The World’s Shortest Laser Pulse

Thomas Gaumnitz, postdoctoral fellow in the group of ETH professor Hans Jakob Wörner with the setup that generates the shortest laser pulses in the world. (Source: ETHZ)

In order to fully under­stand the dynamics during a chemical reaction, scientists must be able to study all movements of atoms and mole­cules on their basic time scale. Mole­cules rotate in the range of pico­seconds, their atoms vibrate in the range of femto­seconds, and the electrons move in the range of atto­seconds. ETH pro­fessor Hans Jakob Wörner and his group have now succeeded in generating the world’s shortest laser pulse with a duration of only 43 atto­seconds. More generally speaking, this laser pulse is the shortest controlled event that has ever been created by humans. The researchers can now observe in high detail how electrons move within a molecule or how chemical bonds are formed.

Starting from an infra­red laser, the researchers generate a soft X-ray laser pulse with a very large spectral band­width. As a result, various elements including phos­phorus and sulphur can be directly observed by exciting their inner-shell electrons. Both elements are present in biomo­lecules, and it is now possible to observe them with unpre­cedented time reso­lution.

But what is the advantage of being able to observe the reaction steps now with even higher reso­lution? “The faster a charge transfer can take place, the more effi­ciently a reaction can proceed”, says Wörner. The human eye for example is very efficient when it comes to conver­ting photons into nerve signals. In rhodopsin, a visual pigment in the retina, the photo­sensitive molecule retinal is prearranged in such a way that its structure can change extremely fast through the absorp­tion of only a single photon. This enables the visual process even in twilight. A much slower reaction would render vision impos­sible, because the energy of the photon would be converted to heat in only a few pico­seconds.

Atto­second spectro­scopy could contribute to the develop­ment of more efficient solar cells since it is now for the first time possible to follow the process of exci­tation through sunlight up to the gene­ration of elec­tricity step by step. A detailed under­standing of the charge transfer pathway could help opti­mizing the effi­ciency of the next gene­ration of photo­sensitive elements.

Atto­second laser spectro­scopy is not only suitable for mere obser­vation, Wörner explains. Chemical reactions can also be directly mani­pulated: Using a laser pulse can alter the course of a reaction – even chemical bonds can be broken by stopping the charge shift at a certain location in the molecule. Such targeted inter­ventions in chemical reactions have not been possible until now, since the time scale of electron movement in molecules was previously unreached. The group is already working on the next gene­ration of even shorter laser pulses. These will make it possible to record even more detailed images, and thanks to a wider X-ray spectrum even more elements can be probed than before. Soon it will be possible to follow the migra­tion of electrons in more complex molecules with an even higher time resolution. (Source: ETHZ)

Reference: T. Gaumnitz et al.: Streaking of 43-attosecond soft-X-ray pulses generated by a passively CEP-stable mid-infrared driver, Opt. Exp. 25, 027506 (2017) DOI: 10.1364/OE.25.027506

Link: Ultrafast Spectroscopy and Attosecond Science, Lab. of Physical Chemistry, ETHZ; Zurich, Switzerland

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