Imaging of chemical species using mid-infrared light is a key technique for bioimaging and environmental monitoring, but the mid-IR detectors needed for the task have tended to 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 at present not on par with silicon-based CCD cameras for visible light.
Researchers have now demonstrated a way to make a standard Si-based CCD used with visible light respond instead to incoming mid-IR irradiation, through a non-degenerate two-photon absorption method. This potentially allows such CCDs to perform mid-IR chemical imaging.
Exploiting non-linear optics (NLO) in the detection of mid-IR signals has been investigated before, but has usually involved the use of a suitable NLO medium or crystal, added to the Si-based detector. This breakthrough has been to arrange instead for non-degenerate two-photon absorption (NTA) to take place. 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 exactly identical.