Light flashes keep electrons in check

CFEL's DESY research published in Science Express

A light field synthesizer divides incident white light into three-colour channels and modifies it afterwards. The composition creates laser pulses with a complex, however fine adjustable waveform
Photo: Thorsten Naeser

An international team of scientists in cooperation with CFEL has taken a major step closer towards the possibility to control electron movements in atoms. For the first time, they generated white laser pulses that are shorter than a complete light oscillation and precisely synthesised their electromagnetic fields. These new tools hold promise for an extensive control of the motion of electrons in the microcosm. Possible future applications might be nanoscopic electronic circuits which by far exceed the switching speed of -+modern electronics. The scientists reported on their results in the scientific magazine Science (Science Express, 08 September 2011).

Electrons in atoms and molecules move with enormous speed and massive forces are acting on the particles. Ultra-short light pulses are required to observe these particles. For the additional control of the electrons, it is necessary to manipulate the pulsestructure of these flashes. This type of manipulation has now, for the first time, been achieved by a team of physicists lead by Dr. Eleftherios Goulielmakis and Prof. Ferenc Krausz of the Laboratory of Attosecond Physics at the Max Planck Institute of Quantum Optics (MPQ) and the Ludwig-Maximilians-University (LMU Munich) in Garching, along with collaborators from the Center for Free-Electron Science (DESY Hamburg) and the King Saud University (Saudi Arabia).

At these experiments, the scientists took advantage of the wave properties of light. They have sculpted fine features into the waveform of these pulses of white light. Among other things, the researchers were able to make their pulses shorter than a complete light oscillation, thereby creating for the first time isolated sub-optical-cycle pulses of high coherence in the visible light spectrum. Not only will these novel tools allow for a precise control of electron motion in the fundamental building blocks of matter, they will also enhance the tracing of subatomic processes and will permit a more precise timing of electronic processes in molecules and atoms.

The interaction of light pulses with electrons is of special importance in this method. Standard methods to describe how light and electrons influence each other are inapplicable in the ultra-fast nano world. The theoretical description of this interaction has been carried out by the team of Robin Santra, CFEL. They developed a state-of-the-art theory to describe the ionising process of a multi-electron atom with such a synthesised light pulse. Unlike the so far existing methods, this theory describes both the influence of spin-orbit coupling in the ion and the electronic correlation, including the quantum-mechanical processes between the ion and the ripped-off electron. With the help of complex computations, it was taken into account that many photons are simultaneously absorbed to release one electron. “With the help of our state-of-the art computations, we are able to understand essential aspects of the experiment, particularly the high level of coherence that was achieved,” said CFEL scientist Robin Santra. “To reach the long-term goal of controlled electron interaction, we must further develop the theoretical insight of the processes which are available using the new light sources.”