WASHINGTON, April 30--Physicists have long chased an elusive goal: the ability to "freeze" and then study the motion of electrons in matter. Such experiments could help confirm theories of electron motion and yield insights into how and why chemical reactions take place. Now a collaboration of scientists from France and Canada has developed an elegant new method to study electrons' fleeting antics using isolated, precisely timed, and incredibly fast pulses of light. The team will describe the technique at the Conference on Lasers and Electro-Optics , taking place in San Jose, Calif. May 6-11. (CLEO: 2012 (http://www.cleoconference.org))
The exchanges of electrons during chemical reactions typically occur on time scales less than one femtosecond, or a millionth of a billionth of a second. The only way to freeze electron motion is using pulses of light with durations that are shorter still than the rapid comings and goings of electrons – on the order of quintillionths of a second, or attoseconds. Once they are frozen, "the dynamics of electrons could then be studied by so-called pump-probe experiments" that use a pair of light pulses, explains physicist Fabien Quéré of the French Commissariat à l'Energie Atomique (CEA). The first pulse – dubbed the pump – kick-starts the motion of the electrons at a well-defined starting time, he explains, "and the second one, the probe, looks at the excited system at different times after the pump, to measure its evolution after excitation."
Attosecond pulses have been produced through the interaction of ultra-powerful laser beams with matter. The resulting light bursts, however, come in "trains" – collections of pulses closely spaced in time – that don't work very well in pump-probe experiments. What would work far better are isolated and precisely-timed pulses. That's exactly what Quéré, along with graduate student Henri Vincenti and colleagues at the Applied Optics Laboratory (LOA, France) and the National Research Council of Canada (NRC), have created using a new method, dubbed the "attosecond lighthouse" effect.
In the technique, the initial laser beam is shaped in order 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 one consisting in the time domain, according to theory and simulations, of a single attosecond pulse," Quéré says – which produces an ideal light source for attosecond pump-probe experiments.
According to the researchers, the attosecond lighthouse effect has several major advantages over previous methods for making isolated attosecond pulses. For example, Quéré says, "it is by far the easiest one to implement experimentally. In practice, it only requires a very small rotation of one optical element – a prism or a grating, typically – in existing laser systems."