High-Flux Ultrafast Photoelectron Spectroscopy

The core element of their new technique is a novel enhancement resonator. Ultrashort, near-infrared laser pulses delivered to the cavity at a rate of 18.4 million per second are converted into extreme ultraviolet attosecond pulse trains, which are ideally suited for experiments in electron dynamics. (Source: T. Naeser, MPQ)

The advent of ultrafast photo­electron spectro­scopy some two decades ago made it possible to observe the motions of electrons in atoms, molecules and solid-state materials on atto­second scales. However, application of the technique has been limited by the need to collect large datasets. This restriction has precluded many experi­ments on the dynamics of electron emission – in particular, efforts to determine their energies and momenta, and localize their sites of origin. Electrons are negatively charged particles and therefore repel each other. Conse­quently, precise measure­ments of their dynamics can best be carried out if each ultrashort light pulse results the photo­emission of only a few electrons from the sample under inves­tigation.

Now, scientists involved in the MEGAS Project, which forms part of the wider programme of coopera­tion between the Max Planck Society and the Frau­nhofer Society, have overcome this obstacle. Together, groups working at the Max Planck Institute of Quantum Optics, the LMU Munich and the Fraunhofer Insti­tutes for Laser Tech­nology and for Applied Optics and Precision Engi­neering have developed a new source of attosecond laser pulses in the extreme ultraviolet (XUV) region of the electro­magnetic spectrum. The set-up allows them to generate trains of high-intensity atto­second pulses at a rate of 18.4 million per second. These pulse sequences are used to trigger the emission of electrons from metal surfaces and film their behavior. “The new laser source generates pulses at rates that are about 1000-fold higher than was previously feasible in this spectral range, which reduces the measure­ment times required by the same factor,” as the leader of the project, Ioachim Pupeza, explains.

The break­through was made possible by the use of a high-effi­ciency enhance­ment resonator cavity, in which trains of input pulses are passively enhanced, before being focused on a sample of argon gas. The upshot of the process is to convert the incoming sub-40-femto­second near-infrared pulses into XUV pulses of atto­second duration, each containing up to 500,000 photons with energies of between 25 and 60 electron volts. Given their repe­tition rate of 18.4 million per second, these pulses have an unpre­cedentedly high energy density. To demonstrate this, the attosecond pulse train was directed onto a tungsten target, detaching electrons from its surface by the photo­electric effect. The team was then able to analyze the pro­perties of these electrons by means of atto­second-resolved photo­electron spectro­scopy.

“The lower repetition rates available up to now meant that one had to wait a while for the next laser pulse in this type of experiment. But the new confi­guration allows us to detect the virtually continuous release of photo­electrons from the tungsten sample,” as Stephan Heinrich and Tobias Saule point out. This reduces the times required to determine the spatial distri­bution of photo­electrons from several days to a matter of minutes. “This advance is of consi­derable signi­ficance for research on condensed-matter systems. It also opens up new opportunities for the inves­tigation of local electric fields in nano­structures, which are of great interest for appli­cations in future infor­mation processing with lightwaves,” Pupeza adds. (Source: MPQ)

Reference: T. Saule et al.: High-flux ultrafast extreme-ultraviolet photoemission spectroscopy at 18.4 MHz pulse repetition rate, Nat. Commun. 10, 458 (2019); DOI: 10.1038/s41467-019-08367-y

Link: Laboratory for Attosecond Physics (LAP), Max-Planck-Institute for Quantum Optics, Garching, Germany • Project MEGAS, Fraunhofer Institute for Applied Optics and Precision Engineering IOF, Jena, Germany

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