Taming light: Mastering the fine structuring of ultrashort light fields

Physicists at the Max Planck Institute of Quantum Optics and LMU Munich have generated for the first time "white" light pulses. They are able to control their field on a time scale shorter than an optical oscillation. These new tools hold promise for unprecedented control of the motion of electrons.

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.

Story Source:

The above story is reprinted (with editorial adaptations ) from materials provided by Ludwig-Maximilians-Universitaet Muenchen (LMU).

Journal Reference:

A. Wirth, M. T. Hassan, I. Grguras, J. Gagnon, A. Moulet, T. T. Luu, S. Pabst, R. Santra, Z. A. Alahmed, A. M. Azzeer, V. S. Yakovlev, V. Pervak, F. Krausz, E. Goulielmakis. Synthesized Light Transients. Science, 2011; DOI: 10.1126/science.1210268

Taming the molecule's Dr. Jekyll and Mr. Hyde

Many organic molecules are non-superimposable with their mirror image. The two forms of such a molecule are called enantiomers and can have different properties in biological systems. The problem is to control which enantiomer you want to produce – a problem that has proved to be important in the pharmaceutical industry. Researchers at the University of Gothenburg have now come up with a new method to control the process.

"Organic chemists think that it's impossible to create only one of the enantiomers without introducing some kind of optical activity into the reaction, but I've succeeded," says Theonitsa Kokoli at the University of Gothenburg's Department of Chemistry. "My method will allow the industry to produce the version they want without the use of a catalyst."

The phenomenon of non-superimposable mirror-image molecular structures is known as chirality. The two enantiomers can be compared to a pair of hands; they are non-superimposable mirror images of each other. A consequence of the different properties in biological systems is that a molecule can behave either as Dr Jekyll or Mr Hyde. The different characteristics in the enantiomers can be harmless, like in the limonene molecule. One enantiomer smells like orange and the other like lemon.

 

"Organic chemists think that it's impossible to create only one of the enantiomers without introducing some kind of optical activity into the reaction, but I've succeeded," says Theonitsa Kokoli at the University of Gothenburg's Department of Chemistry. Credit: University of Gothenburg

Thalidomide is a good example of how different forms of the same molecule can have disastrous consequences. One of the enantiomers was calming and eased nausea in pregnant women, while the other caused serious damage to the foetus. The thalidomide catastrophe is one of the reasons that a lot of research is devoted to chirality, as it is absolutely vital to be able to control which form of the molecule that is produced. Research on chirality has resulted in several Nobel Prizes over the years.

In biomolecules like DNA and proteins only one of the enantiomers exists in nature. In contrast to biomolecules, the same does not apply when chiral compounds are created synthetically in the lab. Generally an equal amount of both enantiomers is produced. One way of creating an excess of one enantiomer is to use a chiral catalyst, but this only transfers the properties that are already present in the catalyst.

"I've been working with absolute asymmetric synthesis instead, where optical activity is created," says Kokoli. "This is considered impossible by many organic chemists. I've used crystals in my reactions, where the two forms have crystallised as separate crystals, which in itself is fairly unusual. The product that was formed after the reactions comprised just one enantiomer."

While the results of Kokoli's research are particularly significant for the pharmaceuticals industry, they can also be used in the production of flavourings and aromas.

Provided by University of Gothenburg (news : web)

Taming the flame: Electrical wave 'blaster' could provide new way to extinguish fires

A curtain of flame halts firefighters trying to rescue a family inside a burning home. One with a special backpack steps to the front, points a wand at the flame, and shoots a beam of electricity that opens a path through the flame for the others to pass and lead the family to safety.

Scientists today described a discovery that could underpin a new genre of fire-fighting devices, including sprinkler systems that suppress fires not with water, but with zaps of electric current, without soaking and irreparably damaging the contents of a home, business, or other structure. Reporting at the 241st National Meeting & Exposition of the American Chemical Society (ACS), Ludovico Cademartiri, Ph.D., and his colleagues in the group of George M. Whitesides, Ph.D., at Harvard University, picked up on a 200-year-old observation that electricity can affect the shape of flames, making flames bend, twist, turn, flicker, and even snuffing them out. However, precious little research had been done over the years on the phenomenon.

"Controlling fires is an enormously difficult challenge," said Cademartiri, who reported on the research. "Our research has shown that by applying large electric fields we can suppress flames very rapidly. We're very excited about the results of this relatively unexplored area of research."

Firefighters currently use water, foam, powder and other substances to extinguish flames. The new technology could allow them to put out fires remotely — without delivering material to the flame — and suppress fires from a distance. The technology could also save water and avoid the use of fire-fighting materials that could potentially harm the environment, the scientists suggest.

In the new study, they connected a powerful electrical amplifier to a wand-like probe and used the device to shoot beams of electricity at an open flame more than a foot high. Almost instantly, the flame was snuffed out. Much to their fascination, it worked time and again.

The device consisted of a 600-watt amplifier, or about the same power as a high-end car stereo system. However, Cademartiri believes that a power source with only a tenth of this wattage could have similar flame-suppressing effect. That could be a boon to firefighters, since it would enable use of portable flame-tamer devices, which perhaps could be hand-carried or fit into a backpack.

But how does it work? Cademartiri acknowledged that the phenomenon is complex with several effects occurring simultaneously. Among these effects, it appears that carbon particles, or soot, generated in the flame are key for its response to electric fields. Soot particles can easily become charged. The charged particles respond to the electric field, affecting the stability of flames, he said.

"Combustion is first and foremost a chemical reaction – arguably one of the most important – but it's been somewhat neglected by most of the chemical community," said Cademartiri. "We're trying to get a more complete picture of this very complex interaction."

Cademartiri envisions that futuristic electrical devices based on the phenomenon could be fixed on the ceilings of buildings or ships, similar to stationary water sprinklers now in use. Alternatively, firefighters might carry the flame-tamer in the form of a backpack and distribute the electricity to fires using a handheld wand. Such a device could be used, for instance, to make a path for firefighters to enter a fire or create an escape path for people to exit, he said.

The system shows particular promise for fighting fires in enclosed quarters, such as armored trucks, planes, and submarines. Large forest fires, which spread over much larger areas, are not as suitable for the technique, he noted.

Cademartiri also reported how he and his colleagues found that electrical waves can control the heat and distribution of flames. As a result, the technology could potentially improve the efficiency of a wide variety of technologies that involve controlled combustion, including automobile engines, power plants, and welding and cutting torches, he said.

Provided by American Chemical Society (news : web)