Institut des Sciences Moléculaires d'Orsay




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Regarder battre le coeur des molécules

In a recent publication in Nature, researchers from the Institut des Sciences Moléculaires d’Orsay (France) report on a new method of rapidly identifying different molecular species under a microscope. Their technique of coherent Raman spectro-imaging with laser frequency combs takes a big step towards the holy grail of real-time label-free biomolecular imaging.

How does a drug influence a living cell ? In which way can signal molecules change the cell metabolism ? Such questions are difficult to answer, since cells are highly complex “chemical factories” which constantly manufacture and break down a large number of different molecular species. Biologists have learnt to attach fluorescent dye labels to certain proteins so that they can distinguish them under a microscope. However, such labels can alter the cell functions. Many molecules of interest have characteristic absorption spectra at mid-infrared wavelengths, but such long wavelengths do not allow for good spatial resolution. Coherent Raman spectroscopy has long been used as an alternative for highly selective label-free imaging. Scanning Raman microscopes mostly focus on a distinct spectral feature of a selected molecular species so that they can provide images quite rapidly. However, for the analysis of a complex mixture of molecules with possibly unknown components, a complete Raman spectrum should be recorded for each image pixels. Existing techniques have been much too slow to accomplish such a feat.

Probing the heart beat of molecules in a liquid sample.

A team of scientists of the Institut des Sciences Moléculaires d’Orsay (France), in a collaboration with the Max Planck Institute of Quantum Optics of Garching and the Ludwig-Maximilian University of Munich, now shows that, using two laser frequency combs, complete well-resolved Raman spectra can be measured on the microsecond timescale. Interestingly, laser frequency combs, initially developed for frequency metrology, are finding exciting interdisciplinary applications far beyond their original purpose They are the key to such achievements : by rapidly changing the time-delay between pump and probe pulse without moving parts, they can acquire a broad Raman spectrum very rapidly.

Furthermore, the technique only uses a single photodetector for the measurement of a complete spectrum. Replacing the detector by a camera would make real-time hyperspectral imaging possible, as one could simultaneously measure as many spectra as there are pixels on the camera. The scientists expect that their proof-of-principle experiments open up new opportunities in spectroscopy and imaging. With further system development, they plan to explore the potential of their technique for observation of biological samples.

Original publication :
T. Ideguchi, S. Holzner, B. Bernhardt, G. Guelachvili, N. Picqué and T.W. Hänsch
Coherent Raman spectro-imaging with laser frequency combs
Nature 502, 355-358, Issue of October 17th, 2013. DOI : 10.1038/nature12607