Nonlinear Coherence Effects in Transient-Absorption Ion Spectroscopy with Stochastic Extreme-Ultraviolet Free-Electron Laser Pulses

We demonstrate time-resolved nonlinear extreme-ultraviolet absorption spectroscopy on multiply charged ions, here applied to the doubly charged neon ion, driven by a phase-locked sequence of two intense free-electron laser pulses. Absorption signatures of resonance lines due to 2$p$--3$d$ bound--bound transitions between the spin-orbit multiplets $^3$P$_{0,1,2}$ and $^3$D$_{1,2,3}$ of the transiently produced doubly charged Ne$^{2+}$ ion are revealed, with time-dependent spectral changes over a time-delay range of $(2.4\pm0.3)\,\text{fs}$. Furthermore, we observe 10-meV-scale spectral shifts of these resonances owing to the AC Stark effect. We use a time-dependent quantum model to explain the observations by an enhanced coupling of the ionic quantum states with the partially coherent free-electron-laser radiation when the phase-locked pump and probe pulses precisely overlap in time

Strong-field extreme-ultraviolet dressing of atomic double excitation

We report on the experimental observation of strong-field dressing of an autoionizing two-electron state in helium with intense extreme-ultraviolet laser pulses from a free-electron laser. The asymmetric Fano line shape of this transition is spectrally resolved, and we observe modifications of the resonance asymmetry structure for increasing free-electron-laser pulse energy on the order of few tens of $\mu$J. A quantum-mechanical calculation of the time-dependent dipole response of this autoionizing state, driven by classical extreme-ultraviolet (XUV) electric fields, reveals a direct link between strong-field-induced energy and phase shifts of the doubly excited state and the Fano line-shape asymmetry. The experimental results obtained at the Free-Electron Laser in Hamburg (FLASH) thus correspond to transient energy shifts on the order of few meV, induced by strong XUV fields. These results open up a new way of performing non-perturbative XUV nonlinear optics for the light-matter interaction of resonant electronic transitions in atoms at short wavelengths

Doubly-Excited Helium Strongly Driven with Short and Long Wavelength Pulses

Within this work, electron dynamics of autoionizing, doubly excited states in helium are investigated. Therefore, strong field couplings of these states are studied in two spectral regimes: in the infrared and in the extreme ultraviolet. A laser system capable of tuning its output-pulses' central wavelength within the infrared spectral regime is installed and characterized by a FROG setup. Furthermore, high harmonics of these output pulses are generated and discussed. First intensity dependent and time resolved experiments in helium with this laser system are presented and supported by numerical calculations. The transition from an unperturbed light induced state, which is generated from the 2p4p state, to a resonant Autler-Townes splitting of the sp2;3+ state is observed. Furthermore, strong field couplings between the ground and a doubly-excited state are studied in the extreme ultraviolet only regime. With the help of simulations Rabi-like couplings and Stark shifts for ultrashort pulses leading to a manipulated absorption line shape are investigated. The same study is carried out for autoionizing states. This theory part is used to explain experimentally measured line-shape changes of the 2s2p resonance in helium interacting with FEL pulses

Elektronen-Korrelation in Helium unter Einuss intensiver XUV-Laserpulse

In this work the interaction of helium with intensive XUV-pulses is investigated numerically to interpret the experiments (june 2016) performed at FLASH (Free-Electron-Laser in Hamburg). For this purpose the helium atom can be regarded as a few-level-model, furthermore the electron-correlation as well as the coupling with the continuum are considered. The time-dependent Schrödinger equation of the system is solved with discrete time steps. The result is an intensity dependent variation of the observed 2s2p resonance's line shape. This variation is explained by an energy-change induced phase-shift. With this, results and explanations of similar experiments using intense IR-pulses and weak high harmonics can be conrmed. First steps of the inuence on this line shape variation due to noisy XUV-pulses produced at FLASH are investigated as well

The ocean wave dynamo: a source of magnetic field fluctuations

The alternating magnetic dynamo field of sea surface waves, a consequence of their Lorentz electric field, has been observed with a pair of simultaneously operated, closely spaced tri-axial magnetometers. Measurements from a magnetometer located in the centre of a tiny, uninhabited island served to compensate measurements from a near-shore magnetometer for magnetic pulsations of ionospheric origin, leaving the ocean wave dynamo field, effective close to shore only, as the dominant residual magnetic field. Amplitude and frequency of waves and swell were recorded with a vertical accelerometer (wave rider buoy) floating nearby on the sea surface. A spectral analysis was performed on ten nighttime intervals of three hours length each, and for every interval, the peak power of the surface waves (obtained from the wave rider) was compared with the peak power of the residual horizontal magnetic field (after the background field had been removed). The results suggest that the dual-sensor magnetic field observations yield, within the limits of statistical significance, a good quantitative description of the amplitude and frequency of sea surface waves and swell.JCR Journalope

The ocean wave dynamo: a source of magnetic field fluctuations

The alternating magnetic dynamo field of sea surface waves, a consequence of their Lorentz electric field, has been observed with a pair of simultaneously operated, closely spaced tri-axial magnetometers. Measurements from a magnetometer located in the centre of a tiny, uninhabited island served to compensate measurements from a near-shore magnetometer for magnetic pulsations of ionospheric origin, leaving the ocean wave dynamo field, effective close to shore only, as the dominant residual magnetic field. Amplitude and frequency of waves and swell were recorded with a vertical accelerometer (wave rider buoy) floating nearby on the sea surface. A spectral analysis was performed on ten nighttime intervals of three hours length each, and for every interval, the peak power of the surface waves (obtained from the wave rider) was compared with the peak power of the residual horizontal magnetic field (after the background field had been removed). The results suggest that the dual-sensor magnetic field observations yield, within the limits of statistical significance, a good quantitative description of the amplitude and frequency of sea surface waves and swell

Flexible experimental platform for dispersion-free temporal characterization of ultrashort pulses

The precise temporal characterization of laser pulses is crucial for ultrashort applica- tions in biology, chemistry, and physics. Especially in femto- and attosecond science, diverse laser pulse sources in different spectral regimes from the visible to the infrared as well as pulse durations ranging from picoseconds to few femtoseconds are employed. In this article, we present a versatile temporal-characterization apparatus that can access these different temporal and spectral regions in a dispersion-free manner and without phase-matching constraints. The design combines transient-grating and surface third-harmonic-generation frequency-resolved optical gating in one device with optimized alignment capabilities based on a noncollinear geometry