Detecting Colors in the Dark

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 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.

Two mode-locked femtosecond laser beams of slightly different pulse repetition frequencies are superimposed with a beam splitter. One output is highly attenuated before passing through a sample and reaching a photon-counting detector. At power levels one billion times weaker than usually employed, the statistics of the detected photons carries the information about the sample with its possibly highly complex optical spectrum. (Source: N. Picqué, MPQ)

A mode-locked femto­second laser emits hundreds of thousands of sharp spectral lines which are spaced precisely evenly in frequency. Such laser frequency combs are now widely used to count the oscil­lations of a laser wave and they serve as clockworks in optical atomic clocks. The frequency comb technique has been high­lighted when the 2005 Physics Nobel Prize was awarded to Theodor Hänsch and John L. Hall. During the past fifteen years, Nathalie Picqué has harnessed frequency combs for new approaches to broadband optical spectro­scopy. In her technique of dual-comb spectro­scopy, all the comb lines of one laser inter­rogate a sample simul­taneously over a broad spectral range, and the comb lines of a second laser with slightly different spacing interfere on a fast photo­detector for read-out. Pairs of comb lines, one from each laser, produce radio­frequency beat notes in the detector signal.

These radio­frequency signals can be digitized and processed by a computer. Any optical spectral structure in the sample reappears as a corres­ponding pattern in the comb of radio-frequency signals. Optical signals are effec­tively slowed down by a large factor equal to the laser repetition frequency divided by the difference in repe­tition frequencies. The unique advantages of this powerful spectro­scopic tool include virtually unlimited spectral resolution, possible cali­bration with an atomic clock, and highly consistent acqui­sition of complex spectra without any need of scanning or mechanically moving parts.

Picqué and Hänsch have now demons­trated that dual-comb spectro­scopy can be extended to extremely low light intensities in the photon counting regime. The inter­ference signals can be observed in the statistics of the clicks of the photon counting detector, even if the power is so low, that only one click is registered over the time of 2000 laser pulses, on average. Under such circum­stances it is extremely unlikely that two photons, one from each laser, are present in the detection path at the same time. The experiment cannot be explained intuitively if one assumes that a photon exists before detection.

The ability to work at light intensities a billion-fold lower than usually employed opens intriguing new prospects for dual-comb spectro­scopy. Nathalie Picqué explains: “The method can now be extended to spectral regions where at most feeble frequency comb sources are available, such as the extreme ultraviolet or soft x-ray region. Spectro­scopic signals can be acquired through highly atte­nuating materials or through back­scattering over large distances. And it becomes feasible to extract dual comb spectra from nano­scopic samples down to single atoms or molecules, which produce only feeble fluores­cence signals.”

Theodor Hänsch remembers the moment in the laboratory when an inter­ference pattern first emerged in the statistics of detector clicks: “I felt thrilled. Even after working in laser spectro­scopy for more than 50 years, it seemed quite counter-intuitive to me that single detected photons could be aware of the two lasers with their large number of comb lines and of the complex spectrum of a sample.” (Source: MPG)

Reference: N. Picqué & T. W. Hänsch: Photon-level broadband spectroscopy and interferometry with two frequency combs, Proc. Nat. Ac. Sc.

Link: Laser Spectroscopy & Quantum Physics (T. W. Hänsch), Max-Planck Institute of Quantum Optics, Garching, Germany

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