Shaping attosecond waveforms
Chemical reactions and complex phenomena occurring in liquids or solids are determined at the most fundamental level by their electronic structure and dynamics. The rearrangement of the electrons due to an initial stimulus (for example the absorption of a sunlight photon) evolves on an extremely short timescale, typically only a few hundred attoseconds (1 attosecond =0.000 000 000 000 000 001 s). Only light pulses with comparable duration can be used to take snapshots of this motion, thus resolving in time the dynamics of the electrons.
Electrons are sensitive to external fields and they can be easily steered when irradiated with light pulses. The capability to shape in time the electric field of an attosecond pulse directly translates into the opportunity to steer on this timescale the electronic dynamics. This intriguing possibility is at the edge of currently available technology, but promises to deliver new approaches for controlling chemical reactions and for developing electronic devices much faster than those currently used (even at petahertz (1 PHz=1 000 000 000 000 000 Hz) clock frequencies).
A large international collaboration, led by scientists from the university of Freiburg, have demonstrated for the first time ever the possibility to completely shape the waveform of an attosecond pulse train. The experiment was performed at the seeded Free-Electron Laser (FEL) FERMI in Trieste, which offers the unique capability to synthesize radiation with different wavelengths in the extreme ultraviolet spectral range with fully controllable relative phases.
The results have been published in the prestigious scientific journal Nature:
P.K. Maroju et al. “Attosecond pulse shaping using a seeded free-electron Laser”
In the experiment, groups of four harmonics of a fundamental wavelength were generated using the set of undulators available at FERMI; their relative phases were changed using phase shifters placed between the undulators. 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. The presence of the latter gives rise to additional peaks in the electron spectra, which are usually referred to as sidebands. A novel technique based on the characterization of the correlation of the variation of the sidebands generated for each laser shot was the key advance for the full characterization of the attosecond pulse train.
The results open new perspectives in attosecond science and technology as, for the first time, we can model and control the electric field of a pulse on this unprecedented timescale. With these pulses, we can investigate how the duration of an attosecond pulse influences the first moment of the electronic response in a molecule or in a crystal. The capability to shape the electric field, moreover, lends itself to enabling control of the electronic motion.
The results have been possible only through the strong collaboration between the group of the University of Freiburg, the team at Elettra, the Russian team of theoreticians, and the international team of theoreticians and experimentalists from United States, Germany, Italy, Austria, Slovenia, Hungary, Japan and Sweden. We have taken fully advantage of the unique characteristics of the FEL FERMI in this experiment. Our results indicate not only that FELs can produce attosecond pulses, 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.