Feb 14 2020
At the most elementary level, chemical reactions are governed by their respective electronic structure and dynamics. Electrons in solids or liquids tend to rearrange themselves upon being driven by a stimulus, such as light irradiation.
Researchers were able to shape the electric field of an attosecond pulse. Image Credit: Jürgen Oschwald and Carlo Callegari.
This process occurs within a few hundred attoseconds, where one attosecond is the billionth part of one-billionth of a second. Since electrons are sensitive to external fields, they can be easily manipulated by irradiating them with light pulses. Once they temporally shape an attosecond pulse’s electric field, the electronic dynamics can be controlled by researchers in real time.
Headed by Prof. Dr. Giuseppe Sansone from the Institute of Physics at the University of Freiburg, a team of researchers has now been able to completely shape the waveform of an attosecond pulse. They have described the process in the scientific journal Nature.
These pulses enable us to study the first moment of the electronic response in a molecule or crystal. 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.
Dr Giuseppe Sansone, Professor, Institute of Physics, University of Freiburg
The team, including experimental physicists and theoreticians from research institutes in the United States, Germany, Russia, Austria, Italy, Hungary, Slovenia, Sweden, and Japan, performed their experiment at the Free-Electron Laser (FEL) FERMI in Trieste/Italy.
This laser is the only one with the exclusive ability to generate radiation with different wavelengths in the extreme ultraviolet range of the spectrum, with completely controllable relative phases.
The attosecond pulse is produced as a result of the temporal overlap of laser harmonics. The researchers used the undulators at FERMI to produce groups of four laser harmonics of a fundamental wavelength. Undulators are technical devices that guide the motion of a relativistic electron cluster, which results in the production of ultraviolet radiation.
Measurement of these relative phases is one of the major difficulties faced during the experiment. Characterization of these phases was performed by capturing the photoelectrons emitted from neon atoms by using a combination of an infrared field and the attosecond pulses.
This results in additional structures in the electron spectra, commonly known as sidebands. The correlation between the different sidebands produced for each laser shot was measured by the researchers. Thus, they were eventually able to fully characterize the attosecond pulse train.
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.
Dr Giuseppe Sansone, Professor, Institute of Physics, University of Freiburg