High-harmonic generation, a nonlinear optical process that occurs when a short burst of intense light irradiates a target, is an important method for producing attosecond () length pulses of extreme ultraviolet (XUV) light. Classically, the light pulse ejects an electron from an atom in the target material into the continuum. When this electron is accelerated and then driven back to the parent ion by the laser field, the recombination of the electron with the ion leads to the emission of XUV light.
The quantum mechanical version of this process is somewhat of a generalization of the classical picture: the process is a coherent sum over all possible electron paths that the electron takes on its excursion from the atom. At high enough intensities, the wave function describing the atom and its associated electron is a sum over interferences between a “short” path (when the electron excursion time is a half laser cycle) and a “long” path (when the excursion time is close to a full laser cycle).
Until now, these interferences have not been observed because they are smoothed out in the collective response of the macroscopic XUV generating medium, which is typically a jet of gas. However, in a paper appearing in Physical Review A, Thierry Auguste and colleagues at the CEA in Saclay, France, and a team of researchers from France, the UK, and Switzerland have now demonstrated both theoretically and experimentally how to observe these interferences. They measure the quantum path interferences by properly spectrally and spatially filtering the XUV light generated from a pulsed argon jet irradiated by a focused high-intensity Ti:Sapphire laser.
These results demonstrate a mechanism to control the electron trajectory—and hence the higher harmonics of light emitted by its oscillation—with an accuracy approaching tens of attoseconds. – Frank Narducci