Scientists at the University of California, Irvine developed a spectral imaging technique that is poised to help researchers and industries across multiple fields, including medical and tech, quickly visualize the chemical composition of tissues and various materials. The mid-infrared (MIR) imaging technique is based on nondegenerate two-photon absorption (NTA) of temporally chirped optical MIR pulses. Called chirped-pulse NTA (CP-NTA), the technique also enables high-speed imaging, acquiring spectral data cubes at high-pixel density in under a second without the need for complex image processing or reconstruction. Spectral imaging in the MIR range provides simultaneous morphological and chemical information for a wide variety of samples. NTA-based MIR detection offers the ability to collect MIR images at high definition (HD). However, in conventional systems, the HD imaging capability does not come at the expense of speed, as NTA benefits from the mature readout technology of visible/near-infrared (NIR) cameras that permit MIR imaging at high frame rates. In their paper, the researchers said, “MIR imaging historically suffers from technological limitations associated with detection of infrared radiation. Traditional detection approaches are based on the linear absorption of MIR light by small-bandgap semiconductors, such as mercury cadmium telluride (MCT) or indium antimonide (InSb).” According to the researchers, thermal noise can negatively affect single-element or matrix arrays such as these. As a result, they require active cooling, which can descend into cryogenic temperature ranges. “In addition, for arrayed detectors, there is a trade-off between pixel density and the overall response time of the detector assembly,” the researchers said. The researchers said a new cohort of MIR imaging approaches has been developed to address limitations of arrayed MIR detectors. In previous work, they said, the researchers used NTA in large-bandgap semiconductor materials to directly detect MIR radiation on the chip of visible or NIR cameras. This allowed the team to collect chemically selective MIR images with a megapixel indium gallium arsenide (InGaAs) camera at 500-Hz frame rates. With the current work, the researchers took advantage of the NTA imaging conditions. Using temporally chirped MIR pulses and a short NIR gate pulse, the researchers acquired MIR spectral data cubes over a spectral range of greater than 400 cm−1 in under one second with an InGaAs camera. The CP-NTA imaging technique has the potential for true video-rate hyperspectral data acquisition of live processes, in which full data cubes could be collected in real time. The first prototype of the imaging technology could only deliver individual image frames of a particular color, and it could not capture the full MIR spectrum. Still, the work demonstrates the ability to rapidly capture images where each pixel of the video frame contains full spectral information. “It’s the difference between a black-and-white television and a color TV,” said professor Eric Potma. One of the keys to successfully developing the CP-NTA technology was providing it with the ability to quickly capture and differentiate IR wavelengths needed to compose images. The CP-NTA imaging technique detects colors in the IR using a process that is similar to how smartphone cameras record different colors in the visible light spectrum to create photographs. “And colors reveal spectroscopic lines, which, in turn, provide a chemical fingerprint,” said professor Dmitry Fishman. Chemical bonds only vibrate and absorb light in the IR portion of the spectrum. Spectral imaging in the MIR allows direct spatial differentiation of chemical composition. “Mid-IR light is associated with chemical bonds,” said researcher Dave Knez. “With this technology, we can more confidently say there’s a particular chemical, or a given chemical bond, in a sample.” The CP-NTA imaging technology will make it easier for scientists to accurately assess the composition of the materials, including human tissues. It could be useful for many other applications in the chemical, medical, and bio-related fields. According to Potma, CP-NTA imaging could also dramatically speed up and improve tests used in the medical field to analyze tissues affected by disease. “We have plans to employ this technology for solving real problems, from visualization of fundamental chemical processes to cancer research, histopathology, tissue dynamics, and a vast number of industry applications — everywhere the ability to see chemistry in real time is critical,” Potma said. The research was published in Science Advances (www.science.org/doi/10.1126/sciadv.ade4247).