Northwestern University engineers found a way to suppress spectral crosstalk between dual-band long-wavelength photodetectors. Their discovery could reduce image distortion and improve IR imaging performance, potentially opening the door for a new generation of high spectral-contrast IR imaging devices. Spectral crosstalk is a type of distortion that occurs when a portion of the light from one wavelength channel is absorbed by the second channel. The distortion becomes more severe as the detection wavelengths get longer. Zoomed-in SEM image showing the air gaps sandwiched by two channels. Courtesy of Northwestern University McCormick School of Engineering. To inhibit spectral crosstalk, the group developed a distributed Bragg reflector (DBR), consisting of 650-nm/650-nm pairs of type II superlattices (T2SLs) and air gaps, and sandwiched the DBR between the two channels of a dual-band detector to improve the optical crosstalk between the channels. The design of the DBR was adapted to enable it to divide two channels in an antimonide T2SL photodetector, an important element of night-vision cameras. To test their design, the researchers compared the quantum efficiency levels of two long-wavelength IR photodetectors with and without the air-gapped DBR. They observed notable spectral suppression, with quantum efficiency levels as low as 10 percent when using the air-gapped DBR. The air-gapped DBR achieved a significant spectral suppression in the 4.5- to 7.5-μm photonic stopband while transmitting the optical wavelengths beyond 7.5 μm. The researchers confirmed these results using theoretical calculations and numerical simulation. High spectral-contrast IR imaging devices have numerous applications in various fields, including medicine, defense and security, planetary sciences, and art preservation. Dual-band imaging in night-vision cameras could help the wearer distinguish between moving targets and objects in the background. “Dual-band photodetectors offer many benefits in infrared imaging, including higher quality images and more available data for image-processing algorithms,” said professor Manijeh Razeghi. “However, performance can be limited by spectral crosstalk interference between the two channels, which leads to poor spectral contrast and prevents infrared camera technology from reaching its true potential.” The research was published in IEEE Journal of Quantum Electronics (http://dx.doi.org/10.1109/JQE.2018.2882808).