Combining active and passive IR imaging

According to the researchers, the discovery could open up the way to lighter and simpler dual-mode active/passive cameras with lower power dissipation.

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.

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.

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.