Taming light: Mastering the fine structuring of ultrashort light fields

An expedition through the fast-paced microscopic world of atoms reveals electrons that spin at enormous speeds and the gigantic forces that act on them. Monitoring the ultrafast motion of these electrons requires ultrashort flashes of light. However, in order to control them, the structure of these light flashes, or light pulses, needs to be tamed as well.

This type of control over light pulses has now been achieved, for the first time, by a team of physicists led by Dr. Eleftherios Goulielmakis and Professor Ferenc Krausz of the Laboratory of Attosecond Physics at the Max Planck Institute of Quantum Optics (MPQ) and Ludwig-Maximilians-Universität (LMU) in Munich, together with collaborators from the Center of Free-Electron Laser Science (DESY Hamburg) and the King Saud University (Saudi Arabia).

Taking advantage of the fact that light possesses both particle-like and wave-like properties, they have sculpted fine features into the waveform of these pulses of white light. Additionally, the researchers were able to make their pulses shorter than a complete light oscillation, thereby creating isolated sub-optical-cycle flashes of light for the first time. Not only will these novel tools allow for the precise control of electron motion in the fundamental building blocks of matter, they will also further our understanding of atomic processes and permit more precise timing of electronic processes in molecules and atoms.

The motion of electrons in the microcosm occurs on an attosecond time scale, where one attosecond is a billionth of a billionth of a second. On such a short scale, only light itself is able to keep up with the motion. Because of the fast oscillations of its own electromagnetic field, light can act on electrons rather like a pair of tweezers, influencing their motions and interactions. The time it takes light generated by modern laser sources to complete one full oscillation amounts to around 2.6 femtoseconds, where one femtosecond is 1000 attoseconds, or one millionth of a billionth of a second.

That is the reason why light is a promising tool for controlling electron dynamics in the microcosm. But before this can become reality, light's field oscillations have to be tamed, i.e. its electromagnetic field must be precisely and completely controllable on a time scale which is shorter than one complete oscillation cycle. In order to achieve this lofty aim, one first has to learn how to develop and perfect these extraordinary tweezers.

The international team assembled at MPQ and LMU Munich by Dr. Eleftherios Goulielmakis and Professor Ferenc Krausz has now taken a big step towards this ambitious goal, managing to sculpt the waveforms of laser pulses with sub-cycle precision. In order to control light pulses on a sub-cycle time scale, it is necessary to use white laser light, as it contains wavelengths (light colors) ranging from the near-ultraviolet through the visible all the way to the near infrared region of the electromagnetic spectrum.

The physicists have created these light pulses and sent them into a newly developed "light field synthesizer." The light field synthesizer is analogous to the sound synthesizers used by many musicians. Just as a sound synthesizer superimposes sound waves of different frequencies to create different sounds and beats, so the light field synthesizer superimposes optical waves of different colors and phases to create various field shapes. The apparatus first splits the incident white laser light into red, yellow and blue color channels. After manipulating the properties of the individual colors, these are recombined to form the synthesized wave form.

Several components of this novel device, e.g. its mirrors and its elaborate beam splitters, were developed in the service center of the Munich Center for Advanced Photonics (MAP) located at LMU. Utilizing this technology, the scientists were able to generate completely new isolated waveforms.

Furthermore, in doing so they managed to compose the shortest pulses ever measured in the visible spectral range, lasting only 2.1 femtoseconds. These pulses are more intense than those commonly afforded by current femtosecond light sources, because all the energy of the electromagnetic field is confined within a tiny temporal window. It is precisely these powerful and specially tailored electromagnetic forces which are necessary to control electrons in atoms and molecules, as they are similar in strength to the forces encountered in such microscopic systems.

However, to steer electron motion on a microscopic scale, strength is not the only prerequisite -- precision is also needed. The desired level of precision is provided by the well-controlled waveforms of the synthesized light pulses. Thanks to these latest results, the scientists have accomplished a major step towards the control of the microcosm. "These newly developed tools allow us to initiate, control and therefore further understand sub-atomic processes. With these devices, we can master the fine structuring of ultrashort light fields and reliably measure the newly formed light," explains Dr. Adrian Wirth, a Postdoctoral Fellow in the research team of Dr. Eleftherios Goulielmakis, leader of the ERC-research group "Attoelectronics."

As a matter of fact, the physicists have already applied this novel technique in an experiment. By shining the newly designed light pulses onto krypton atoms, the outermost electron was ripped away within less than 700 attoseconds, the fastest electronic process which has yet been initiated by visible light.

Similar processes can certainly be induced with similar precision in more complex systems such as molecules, solids and nanoparticles. This new technology may very well lead the way towards light-based electronics in the future. Light fields are expected to drive electrons not only in isolated systems such as atoms or molecules, but even on microscopic circuits so as to perform logic operations at unprecedented speeds" said Dr. Goulielmakis, whose group is exploring the principles of electronics on these extreme time scales. "We are progressively increasing our understanding of the principles in the microcosm and learning how to control it," adds Ferenc Krausz.