Speaker
Description
The interaction of atoms with attosecond ($10^{-18}$ s) light pulses has renewed interest in studying photoionization dynamics. The ionized electrons exhibit a wave-like behaviour that is commonly used to retrieve the temporal dynamics of the process using optical cross-correlation techniques between light pulses. In this work, we study the photoionization dynamics of helium in a new regime with just few-cycle pulses[1,2].
The experiment is carried out with a 200 kHz laser system that delivers carrier-to-envelope phase stabilized near-infrared (NIR) pulses of ~ 6 femtoseconds[3,4]. The laser beam is separated into a probe beam and a pump beam. The latter is used to generate two or three eXtreme UltraViolet (XUV) pulses, and is recombined and focused with the former in an experimental chamber, where helium is ionized. Cross-correlation scans between the XUV and NIR pulse are recorded using a three-dimensional momentum spectrometer. The scan exhibits complex interference patterns, which oscillate at a multiple of the NIR laser frequency, $ω_{IR}$. The oscillation at $2ω_{IR}$ have been the source of extensive studies and forms the basis of the so-called RABBIT technique, which stands for the reconstruction of attosecond beating by interference of two-photon transition[5].
Here, by means of spectral analysis based on Fourier transforms, an additional oscillation frequency at $ω_{IR}$ is found. We investigate its physical origins by modeling photoionization using the Strong Field Approximation. Finally, we quantify the influence of experimental parameters on the $ω_{IR}$ and $2ω_{IR}$ oscillations providing new insights into this new few-cycle regime of light-matter interaction.
References
[1] Gramajo A. A. et al., Phys. Rev. A 94, 053404 (2016).
[2] Yu-Chen C. et al., PNAS 10, 10727 (2020).
[3] Mikaelsson S. et al., Nanophotonics 10, 117 (2021).
[4] Guo C. et al., J. Phys. B: At. Mol. Opt. Phys. 51, 034006 (2018)
[5] Paul P. M. et al., Science 292, 1689 (2001).
