Speaker
Description
Free-electron lasers (FELs) providing coherent light pulses ranging from mid-infrared to hard X-ray domain have opened up novel ways to study fundamental aspects of light-matter interactions. The ultrashort (10$^{-15}$ s to 10$^{-18}$ s) and ultra-intense (>10$^{13}$ W/cm$^2$) nature of these pulses allow probing the coherent nature of these interactions. One such example is the Rabi oscillation [1] $-$ the periodic transfer of population between two levels driven by an external electromagnetic field. While these oscillations have been studied mainly in the long wavelength regime, such as from infrared to visible, using femtosecond extreme ultraviolet (XUV) pulses from the seeded FEL FERMI [2], we could study Rabi oscillations in bound-bound transitions for neutral helium [3] as well as for singly-charged helium ion [4]. For the latter case, the high intensity of the FEL pulse allows us to create quantum entanglement between the emitted photoelectron and the XUV-dressed He$^+$ ion undergoing Rabi oscillations. The emitted photoelectron wave-packet entangled with the He$^+$-ion expands to a distance of few hundred nanometers, thus providing indirect evidence of quantum entanglement at mesoscopic length-scale [4]. While seeded FELs are known for their superior temporal coherence properties, they mostly operate in the XUV domain. Compared to that, SASE FELs, despite their low temporal coherence, can go high up in photon energies allowing us to probe ultrafast dynamics in complex molecular systems close to different K$_{\alpha}$-edges. We used a UV-pump $-$ soft X-ray-probe scheme at the European XFEL [5] to study ring opening and subsequent isomerization processes inside a photo-switch molecule in gas phase. The time-dependent mass spectra as well as the X-ray photoelectron spectra show the signature of an ultrafast (~500 fs) conversion from one isomer to another, following excitation of the neutral molecule to a highly-excited state [6].
References:
[1] I.I. Rabi, Space quantization in a gyrating magnetic field. Phys. Rev. 51, 652-654 (1937)
[2] E. Allaria et al., Highly coherent and stable pulses from the FERMI seeded free-electron laser in the extreme ultraviolet. Nat. Photon. 6, 699-704 (2012)
[3] S. Nandi et al., Observation of Rabi dynamics with a short-wavelength free-electron laser. Nature 608, 488-493 (2022)
[4] S. Nandi et al., Generation of entanglement using a short-wavelength seeded free-electron laser. Sci. Adv. 10, eado0668 (2024)
[5] W. Decking et al., A MHz-repetition-rate hard X-ray free-electron laser driven by a superconducting linear accelerator. Nat. Photon. 14, 391-397 (2020)
[6] S. Nandi et al., under preparation (2026).
| Scientific Topics | AMO Physics |
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