Shaping Waveforms

Chemical reactions are determined at their most funda­mental level by their respective electronic structure and dynamics. Steered by a stimulus such as light irra­diation, electrons rearrange themselves in liquids or solids. This process takes only a few hundred atto­seconds. Electrons are sensitive to external fields, so researchers can easily control them by irra­diating the electrons with light pulses. As soon as they thus temporally shape the electric field of an attosecond pulse, researchers can control the electronic dynamics in real time. A team led by Giuseppe Sansone from the Institute of Physics at the Uni­versity of Freiburg now shows how they were able to completely shape the waveform of an atto­second pulse.

Shaping the electric field of an attosecond pulse. (Source: J. Oschwald & C. Callegari)

“These pulses enable us to study the first moment of the electronic response in a molecule or crystal,” explains Sansone. “With the ability to shape the electric field enables us to control electronic movements – with the long-term goal of optimising basic processes such as photo­synthesis or charge separation in materials.” The team, consisting of theoreticians and experimental physicists from research institutes in the USA, Russia, Germany, Italy, Austria, Slovenia, Hungary, Japan and Sweden, carried out their experiment at the Free-Electron Laser (FEL) FERMI in Trieste/Italy. This laser is the only one which offers the unique capa­bility to synthesize radiation with different wave­lengths in the extreme ultra­violet spectral range with fully control­lable relative phases.

The attosecond pulse results from the temporal overlap of laser harmonics. The scientists generated groups of four laser harmonics of a funda­mental wavelength using the undulators available at FERMI. These are technical devices, which steer the motion of a rela­tivistic electron bunch, thus leading to the production of ultra­violet radiation. One of the main challenges of the experiment was the measurement of these relative phases, which were charac­terized by acquiring the photo­electrons released from neon atoms by the combination of the attosecond pulses and an infrared field. This leads to addi­tional structures in the electron spectra, usually referred to as sidebands. The scientists measured the correlation between the different sidebands generated for each laser shot. This finally enabled them to fully characterize the atto­second pulse train.

“Our results indicate not only that FELs can produce atto­second pulses”, says Sansone, “but, due to the approach implemented for the waveform generation, such pulses are fully control­lable and attain high peak inten­sities. These two aspects represent key advantages of our approach. The results will also influence the planning and design of new Free-Electron Lasers worldwide.” (Source: U. Freiburg)

Reference: P. K. Maroju et al.: Attosecond pulse shaping using a seeded free-electron laser, Nature, online 10 February 2020; DOI: 10.1038/s41586-020-2005-6

Link: Attosecond- and Strong Field Physics (G. Sansone), Institute of Physics, University of Freiburg, Germany

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