Phase-locking of time-delayed attosecond XUV pulse pairs

State-resolved femtosecond phase control in dense-gas laser-atom interaction enabled by attosecond XUV interferometry

We perform an interferometric measurement of an extreme ultraviolet (XUV) pulse passing through a dense gas-phase target. Utilizing the interaction with strong and narrow atomic resonances, we demonstrate the femtosecond phase control of the resonant XUV pulse propagation by means of a time-delayed intense near-infrared (NIR) laser pulse. XUV spectral interferometry provides direct access to the amplitude and phase of the transmitted pulse and thus enables the full reconstruction of the ultrafast dynamics. We benchmark our measurement approach with the singly excited 1s4p Rydberg state of helium to directly reconstruct the temporal structure of the transmitted XUV pulse beyond the single-atom response. An NIR-intensity-controlled variable phase step between 0 and 2 rad is measured on the XUV pulse after passage through the medium, originating from laser-induced transient Stark shifts.

Wave packet dynamics and control in excited states of molecular nitrogen

Wave packet interferometry with vacuum ultraviolet light has been used to probe a complex region of the electronic spectrum of molecular nitrogen, N2. Wave packets of Rydberg and valence states were excited by using double pulses of vacuum ultraviolet (VUV), free-electron-laser (FEL) light. These wave packets were composed of contributions from multiple electronic states with a moderate principal quantum number (n ∼ 4-9) and a range of vibrational and rotational quantum numbers. The phase relationship of the two FEL pulses varied in time, but as demonstrated previously, a shot-by-shot analysis allows the spectra to be sorted according to the phase between the two pulses. The wave packets were probed by angle-resolved photoionization using an infrared pulse with a variable delay after the pair of excitation pulses. The photoelectron branching fractions and angular distributions display oscillations that depend on both the time delays and the relative phases of the VUV pulses. The combination of frequency, time delay, and phase selection provides significant control over the ionization process and ultimately improves the ability to analyze and assign complex molecular spectra.

Frequency-domain interferometry for the determination of time delay between two extreme-ultraviolet wave packets generated by a tandem undulator

Synchrotron radiation, emitted by relativistic electrons traveling in a magnetic field, has poor temporal coherence. However, recent research has proved that time-domain interferometry experiments, which were thought to be enabled by only lasers of excellent temporal coherence, can be implemented with synchrotron radiation using a tandem undulator. The radiation generated by the tandem undulator comprises pairs of light wave packets, and the longitudinal coherence within a light wave packet pair is used to achieve time-domain interferometry. The time delay between two light wave packets, formed by a chicane for the electron trajectory, can be adjusted in the femtosecond range by a standard synchrotron technology. In this study, we show that frequency-domain spectra of the tandem undulator radiation exhibit fringe structures from which the time delay between a light wave packet pair can be determined with accuracy on the order of attoseconds. The feasibility and limitations of the frequency-domain interferometric determination of the time delay are examined.