A photon-counting dual-comb spectrometer. Source: Picqué/MPQ
Our eyes are sensitive to only three spectral color bands (red, green, blue), and we all know that we can no longer distinguish colors if it becomes very dark. Spectroscopists can identify many more colors by the frequencies of the light waves, so that they can distinguish atoms and molecules by their spectral fingerprints. In a proof-of-principle experiment, Nathalie Picqué and Theodor Hänsch from the Max-Planck Institute of Quantum Optics (MPQ) and the Ludwig-Maximilian University (LMU) have now recorded broad spectra with close to one hundred thousand colors in almost complete darkness. The experiment employs two mode-locked femtosecond laser and a single photon counting detector. Picqué and Hänsch have now demonstrated that dual-comb spectroscopy can be extended to extremely low light intensities in the photon counting regime.
