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 3×1016 to 3 × 1018 ) 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.
Highlights
Table-top sources based on high-order harmonic generation (HHG) provide coherent extreme ultraviolet (XUV) radiation on the femtosecond or even attosecond timescale [1]
We have investigated high-order harmonic generation in a supersonic argon gas jet
To identify the contributions of the generated high-order harmonics from both clusters and gas monomers, we measured the harmonic spectra over a broad range of the total atomic number densities in the jet at two different reservoir temperatures (303 K and 363 K)
Summary
Table-top sources based on high-order harmonic generation (HHG) provide coherent extreme ultraviolet (XUV) radiation on the femtosecond or even attosecond timescale [1]. Such radiation is of great interest for various applications such as probing the ultrafast dynamics of atomic, molecular and solid systems [2], lensless diffractive imaging of objects at the nanoscale [3], as well as seeding free-electron lasers [4]. The generation mechanism of HHG in solids differs fundamentally from that in gas atoms. Unlike the atomic three-step model, knowledge of the complex electron dynamics inside the periodic structure of solids, responsible for the generation of harmonics, is missing. In order to prevent permanent damage of the crystal, the drive laser intensities in those experiments are at least one order of magnitude lower than those conventionally applied in the gas medium, which results in a rather low cut-off energy [12]
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