Nonlinear interferometers based on spontaneous parametric down-conversion for Fourier-transform mid-infrared spectroscopy

The interference effects of correlated photon pairs inside nonlinear interferometers allow measurements with undetected photons. One application of these effects is mid-infrared spectroscopy with visible or near-infrared light detection. In this work, the concept of classical Fourier-transform infrared spectroscopy is adapted to nonlinear interferometers based on induced coherence. The mid-infrared spectrum and its modification by a spectroscopic sample can then be obtained from a Fourier-transform analysis of the detected visible or near-infrared interferogram. This newly developed measurement approach allows improving the spectral resolution and accuracy of nonlinear interferometers - for the first time, rotational-line-resolving spectroscopy with undetected photons is demonstrated. In addition, this work aims at a thorough understanding of the physical properties and effects of the correlated photon source and the nonlinear interferometer.