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Taking Time’s pulse

Munich, 07/18/2013

A journey to the briefest instants of time: Researchers at the Munich-Centre for Advanced Photonics use ultrashort light pulses to study the motion of electrons in atoms – and are finding applications for “attosecond physics”.

(Photo: Jan Greune)
An electron "camera": Ferenc Krausz with one of the instruments used in attosecond experiments (Photo: Jan Greune)

When Ferenc Krausz turns the light on, it goes out again in very, very short order. For the light pulses generated in his laboratory on the Research Campus in Garching last for only a few billionths of a nanosecond (10-18 s). This made them record-breakers, as the Guinness Book of Records confirms. The certificate to this effect hangs, unframed and inconspicuous, on a magnet board in Krausz’s office.

Krausz is Chair of Experimental Physics at LMU and Director at the Max-Planck Institute of Quantum Optics (MPQ) in Garching, and he is not at all interested in setting records. “The ultrashort light pulses we produce are not an end in themselves, but the means to an end,” the Hungarian-born physicist remarks. “And the range of their potential real-life application is growing,” he adds.

Ultrashort flashes
Now the researchers involved in the MAP Excellence Cluster (“MAP” stands for “Munich-Centre for Advanced Photonics”) have begun to explore in detail what exactly can be done with their ultrashort and extremely energetic light flashes. Ferenc Krausz is the Speaker for the Cluster. In collaboration with researchers at the MPQ, the Technische Universität München (TUM) and other partners, he and his LMU colleagues want to use the incredibly brief pulses to capture and illuminate the motions of electrons in atoms, molecules and condensed matter.

But they also hope to exploit the technology to develop new sources of radiation for medical diagnostics, and particle generators for targeted cancer therapy. “The field of biomedicine offers particularly interesting potential applications for this new laser-generated radiation,” says Franz Pfeiffer, Chair of Biomedical Physics at the TUM.

Not what you find in the corner shop
The indispensable basis for everything that follows is the laser – though not the kind you can get from your local purveyor of office supplies. Conventional laser pointers are designed to produce a steady, stable beam of monochromatic light. The scientists in Garching are in pursuit of the other extreme – a laser that packs as much punch as possible into the shortest possible pulse.

For the past several decades so-called femtosecond lasers have been able to produce pulses lasting a few millionths of a billionth of a second. Such instruments are now available commercially. In fact, there’s one in Krausz’s laboratory, which serves as the workhorse for production of much shorter flashes. This laser emits red light, which consists of electromagnetic waves with a period of just over 2 femtoseconds (fs). The laser can produce a train of “wave packets”, each consisting of a few complete cycles, with a pulse-length of 20-30 fs. “For our purposes, however, this is far too long,” Krausz points out.

Summing wave-crests
To produce transient, high-intensity pulses, the physicists use this highly pure red laser light to generate a whole spectrum of colors, and separate it into infrared, visible and ultraviolet wavelengths. They then add these together again in such a way that the wave crests coincide at a predetermined time, and their amplitudes add up. Prior to, and immediately after this point, the waves are no longer in phase, peaks coincide with troughs and cancel each other out. What is left is a single intense pulse that lasts only a little longer than the fundamental oscillation period of red light waves – around 2 fs.

“With such light pulses, one can photograph almost all of the physical processes that occur at the atomic level,” says Krausz. However, electrons move 100 to 1000 times faster, and a snapshot of their motions taken with a femtosecond pulse would yield only a blur, as if one had tried to image a passing fighter jet using an exposure time of several seconds. To get around this problem, Krausz and his team use a clever trick. Alexander Stirn, Translation: Paul Hardy

The complete article is available here.

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