Ultra-short light pulses enable high-precision ‘artificial nose’

14 Mar 2024

TU Vienna improves spectroscopy with “stimulated Raman emission” method; claims faster, more precise results.

Researchers at the Technical University of Vienna (“TU Vienna”), Austria, have developed a novel spectroscopy method. Using a series of laser pulses, chemical analyses can be carried out much faster and more precisely than before, says the group’s statement.

“Whether you want to analyze environmental samples in nature or monitor a chemical experiment, you often need highly sensitive sensors that can sniff out even tiny traces of a certain gas with extreme accuracy,” explains the group’s announcement.

“Variants of Raman spectroscopy are often used for this purpose: different molecules react in characteristic ways to light of different wavelengths. If you irradiate a sample with the appropriate light and measure exactly how the light is modified by the sample, you can find out whether the sample contains a certain gas or not.”

The new TU Vienna method has been developed to generate and precisely control suitable light for such experiments. This not only enables much greater accuracy than before, the method also works without moving parts and is therefore much faster than the best technologies to date is described in Nature’s Light: Science and Applications journal.

Stimulated Raman emission

The basis of the new technology is so-called “stimulated Raman emission”. A sample is irradiated with light that consists of two slightly different wavelengths. A molecule in the sample can therefore be hit simultaneously by two photons that carry slightly different amounts of energy. The high-energy photon and the lower-energy photon suddenly become two lower-energy photons – the remaining energy difference leads to the molecule suddenly having a little more energy than before.

The atoms of the molecule can be stimulated to wobble or rotate, for example. Observing this process can indicate whether the target molecule is actually present in the sample.

Researcher Hongtao Hu, from the Institute of Photonics at TU Vienna, and first author of the Nature paper, explained, “You have to carefully try out one wavelength after another – for example, by directing the light at a crystal and then slowly rotating the angle or changing the temperature of the crystal so that the sample is hit by many different wavelengths over time.”

At TU Vienna, Prof. Andrius Baltuska’s research group cooperated with Dr. Xinhua Xie from SwissFEL at Paul Scherrer Institute, Switzerland, and Prof. Alexei Zheltikov from the Department of Physics and Astonomy at Texas A&M University, USA, to measure the Raman using a special light source.

Prof. Baltuska’s group had been working on this light source for years. Hongtao Hu and the co-authors have now been able to show through extensive computer simulations that it can achieve much higher precision than conventional methods. “We don’t just produce one wavelength, but a series of ultra-short light pulses,” said Prof. Baltuska. “Each of these pulses has a duration in the femtosecond range.”

The light pulse series do not have a specific wavelength; they comprise many different wavelengths. The decisive factor is the phase of the light waves. “By changing the phase, we can shift all these wavelengths that make up the pulse a little at the same time,” said Hongtao Hu. “You then get a Raman signal at very specific wavelengths, but not at others. Our method therefore allows us to examine a specific wave range in a very elegant way without having to adjust any moving parts. In principle, this allows us to differentiate between very different molecules.”

Higher spectral resolution

Hongtao Hu was able to show the longer the series of light pulses, the higher the precision achieved. He said, “You can therefore achieve a significantly higher spectral resolution with a series of many individual pulses than ever before. In principle, it is also possible to distinguish Raman transitions from each other that come from different molecules, whose signals looked almost exactly the same if the spectral resolution is not high enough. Possible applications for the new technology range from environmental analysis to quality assurance in the chemical industry and biological imaging.”