Infrared chemical imaging through non-degenerate two-photon absorption in silicon-based cameras

Chemical imaging based on mid-infrared (MIR) spectroscopic contrast is an important technique with a myriad of applications, including biomedical imaging and environmental monitoring. Current MIR cameras, however, lack performance and are much less affordable than mature Si-based devices, which operate in the visible and near-infrared regions. Here, we demonstrate fast MIR chemical imaging through non-degenerate two-photon absorption (NTA) in a standard Si-based charge-coupled device (CCD). We show that wide-field MIR images can be obtained at 100ms exposure times using picosecond pulse energies of only a few femtojoules per pixel through NTA directly on the CCD chip. Because this on-chip approach does not rely on phase matching, it is alignment-free and does not necessitate complex postprocessing of the images. We emphasize the utility of this technique through chemically selective MIR imaging of polymers and biological samples, including MIR videos of moving targets, physical processes and live nematodes. Mid-infrared imaging: silicon successStandard silicon CCD cameras can be used for capturing mid-infrared chemical images by harnessing two-photon absorption within the camera's image sensor. David Knez and coworkers from the University of California at Irvine, USA, used a mid-IR beam in combination with a near-infrared pump beam to induce a two-photon absorption process in silicon. The pump beam is sent straight to the camera, whereas the mid-IR beam is sent through the sample to be imaged prior to reaching the camera. Two-photon absorption of the pump and mid-IR light in the camera's CCD sensor produces an image. The team successfully generated both static images and movies of various samples including polymer films, oil drops, a bee wing and live nematodes at several different mid-IR wavelengths.