Improving Infrared Imaging

Zoomed-in SEM image showing the air-gaps sandwiched by two channels. (Source: NWU)

A new method developed by North­western Engi­neering’s Manijeh Razeghi has greatly reduced a type of image distortion caused by the presence of spectral cross-talk between dual-band long-wavelength photo­detectors. The work opens the door for a new gene­ration of high spectral-contrast infrared imaging devices with appli­cations in medicine, defense and security, planetary sciences, and art preser­vation.

“Dual-band photo­detectors offer many benefits in infrared imaging, including higher quality images and more available data for image processing algo­rithms,” said Razeghi, Walter P. Murphy Professor of Electrical and Computer Engi­neering in the McCormick School of Engi­neering. “However, perfor­mance can be limited by spectral cross-talk inter­ference between the two channels, which leads to poor spectral contrast and prevents infrared camera tech­nology from reaching its true potential.”

Dual-band imaging allows for objects to be seen in multiple wave­length channels through a single infrared camera. The use of dual-band detection in night-vision cameras, for example, can help the wearer better dis­tinguish between moving targets and objects in the background. Spectral cross-talk is a type of distortion that occurs when a portion of the light from one wavelength channel is absorbed by the second channel. The issue becomes more severe as the detec­tion wave­lengths get longer.

To suppress that, Razeghi and her group in the Center for Quantum Devices developed a novel version of a distri­buted Bragg reflector (DBR), a highly-refrac­tive, layered material placed between channels that separates the two wave­lengths. While DBRs have been widely used as optical filters to reflect target wave­lengths, Razeghi’s team is the first to adapt the structure to divide two channels in an anti­monide type-II superlattice photo­detector, an important element of night-vision cameras that the researchers previously studied.

To test their design, the team compared the quantum efficiency levels of two long-wave­length infrared photo­detectors with and without the air-gapped DBR. They found notable spectral suppression, with quantum effi­ciency levels as low as ten percent, when using the air-gapped DBR. The results were confirmed using theo­retical calcu­lations and numerical simulation. (Source: NWU)

Reference: Y. Zhang et al.: Suppressing Spectral Crosstalk in Dual-Band Long- Wavelength Infrared Photodetectors With Monolithically Integrated Air-Gapped Distributed Bragg Reflectors, IEEE J. of Quant. Elec. 55, 4000106 (2019); DOI: 10.1109/JQE.2018.2882808

Link: Center for Quantum Devices, Northwestern University, Evanston, USA

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