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Abstract We present the results of an experiment investigating the generation of high-order harmonics by a femtosecond near-infrared (NIR) laser pulse in the presence of an extreme ultraviolet (XUV) field provided by a free-electron laser (FEL), a process referred to as XUV-assisted high-order harmonic generation (HHG). Our experimental findings show that the XUV field can lead to a small enhancement in the harmonic yield when the XUV and NIR pulses overlap in time, while a strong decrease of the HHG yield and a red shift of the HHG spectrum is observed when the XUV precedes the NIR pulse. The latter observations are in qualitative agreement with model calculations that consider the effect of a decreased number of neutral emitters but are at odds with the predicted effect of the correspondingly increased ionization fraction on the phase matching. Our study demonstrates the technical feasibility of XUV-assisted HHG experiments at FELs, which may provide new avenues to investigate correlation-driven electron dynamics as well as novel ways to study and control propagation effects and phase matching in HHG.
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Macroscopic properties of high-order harmonic generation from molecular ions
High harmonic spectroscopy utilizes the extremely nonlinear optical process of high-order harmonic generation (HHG) to measure complex attosecond-scale dynamics within the emitting atom or molecule subject to a strong laser field. However, it can be difficult to compare theory and experiment, since the dynamics under investigation are often very sensitive to the laser intensity, which inevitably varies over the Gaussian profile of a typical laser beam. This discrepancy would usually be resolved by so-called macroscopic HHG simulations, but such methods almost always use a simplified model of the internal dynamics of the molecule, which is not necessarily applicable for high harmonic spectroscopy. In this Letter, we extend the existing framework of macroscopic HHG so that high-accuracyab initiocalculations can be used as the microscopic input. This new (to the best of our knowledge) approach is applied to a recent theoretical prediction involving the HHG spectra of open-shell molecules undergoing nonadiabatic dynamics. We demonstrate that the predicted features in the HHG spectrum unambiguously survive macroscopic response calculations, and furthermore they exhibit a nontrivial angular pattern in the far field.
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- Award ID(s):
- 1734006
- PAR ID:
- 10140323
- Publisher / Repository:
- Optical Society of America
- Date Published:
- Journal Name:
- Optics Letters
- Volume:
- 45
- Issue:
- 7
- ISSN:
- 0146-9592; OPLEDP
- Format(s):
- Medium: X Size: Article No. 1954
- Size(s):
- Article No. 1954
- Sponsoring Org:
- National Science Foundation
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