Terahertz spectroscopy by only detecting visible light

New concepts of sensing inspired by quantum optics provide a highly attractive alternative for accessing spectral regions in which photon detection is technically challenging. By using nonlinear interferometers, it is possible to transfer the properties of photons after interaction with a sample to visible photons. Then detection can be realized by widely available and highly developed imaging sensors without the need of cooling or expensive pulsed lasers. We report on the demonstration of spectroscopy in the terahertz frequency range using this new concept measuring absorption features of chemicals in the sub-terahertz frequency range by only detecting visible photons [1].