[Enlarge image]Top: An ultraviolet photon-counting dual-comb spectrometer [B. Xu et al. Nature 627, 289 (2024)] . Bottom: Photon-counting near-ultraviolet dual-comb spectrum of weak transitions in 133Cs at an optical power of 90 pW.
A frequency comb is a spectrum of evenly spaced, phase-coherent laser lines that acts like a frequency ruler. Such optical combs are now widely used to count the oscillations of a laser wave and serve as the clockwork in optical atomic clocks. As an application beyond this original purpose, dual-comb spectroscopy has emerged as a powerful technique for precise spectroscopy over broad spectral bandwidths.1 It has mainly been used for infrared linear absorption of small molecules in the gas phase. It is based on measuring the time-dependent interference between two frequency combs with slightly different line spacings, and it does not suffer from the geometric limitations associated with traditional spectrometers, thus offering great potential for high precision and accuracy.2
However, dual-comb spectroscopy typically requires intense laser beams, making it less suitable for scenarios where low light levels are critical. We have now shown experimentally that dual-comb spectroscopy can be effectively used in starved-light conditions, at power levels more than a million times weaker than those typically used. The interference signals can be observed in the statistics of the clicks of a photon-counting detector, even when the power is so low that, on average, only one click is registered over the time of 100 laser pulses. Under such circumstances, it is extremely unlikely that two photons, one from each laser, are simultaneously present in the detection path. The experiment cannot be explained intuitively by assuming that a photon exists before detection.3
We showcased our results using two distinct experimental setups with different types of frequency-comb generators, with a signal-to-noise ratio at the fundamental limit.4 Our achievement highlights the optimal use of available light for experiments and increases the prospects for success in challenging scenarios where low light levels are essential. One of our experiments was performed in the near-ultraviolet region, where we obtained spectra with resolved-comb lines for the first time, as a step toward shorter wavelengths. Indeed, a particularly compelling future application is precise vacuum- and extreme-ultraviolet molecular spectroscopy over broad spectral spans. Currently, broadband extreme-UV spectroscopy is limited in resolution and accuracy, and it relies on unique instrumentation at specialized facilities.
Dual-comb spectroscopy at short wavelengths is particularly challenging, and our work provides a promising solution to the pressing problem of dealing with the low power of ultraviolet frequency-comb generators produced by nonlinear frequency conversion of near-infrared sources. Our results extend the full capabilities of dual-comb spectroscopy to low light conditions, unlocking novel applications in precision spectroscopy, biomedical sensing and environmental atmospheric sounding.
Researchers
Bingxin Xu, Max Planck Institute of Quantum Optics, Germany
Zaijun Chen, University of Southern California, Los Angeles, USA
Theodor W. Hänsch, Ludwig-Maximilian University of Munich, Germany
Nathalie Picqué, Max-Born-Institute for Nonlinear Optics and Short Pulse Spectroscopy, Germany
References
1. N. Picqué et al. Nat. Photonics 13, 146 (2019).
2. N. Picqué et al. Photoniques 113, 38 (2022).
3. N. Picqué et al. Proc. Natl. Acad. Sci. USA 117, 26688 (2020).
4. B. Xu et al. Nature 627, 289 (2024).