To avoid chemical staining and labeling of biomedical or environmental monitoring samples, mid-infrared (MIR) spectroscopy makes use of the molecular fingerprints of the samples. This is based on the highly specific vibrational states of the different molecules contained in the samples. In principle, infrared spectroscopy allows almost any sample to be examined non-destructively (bio)chemically without pretreatment. However, due to the relatively complex and cost-inefficient MIR technology with the necessary light sources and detectors, such spectroscopic methods are often not yet as practical as established label-based analysis and imaging for use in clinics, for example.

Taking advantage of the quantum optical effect of “sensing with undetected photons”, researchers at Humboldt University in Berlin and FBH-Berlin, in collaboration with Fraunhofer IPM Freiburg, have now developed a spectral imaging method that transfers the measurement of molecular mid-infrared fingerprints of biological and industrial samples to the detection range of silicon-based cameras. All that is needed for this is a cost-effective red laser that generates entangled photon pairs, each consisting of one near-infrared and one mid-infrared photon, in a special nonlinear crystal. The transfer of the molecular fingerprints to the near-infrared range is achieved by the interference of two such pair generation processes. By combining imaging optics and Fourier transform spectroscopy, it is possible to capture not only the two-dimensional imaging information but also the MIR transmission spectrum of the respective sample. The result is a three-dimensional data set consisting of spatial-spectral information, which is used, among other things, to visualize the biochemical composition of tissue samples, for example.