Tabletop Attosecond Four-Wave Mixing Spectroscopy of Core-Exciton Dynamics in Solids: Studies of NaCl

Nonlinear wave-mixing in solids with ultrafast extreme ultraviolet (XUV) and X-rays can provide new insight into complex electronic dynamics of materials. This study demonstrates that tabletop-based attosecond four-wave mixing (FWM) spectroscopy is a viable approach to studying atom-like, highly localized core-excitonic dynamics in solid-state materials. Attosecond FWM uses one XUV pulse obtained by high harmonic generation and two separately timed few-cycle near-infrared (NIR) pulses arranged non-collinearly to generate a spatially distinct background-free emission. The dynamics of the Na⁺ L_2,3 edge core-excitons in NaCl around 33.5 eV are characterized here. An inhomogeneous distribution of core-excitons underlying the well-known doublet absorption of the Na⁺ -point core-exciton spectrum is deconvoluted by the resonance-enhanced nonlinear FWM signals. Dark core-excitonic states that are coupled to the XUV-allowed levels through the NIR pulses are also characterized spectrally and temporally. Approximate sub-10 femtosecond coherence lifetimes of the core-exciton states are reported. The observed dynamics are discussed in the context of electron-hole exchange interactions, electron-electron correlation, and electron-phonon broadening. This investigation successfully indicates that tabletop attosecond FWM spectroscopies represent a viable technique for time-resolved solid-state measurements.