Optical Frequency Combs Make Ultraviolet Spectroscopy More Sensitive and More Precise

New technique enables experiments in the ultraviolet spectral region at very feeble light levels

Figure 1: An ultraviolet photon-counting dual-comb spectrometer. Two ultraviolet frequency combs of slightly different pulse repetition frequencies are generated at very low light levels by nonlinear frequency conversion of near-infrared combs. One ultraviolet comb passes through a sample. The two feeble combs are then superimposed with a beam splitter and detected by a photon-counting detector. At power levels more than one million times weaker than usually employed, the statistics of the detected photons carries the information about the sample with its possibly highly complex optical spectrum. (reproduced from doi.org/10.1038/s41586-024-07094-9)

In a recent publication in Nature, researchers at the Max Born Institute (MBI) in Berlin, Germany, and at Max-Planck Institute of Quantum Optics in Garching report on a new technique for deciphering the properties of matter with light, that can simultaneously detect and precisely quantify many substances with a high chemical selectivity. Their technique interrogates the atoms and molecules in the ultraviolet spectral region at very feeble light levels. Using two optical frequency combs and a photon counter, the experiments open up exciting prospects for conducting dual-comb spectroscopy in low-light conditions and they pave the way for novel applications of photon-level diagnostics, such as precision spectroscopy of single atoms or molecules for fundamental tests of physics and ultraviolet photochemistry in the Earth’s atmosphere or from space telescopes.

Ultraviolet spectroscopy plays a critical role in the study of electronic transitions in atoms and rovibronic transitions in molecules. These studies are essential for tests of fundamental physics, quantum-electrodynamics theory, determination of fundamental constants, precision measurements, optical clocks, high-resolution spectroscopy in support of atmospheric chemistry and astrophysics, and strong-field physics. Scientists in the group of Nathalie Picqué have now made a significant leap in the field of ultraviolet spectroscopy by successfully implementing high-resolution linear-absorption dual-comb spectroscopy in the ultraviolet spectral range. This groundbreaking achievement opens up new possibilities for performing experiments under low-light conditions, paving the way for novel applications in various scientific and technological fields.

Dual-comb spectroscopy, a powerful technique for precise spectroscopy over broad spectral bandwidths, has been mainly used for infrared linear absorption of small molecules in the gas phase. It relies on measuring the time-dependent interference between two frequency combs with slightly different repetition frequencies. A frequency comb is a spectrum of evenly spaced, phase-coherent laser lines, that acts like a ruler to measure the frequency of light with extreme precision. The dual-comb technique does not suffer from the geometric limitations associated with traditional spectrometers, and offers great potential for high precision and accuracy.

However, dual-comb spectroscopy typically requires intense laser beams, making it less suitable for scenarios where low light levels are critical. The team have now experimentally demonstrated that dual-comb spectroscopy can be effectively employed in starved-light conditions, at power levels more than a million times weaker than those typically used. This breakthrough was achieved using two distinct experimental setups with different types of frequency-comb generators. The team developed a photon-level interferometer (Fig.1) that accurately records the statistics of photon counting, showcasing a signal-to-noise ratio at the fundamental limit. This achievement highlights the optimal use of available light for experiments, and opens up the prospect of dual-comb spectroscopy in challenging scenarios where low light levels are essential.

Publication

Near-ultraviolet photon-counting dual-comb spectroscopy
Bingxin Xu, Zaijun Chen, Theordor W. Hänsch, Nathalie Picqué
Nature 627, 289-294 (2024), https://doi.org/10.1038/s41586-024-07094-9

Contact:

Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy
Prof. Dr. Nathalie Picqué
Director
030 6392-1400
Nathalie.Picque(at)mbi-berlin.de
mbi-berlin.de

Press release MBI, 13 March 2024