Credit: © 2008 APS

High-harmonic generation (HHG) is a useful technique for producing extreme UV (EUV) light or trains of attosecond pulses and is now a popular area of research. The process, which results from the interaction of an intense laser field with a collection of atoms or molecules, is often described by classical physics, but more recently models based on quantum mechanics have been developed. The strength of the process is characterized by a harmonic dipole moment, which can be described as a sum of different quantum path contributions, which represent electron trajectories. The relative phase between the different quantum paths (electron trajectories) leads to interference in the total single-atom dipole moment.

A collaboration of scientists from Switzerland, the UK and France now claim to have observed this quantum path interference (QPI) for the first time, by studying the intensity dependence of HHG in an argon gas jet (Phys. Rev. Lett. 100, 143902; 2008).

Such QPI is hard to observe because temporal and spatial averaging usually blurs the effects on a macroscopic scale. However, the researchers used spectral and far-field spatial filtering to overcome this problem and allow direct observation.

The team from ETH Zurich, the Clarendon Laboratory at Oxford University and the CNRS–CEA University of Bordeaux and CEA Saclay in France say that their measurements have sufficient contrast to resolve QPI in the generated harmonics (see image). The measurements also provide access to the relative quantum path phases that are needed to reconstruct the full atom dipole moment.

By slightly changing the intensity of the incident laser beam the team were able to control the quantum paths on an attosecond timescale. For example a 10% change in incident laser intensity was able to shift the QPI from constructive to destructive for the 15th harmonic.

The research provides a valuable insight into the ultrafast electron dynamics involved in the emission process and is the first step towards a direct experimental characterization of a single atom dipole.