39 research outputs found
Chirped seeded free-electron lasers: self-standing light sources for two-colour pump-probe experiments
We demonstrate the possibility to run a single-pass free-electron laser in a
new dynamical regime, which can be exploited to perform two-colour pump-probe
experiments in the VUV/X-ray domain, using the free-electron laser emission
both as a pump and as a probe. The studied regime is induced by triggering the
free-electron laser process with a powerful laser pulse, carrying a significant
and adjustable frequency chirp. As a result, the emitted light is eventually
split in two sub-pulses, whose spectral and temporal separations can be
independently controlled. We provide a theoretical description of this
phenomenon, which is found in good agreement with experiments performed on the
FERMI@Elettra free-electron laser
FEL stochastic spectroscopy revealing silicon bond softening dynamics
Time-resolved X-ray Emission/Absorption Spectroscopy (Tr-XES/XAS) is an
informative experimental tool sensitive to electronic dynamics in materials,
widely exploited in diverse research fields. Typically, Tr-XES/XAS requires
X-ray pulses with both a narrow bandwidth and sub-picosecond pulse duration, a
combination that in principle finds its optimum with Fourier transform-limited
pulses. In this work, we explore an alternative xperimental approach, capable
of simultaneously retrieving information about unoccupied (XAS) and occupied
(XES) states from the stochastic fluctuations of broadband extreme ultraviolet
pulses of a free-electron laser. We used this method, in combination with
singular value decomposition and Tikhonov regularization procedures, to
determine the XAS/XES response from a crystalline silicon sample at the
L2,3-edge, with an energy resolution of a few tens of meV. Finally, we combined
this spectroscopic method with a pump-probe approach to measure structural and
electronic dynamics of a silicon membrane. Tr-XAS/XES data obtained after
photoexcitation with an optical laser pulse at 390 nm allowed us to observe
perturbations of the band structure, which are compatible with the formation of
the predicted precursor state of a non-thermal solid-liquid phase transition
associated with a bond softening phenomenon
Attosecond pulse shaping using a seeded free-electron laser
Attosecond pulses are central to the investigation of valence- and core-electron dynamics on their natural timescales1–3. The reproducible generation and characterization of attosecond waveforms has been demonstrated so far only through the process of high-order harmonic generation4–7. Several methods for shaping attosecond waveforms have been proposed, including the use of metallic filters8,9, multilayer mirrors10 and manipulation of the driving field11. However, none of these approaches allows the flexible manipulation of the temporal characteristics of the attosecond waveforms, and they suffer from the low conversion efficiency of the high-order harmonic generation process. Free-electron lasers, by contrast, deliver femtosecond, extreme-ultraviolet and X-ray pulses with energies ranging from tens of microjoules to a few millijoules12,13. Recent experiments have shown that they can generate subfemtosecond spikes, but with temporal characteristics that change shot-to-shot14–16. Here we report reproducible generation of high-energy (microjoule level) attosecond waveforms using a seeded free-electron laser17. We demonstrate amplitude and phase manipulation of the harmonic components of an attosecond pulse train in combination with an approach for its temporal reconstruction. The results presented here open the way to performing attosecond time-resolved experiments with free-electron lasers
A new method for measuring angle-resolved phases in photoemission
Quantum mechanically, photoionization can be fully described by the complex
photoionization amplitudes that describe the transition between the ground
state and the continuum state. Knowledge of the value of the phase of these
amplitudes has been a central interest in photoionization studies and newly
developing attosecond science, since the phase can reveal important information
about phenomena such as electron correlation. We present a new
attosecond-precision interferometric method of angle-resolved measurement for
the phase of the photoionization amplitudes, using two phase-locked Extreme
Ultraviolet pulses of frequency and , from a Free-Electron
Laser. Phase differences between one- and two-photon
ionization channels, averaged over multiple wave packets, are extracted for
neon electrons as a function of emission angle at photoelectron energies
7.9, 10.2, and 16.6 eV. is nearly constant for emission
parallel to the electric vector but increases at 10.2 eV for emission
perpendicular to the electric vector. We model our observations with both
perturbation and \textit{ab initio} theory, and find excellent agreement. In
the existing method for attosecond measurement, Reconstruction of Attosecond
Beating By Interference of Two-photon Transitions (RABBITT), a phase difference
between two-photon pathways involving absorption and emission of an infrared
photon is extracted. Our method can be used for extraction of a phase
difference between single-photon and two-photon pathways and provides a new
tool for attosecond science, which is complementary to RABBITT
Femtosecond polarization shaping of free-electron laser pulses
We demonstrate the generation of extreme-ultraviolet (XUV) free-electron laser (FEL) pulses with time-dependent polarization. To achieve polarization modulation on a femtosecond timescale, we combine two mutually delayed counterrotating circularly polarized subpulses from two cross-polarized undulators. The polarization profile of the pulses is probed by angle-resolved photoemission and above-threshold ionization of helium; the results agree with solutions of the time-dependent Schrödinger equation. The stability limit of the scheme is mainly set by electron-beam energy fluctuations, however, at a level that will not compromise experiments in the XUV. Our results demonstrate the potential to improve the resolution and element selectivity of methods based on polarization shaping and may lead to the development of new coherent control schemes for probing and manipulating core electrons in matter
A detailed investigation of single-photon laser enabled Auger decay in neon
Single-photon laser enabled Auger decay (spLEAD) is an electronic de-excitation process which was recently predicted and observed in Ne. We have investigated it using bichromatic phase-locked free electron laser radiation and extensive angle-resolved photoelectron measurements, supported by a detailed theoretical model. We first used separately the fundamental wavelength resonant with the Ne+ 2s?2p transition, 46.17 nm, and its second harmonic, 23.08 nm, then their phase-locked bichromatic combination. In the latter case the phase difference between the two wavelengths was scanned, and interference effects were observed, confirming that the spLEAD process was occurring. The detailed theoretical model we developed qualitatively predicts all observations: branching ratios between the final Auger states, their amplitudes of oscillation as a function of phase, the phase lag between the oscillations of different final states, and partial cancellation of the oscillations under certain conditions