The principle of a tuning fork
The new laser spectrometer builds on technologies that were originally developed in the LAP for the production of ultrashort laser pulses, which are used to study the ultrafast dynamics of subatomic systems. The instrument, which was built by Dr. Ioachim Pupeza and his colleagues, is designed to emit trains of extremely powerful pulses of laser light that cover a broad segment of the spectrum in the infrared wavelength. Each of these pulses lasts for a few femtoseconds (in scientific notation 1 fs = 10-15s, one millionth of a billionth of a second). These extremely brief flashes of infrared light cause the bonds that link atoms together to vibrate. The effect is analogous to that of striking a tuning fork. After the passage of the pulse, the vibrating molecules emit coherent light at highly characteristic wavelengths or, equivalently, oscillation frequencies. The new technology makes it possible to capture the complete ensemble of wavelengths emitted. Since every distinct compound in the sample vibrates at a specific set of frequencies, it contributes its own well defined ‘subspectrum’ to the emission. No molecular species has anywhere to hide.
“With this laser, we can cover a wide range of infrared wavelengths – from 6 to 12 micrometers – that stimulate vibrations in molecules,” says Marinus Huber, joint first author of the study and a member of biologist Dr. Mihaela Zigman’s group, which was also involved in the experiments carried out in the LAP. “Unlike mass spectroscopy, this method provides access to all the types of molecules found in biological samples,” she explains.
Each of the ultrashort laser pulses used to excite the molecules consists of only a few oscillations of the optical field. Moreover, the spectral brightness of the pulse (i.e. its photon density) is up to twice as high as those generated by conventional synchrotrons, which have hitherto served as radiation sources for comparable approaches to molecular spectroscopy. In addition, the infrared radiation is both spatially and temporally coherent. All of these physical parameters together account for the new laser system’s extremely high sensitivity, enabling molecules present in very low concentrations to be detected and high-precision molecular fingerprints to be produced. Not only that, samples of living tissue up to 0.1 mm thick can, for the first time, be illuminated with infrared light and analyzed with unparalleled sensitivity. In initial experiments, the team at the LAP has applied the technique to leaves and other living cells, as well as blood samples.
Ioachim Pupeza and Marinus Huber are of one accord: It’s fascinating to be able to detect the signals emitted by excited molecules with such extraordinarily high sensitivity, they say. This ability to accurately measure variations in the molecular composition of body fluids opens up new possibilities in biology and medicine, and in the future the technique could find application in the early detection of disorders,” Zigman adds.
Ioachim Pupeza, Marinus Huber, Michael Trubetskov, Wolfgang Schweinberger, Syed A. Hussain, Christina Hofer, Kilian Fritsch, Markus Poetzlberger, Lenard Vamos, Ernst Fill1, Tatiana Amotchkina, Kosmas V. Kepesidis, Alexander Apolonski, Nicholas Karpowicz, Vladimir Pervak, Oleg Pronin, Frank Fleischmann, Abdallah Azzeer, Mihaela Zigman, Ferenc Krausz
Field-resolved infrared spectroscopy of biological systems
Nature, 2nd January 2020 doi 10.1038/s41586-019-1850-7