military applications, the cameras often fall into one of two types: active, which use an invisible IR source to illuminate the scene, usually in the near or short-wavelength IR; and passive, which detect the thermal radiation given off by a warm object, typically in the mid- or long-wavelength IR, without the need for any illumination.

Until now, dual-mode active and passive IR cameras needed either two different IR detectors or complex controllable filters to accommodate the different wavelengths and then required additional signal processing to reconstruct a single image from the two modes. 

However, Northwestern University’s Center for Quantum Devices claims to have found a way to integrate active and passive IR imaging capability into a single chip. According to Northwestern, this opens the way to lighter and simpler dual-mode active/passive cameras with lower power dissipation.

A paper about the findings, ‘Active and passive infrared imager based on short-wave and mid-wave type-II superlattice dual-band detectors’, was published on 1 January in the journal Optic Letters. The work was led by Manijeh Razeghi, Walter P Murphy professor of electrical engineering and computer science in Northwestern’s McCormick School of Engineering and Applied Science. 

The researchers achieved this by engineering the quantum properties of the novel semiconductor materials indium arsenide/gallium antimonide (InAs/GaSb) type-II superlattices.

Researchers at the centre are said to have been pioneering the development of type-II superlattices as a superior replacement of ageing mercury-cadmium-telluride (HgCdTe) IR camera technology in terms of performance and cost. 

Using the unique band-structure engineering capabilities of type-II superlattices, they have developed a new structure incorporating two different superlattices with different layer spacings, thus enabling detection with a cut-off wavelength of either 2.2µm (active mode) or 4.5µm (passive mode).

This new device can simply switch from passive to active mode by a very small change in bias.

The work was funded by the Defense Advanced Research Projects Agency.