Researchers Develop Spectroscope with Unrivalled Performance

A spectroscope with unrivalled performance, able to identify tiny amounts of trace gases in real time, has been developed by researchers from CNRS's Molecular Photophysics Institute (Laboratoire de Photophysique moléculaire, LPPM) and the Max Planck Institute of Quantum Optics (Germany), within the framework of the European Laboratory for Frequency Comb Spectroscopy. Led by Theodor W. Hänsch, Nobel Laureate in Physics (2005) and Nathalie Picqué of LPPM, the international team has designed an instrument based on two femtosecond laser frequency combs. Thanks to the breakthrough in performance it has achieved, this spectrometer could become the new reference in ultrasensitive spectroscopy. This is a major step forward both for fundamental research and for many applied fields, and is explained in detail in the advance online edition of the journal Nature Photonics.

There has been much interest in the spectroscopy of trace gases in recent years. Due to its very great sensitivity, such absorption spectroscopy can be used to identify compounds at very low concentrations. It is used not only in fundamental research but also in fields such as metrology, the physical chemistry of the interstellar medium, in situ detection of trace amounts of air pollutants (whether accidental or illegal), monitoring of industrial processes, etc. However, to develop an effective spectrometer a number of characteristics need to be brought together. It must:

• be able to explore a wide range of wavelengths with a single measurement, which makes it possible to obtain information about many energy levels simultaneously in fundamental spectroscopy, or measure the presence of several molecules simultaneously in applied fields;
• have a good resolution limit, in other words be able to finely distinguish between the different wavelengths that make up the spectrum. This is necessary in fundamental spectroscopy in order to understand the dense spectrum of complex molecules, and in applied fields in order to unambiguously distinguish between the various molecules present in the medium under study;
• have a rapid measurement time so as to be able to observe transient phenomena (chemical reactions, explosions, etc) in real time;
• be highly sensitive in order to observe weak molecular transitions in fundamental spectroscopy, and compounds at low concentrations in the medium under study in applied fields.

Until now, no instrument had combined these four criteria simultaneously.

Now for the first time, the spectrometer developed by the French-German team has found a compromise between these constraints by being based on a high-finesse cavity and two frequency combs (2). This technique makes it possible to record spectra with great sensitivity, and a million times faster than the best current spectrometers (3). In a demonstration, the spectrum of ammonia, a molecule of great importance in environmental and planetary science, was measured in a mere 18 µs: the sensitivity obtained was 20 times better, with a measuring time 100 times shorter, than the demonstration of feasibility that held the previous record. With such high sensitivity, and the possibility of being extended to all the regions of the electromagnetic spectrum, this method would be able to explore dynamically the mid-infrared, the region of ‘molecular fingerprints', where no effective spectroscopic technique in real time exists.

There are many other potential applications in a wide range of fields, such as analytical chemistry, plasma physics and laboratory astrophysics, as well as biomedicine, environmental surveys, safety, etc.