The Lund group presently holds the world record for producing short laser pulses. In the High-power laser facility at the Lund University, trains of pulses where each pulse is 200 attoseconds long and separated from the next pulse by 1.3 femtoseconds, are routinely produced. A femtosecond is 10-15 seconds, i.e. one-millionth-of-a-billionth of a second, while an attosecond is still one thousand times shorter. These incredibly short light pulses allow scientists to make snapshots of the most rapidly moving constituents of atoms and molecules, the electrons. In a paper published in this month’s issue of Nature Physics, the scientists demonstrate that attosecond pulses are an extremely powerful tool for studying the wave-like nature of electrons.
Quantum mechanics describes all the properties of matter in a probabilistic manner with so-called wave functions. Wave functions describe, for example, the probability that an electron is found at a particular position or that an electron moves with a particular velocity. They also describe how – similar to light – matter sometimes behaves more like a particle, and sometimes more like a wave. Importantly, the wave function is – in mathematical terms – a complex quantity, that it is characterized by both an amplitude and a phase. Though theorists can calculate complex valued wave functions and use them to make precise predictions about the behaviour of matter, the complete measurement of a wave function, both its amplitude and phase, is notoriously difficult. This is why most experiments only give information about the amplitudes of wave functions and not their phase.
In their paper, the scientists now report that they have developed a technique for measuring the phase of an electronic wave function, making use of attosecond pulses. The technique is based on interferences between electrons that are created by two attosecond pulses that quickly follow each other. The technique combines the ultrashort light pulses generated in Lund with an electron imaging detector that was built in Amsterdam and moved to Lund for the experiment. In the experiments, argon atoms were ionized by a series of attosecond pulses in the extreme ultraviolet wavelength range in the presence of longer pulses of intense infrared laser light. When the argon atoms absorb the extreme ultraviolet light of the attosecond pulses, electrons escape in bunches (called wave packets). The intense infrared light changes the velocity of the electron wave packets, and they start to interfere with each other and form complicated interference patterns. The analysis of the interference patterns allowed the scientists to get unprecedented insight into the wave-nature of the electron and to extract information on the phase of the electronic wave function.
The experiment is presented in an article titled “Attosecond electron wave packet interferometry”. It is published online at www.nature.com/naturephysics.