Fraunhofer Institute for Physical Measurement Techniques IPM
Infrared spectroscopy without infrared detectors
The mid-infrared spectral range contains a particularly large amount of information on the composition of a spectroscopic sample. Detectors for this spectral range, however, are usually technologically complex, expensive, and often require cooling. In the visible to near-infrared spectral range, on the other hand, high-performance and cost-effective silicon detectors are available. Interference effects of correlated photon pairs enable infrared spectroscopy by detecting visible light. In our »Nonlinear Optics and Quantum Sensing« team, we are conducting research on the use of this quantum technology for performant spectroscopic analytics.
Measuring with »undetected photons«
At the heart of the technology is the correlated photon source. This is realized as a nonlinear optical crystal in which photons of a pump laser beam can »split« into two correlated photons, called signal and idler. The idler photon then lies in the infrared spectral range (for spectroscopy), the corresponding signal photon in the visible or near-infrared (for detection).
In a nonlinear interferometer, the light from two such photon sources is superimposed. Since the signal and idler photons of the two processes are not distinguishable, interference occurs for both photons. If now a sample absorbs mid-infrared idler photons from the first process, the light sources become distinguishable and the idler interference contrast decreases. Since signal and idler photons are correlated, the signal interference contrast is also reduced. It is thus possible to determine the transmission of the sample for the mid-infrared idler light solely by detecting the (visible or near-infrared) signal interference pattern. This quantum effect can be used for various measurements with »undetected photons«.
The quantum Fourier transform spectrometer
The quantum Fourier transform spectrometer Q-FTIR developed at Fraunhofer IPM enables precise spectroscopy in the mid-infrared with only near-infrared light detection. For this, extremely low optical power on the sample is sufficient – more than six orders of magnitude less infrared power than a typical Fourier transform spectrometer. The Q-FTIR is based on the same measuring principle as classical instruments and thus achieves high spectral resolution. As part of the Fraunhofer lighthouse project QUILT, a highly performant mobile demonstrator for spectroscopic analysis has been realized.