Four-Wave Mixing

Extracting doubly excited state lifetimes in helium directly in the time domain with attosecond noncollinear four-wave-mixing spectroscopy

Phys. Rev. Res. 6, 043100 (2024)

Patrick Rupprecht, Nicolette G. Puskar, Daniel M. Neumark, and Stephen R. Leone

The helium atom, with one nucleus and two electrons, is a prototypical system to study quantum many-body dynamics. Doubly excited states, or quantum states in which both electrons are excited by one photon, showcase electronic-correlation mediated effects. In this paper, the natural lifetimes of the doubly excited 1⁢𝑃𝑜2⁢𝑠⁢𝑛⁢𝑝 Rydberg series and the 1⁢𝑆𝑒2⁢𝑝2 dark state in helium in the 60–65 eV region are measured directly in the time domain with extreme-ultraviolet/near-infrared noncollinear attosecond four-wave-mixing (FWM) spectroscopy. The measured lifetimes agree with lifetimes deduced from spectral linewidths and theoretical predictions, and the roles of specific decay mechanisms are considered. While complex spectral line shapes in the form of Fano resonances are common in absorption spectroscopy of autoionizing states, the background-free and thus homodyned character of noncollinear FWM results exclusively in Lorentzian spectral features in the absence of strong-field effects. The onset of strong-field effects that would affect the extraction of accurate natural lifetimes in helium by FWM is determined to be approximately 0.3 Rabi cycles. This study provides a systematic understanding of the FWM parameters necessary to enable accurate lifetime extractions, which can be utilized in more complex quantum systems in the future.

Deciphering core-exciton dynamics in Ca⁢F2 with attosecond spectroscopy

Rafael Quintero-Bermudez and Stephen R. Leone

Phys. Rev. B 109, 024308 (2024)

Core excitons in solids have garnered increasing interest, yet their behavior and decay mechanisms are not fully understood. Here, we use attosecond extreme ultraviolet (XUV) transient absorption spectroscopy, performed with a broadband 25–45-eV sub-fs XUV pump pulse and a 500–1000-nm sub-5-fs near-infrared (NIR) supercontinuum probe pulse to monitor the excitation, dynamics, and decay of core excitons in Ca⁢F2 at the Ca2+ 𝑀2,3 edge. The XUV pulses are used to excite core excitons in Ca⁢F2 based around the Ca2+ and the polarization of the medium is subsequently perturbed by the time-delayed NIR pulses to measure the spectral changes and decays. A number of features are identified in the transient absorption spectrum, which suggest transfer between excitonic states, Stark shifts, and the emergence of light-induced states. We find that various core excitons identified exhibit coherence lifetimes spanning 3–7 fs. Furthermore, a NIR-intensity-dependent analysis finds a negative correlation with the coherence lifetime of various identified excitonic features, supporting a phonon-mediated mechanism as responsible for the core-exciton decoherence. We present a computational band structure projection analysis strategy to estimate the orbital structure of the core excitons and determine which core-excitonic transitions should be allowed by selection rules with the probe beam. This strategy is found to successfully describe the observed spectroscopic data. The outlined joint spectroscopic and computational investigation of core excitons is a powerful technique that explains the complex behavior of core excitons in solid-state materials.