Attosecond Lighthouses Illuminate Electron Exchange

A new method of studying the motion of electrons in matter using isolated, precisely timed and incredibly fast pulses of light could help to confirm theories of electron motion and yield insights into how and why chemical reactions take place.

The exchange of electrons during chemical reactions typically occurs on timescales of less than 1 fs. The only way to freeze electron motion is using pulses of light with durations that are still shorter than the rapid comings and goings of electrons. The dynamics of electrons can be studied in a pump-probe experiment that uses a pair of light pulses. The first pulse, dubbed the pump, kick-starts the electron motion at a well-defined starting time, explained physicist Fabien Quéré of the French Commissariat à l'Energie Atomique (CEA). The second pulse, the probe, looks at the excited system at different times after the pump to measure its evolution after excitation.

Here, wavefront rotation has been induced on the laser beam, leading to a spatial splitting of this attosecond pulse train into a collection of beamlets containing isolated attosecond pulses. (Image: Fabien Quéré, CEA)

Now, Quéré and colleagues at the Applied Optics Laboratory and the National Research Council of Canada have developed an “attosecond lighthouse” effect to create isolated and precisely timed pulses.

In the method, the initial laser beam is shaped to induce an ultrafast rotation of its wavefront so that its interaction with matter produces a train of individual attosecond pulses that is streaked angularly — like the sweeping beam of a lighthouse. The researchers can thus generate a collection of beamlets, “each consisting in the time domain, according to theory and simulations, of a single attosecond pulse,” Quéré said. These produce an ideal light source for attosecond pump-probe experiments.

The attosecond lighthouse effect has several major advantages over previous methods for making isolated attosecond pulses, the researchers said.

“It is by far the easiest one to implement experimentally,” Quéré said. “In practice, it only requires a very small rotation of one optical element — a prism or a grating, typically — in existing laser systems.”

A presentation on the technique will be given May 8 at the Conference on Lasers and Electro-Optics (CLEO) in San Jose, Calif.

For more information, visit: www.cea.fr