Inhomogeneous High Harmonic Generation in Krypton Clusters

High order harmonic generation from clusters is a controversial topic: conflicting theories exist, with different explanations for similar experimental observations. From an experimental point of view, separating the contributions from monomers and clusters is challenging. By performing a spectrally and spatially resolved study in a controlled mixture of clusters and monomers, we are able to isolate a region of the spectrum where the emission purely originates from clusters. Surprisingly, the emission from clusters is depolarized, which is the signature of statistical inhomogeneous emission from a low-density source. The harmonic response to laser ellipticity shows that this generation is produced by a new recollisional mechanism, which opens the way to future theoretical studies.

Figure 1

(a) High harmonic spectrum at 350 K. The vertical lines correspond to electronic transitions in krypton ions. (b) Normalized difference signal of a spectrum at 350 K and a spectrum at 510 K. The signal from the pink areas dominates at 350 K. (c) Integrated signal of harmonic 15, harmonic 25 and the plasma line centered at 49.4 nm as a function of the temperature at the gas nozzle.

Figure 2

(a) Polarimetry map depicting the oscillation amplitude of the the Malus’ law for each pixel at 350 K. (b) Simulated harmonic spatial profile for a single laser shot (left), 1000 shots (middle), and degree of linear polarization for 1000 shots (right). The first row corresponds to HHG in atoms and the second row to HHG in large clusters.

Figure 3

(a) Harmonic decay rate $\beta$ at 510 K. (b) Evolution of $\beta$ with temperature. The values are an average over the areas depicted above.