Cluster size dependence of high-order harmonic generation

Abstract

We investigate high-order harmonic generation (HHG) from noble gas clusters in a supersonic gas jet. To identify the contribution of harmonic generation from clusters versus that from gas monomers, we measure the high-order harmonic output over a broad range of the total atomic number density in the jet (from 3×10 16 to 3×10 18cm 3 ) at two different reservoir temperatures (303 and 363 K). For the first time in the evaluation of the harmonic yield in such measurements, the variation of the liquid mass fraction, g, versus pressure and temperature is taken into consideration, which we determine, reliably and consistently, to be below 20% within our range of experimental parameters. By comparing the measured harmonic yield from a thin jet with the calculated corresponding yield from monomers alone, we find an increased emission of the harmonics when the average cluster size is less than 3000. Using g, under the assumption that the emission from monomers and clusters add up coherently, we calculate the ratio of the average single-atom response of an atom within a cluster to that of a monomer and find an enhancement of around 100 for very small average cluster size (∼200). We do not find any dependence of the cut-off frequency on the composition of the cluster jet. This implies that HHG in clusters is based on electrons that return to their parent ions and not to neighboring ions in the cluster. To fully employ the enhanced average single-atom response found for small average cluster sizes (∼200), the nozzle producing the cluster jet must provide a large liquid mass fraction at these small cluster sizes for increasing the harmonic yield. Moreover, cluster jets may allow for quasi-phase matching, as the higher mass of clusters allows for a higher density contrast in spatially structuring the nonlinear medium.

High harmonic generation on noble gas clusters

“…Whereas in Xe the cutoff frequency shows an increase with increasing backing pressure, this is not so clear in high-pressure Ar, as it shows saturation. A possible explanation is that the conditions for ideal phase matching may improve with increasing backing pressure, similarly to atoms [29]. Additionally, the monotonic increase for the largest clusters can partly be caused by increasing space charge of multiply ionized Xe (providing the largest clusters), in accordance with results of Ruf et al on multiply ionized Kr clusters [23].…”

“…However, a fundamental research question related to cluster-based HHG remained, namely whether the electron recombines to the parent ion in a cluster ("atom-to-itself" recombination) or recollision with neighbouring atoms is also possible resulting in "atom-to-neighbour recombination". This possible scenario was indirectly studied before [28,29] and further support for this hypothesis was provided by independent theory work [33][34][35][36]. It is also possible that harmonics come from a delocalized wavefunction over the whole cluster and to which they recombine coherently after tunneling out ("cluster-to-itself" recombination) [23].…”

“…The use of clusters as HHG targets has a lot of potential compared to atomic monomers, e.g clusters provide higher photon yield [24][25][26] and higher cutoff [26,27], although there is also ambiguity in the literature concerning the latter [28,29]. Some numerical studies [30] predicted an extended cutoff even beyond a photon energy corresponding to I p + 8U p , but this was never confirmed by experiments [26,27].…”

EUV Source Metrology

“…Besides the grating qualification, the HHG source is verified in terms of the cut-off wavelength for further experiments. [41][42][43]…”

Nonlinear wakefields and electron injection in cluster plasma

“…The interactions between short laser pulses and highdensity clusters has been studied extensively [8][9][10] due to the variety of applications from sources of soft x-ray or EUV emission [11][12][13] to high harmonic generation [14][15][16][17]. These articles found that differences in cluster density, cluster diameter, laser amplitude, or electric field gradient have a significant impact on the expansion of the cluster.…”