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Abstract: Intense attosecond pulse trains generated by the seeded FEL FERMI have been temporally characterized using a covariance-based approach with single-shot photoelectron spectra [1,2]. Due to the high intensities achieved by FEL pulses, such attosecond pulse trains are ideal candidates for investigating nonlinear processes and, eventually, controlling them by varying the phase of individual harmonics composing the comb. In this work, we investigate third-order nonlinearities induced by an attosecond pulse train in a non-collinear four-wave mixing (FWM) process. We report the first experimental demonstration, to our knowledge, of third-order nonlinear signals generated by the combination of coherent extreme-ultraviolet (XUV) harmonics. Two time coincident replicas of multiharmonic XUV beams were crossed at an angle in a MgF$_2$ film to generate FWM signals [3,4]. Using self-diffraction as a probe of the nonlinear response spectrum, the Mg L$_{2,3}$ edge and several other resonant features were identified in the 50–70 eV range [5]. We observed and identified the FWM pathways of several nonlinear signals depending on multiple harmonics, with enhanced response when individual harmonics are resonant with one or more excitonic states in MgF$_2$. The results shows the presence of unintended neighboring harmonics generated by the FEL undulators, leading to new FWM pathways. We also discuss the possibility of using FWM processes for the temporal characterization of attosecond pulse trains.
[1] P. K. Maroju et al., Nature 578, 386 (2020).
[2] P. K. Maroju et al., New Journal of Physics 23, 043046 (2021).
[3] F. Bencivenga et al., Nature 520, 205 (2015)
[4] R. Mincigrucci et al., Nucl. Instrum. Methods Phys. Res. A 907, 132 (2018)
[5] Luca Assogna et al., Opt. Lett. 51, 1832–1835 (2026)
| Scientific Topics | Solid State Physics |
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