Imaging chemical species using mid-infrared light is a key bioimaging and environmental monitoring technique. Still, the mid-IR detectors needed for the task have tended to be expensive, complex, and relatively inefficient. Current mid-IR cameras are based on low bandgap semiconductor materials that require cryogenic cooling to reduce thermally excited electronic noise, and electronically cooled alternatives are not on par with silicon-based CCD cameras for visible light.
Researchers have now demonstrated a way to make a standard Si-based CCD with visible light respond instead to incoming mid-IR irradiation through a non-degenerate two-photon absorption method. It allows such CCDs to perform mid-IR chemical imaging.
Exploiting non-linear optics (NLO) in detecting mid-IR signals has been investigated before. Still, it usually involves using a suitable NLO medium or crystal added to the Si-based detector. This breakthrough has been to arrange for non-degenerate two-photon absorption (NTA) instead. NTA involves the absorption of two photons by a molecule but refers specifically to a case where the energies of the photonics are similar but not identical.
NTA in wide bandgap semiconductor materials has been shown to permit the detection of MIR radiation at room temperature with the help of an additional visible or NIR probe. NTA does not depend on phase matching and avoids the need for an NLO crystal altogether, offering a much simpler detection strategy.
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