We review our recent progress toward attosecond-precision ultrafast photonics based on ultra-low timing jitter optical pulse trains from mode-locked lasers. In femtosecond mode-locked lasers, the concentration of a large number of photons in an extremely short pulse duration enables the scaling of timing jitter into the attosecond regime. To characterize such jitter levels, we developed new attosecond-resolution measurement techniques and show that standard fiber lasers can achieve sub-fs high-frequency jitter. By leveraging the ultra-low jitter of free-running mode-locked lasers, we pursued high-precision optical-optical and optical-microwave synchronization techniques. Optical signals spanning 1.5 octaves were synthesized by attosecond-precision timing and phase synchronization of two independent mode-locked lasers. High-stability microwave signals were also synthesized from mode-locked lasers with drift-free sub-10-fs precision. We further demonstrated the attosecond-precision distribution of optical pulse trains to remote locations via timing-stabilized fiber links. Finally, the application of optical pulse trains for high-resolution sampling and analog-to-digital conversion is discussed.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/lpor.200910003

Attosecond‐precision ultrafast photonics

Citation: Savelyev, E., Boll, R., Bomme, C., Schirmel, N., Redlin, H., Erk, B., . . . Rolles, D. (2017). Jitter-correction for IR/UV-XUV pump-probe experiments at the FLASH free-electron laser. New Journal of Physics, 19, 13. doi:10.1088/1367-2630/aa652d

/pdf/jitter-correction-for-ir-uv-xuv-pump-probe-experiments-at-1t6pgwh58w.pdf

Jitter-correction for IR/UV-XUV pump-probe experiments at the FLASH free-electron laser

We present a systematic study of the photoionization of noble gas atoms exposed simultaneously to ultrashort (20 fs) monochromatic (1–2% spectral width) extreme ultraviolet (XUV) radiation from the Free-electron Laser in Hamburg (FLASH) and to intense synchronized near-infrared (NIR) laser pulses with intensities up to about 1013 W cm−2. Already at modest intensities of the NIR dressing field, the XUV-induced photoionization lines are split into a sequence of peaks due to the emission or absorption of several additional infrared photons. We observed a plateau-shaped envelope of the resulting sequence of sidebands that broadens with increasing intensity of the NIR dressing field. All individual lines of the nonlinear two-color ionization process are Stark-shifted, reflecting the effective intensity of the NIR field. The intensity-dependent cut-off energies of the sideband plateau are in good agreement with a classical model. The detailed structure of the two-color spectra, including the formation of individual sidebands, the Stark shifts and the contributions beyond the classical cut-off, however, requires a fully quantum mechanical description, as is demonstrated with time-dependent quantum calculations in single-active electron approximation.

https://iopscience.iop.org/article/10.1088/1367-2630/14/4/043008/pdf

Atomic photoionization in combined intense XUV free-electron and infrared laser fields

The dissociation dynamics induced by a 100 fs, 400 nm laser pulse in a rotationally cold Br-2 sample was characterized by Coulomb explosion imaging (CEI) using a time-delayed extreme ultra-violet (XUV) FEL pulse, obtained from the Free electron LASer in Hamburg (FLASH). The momentum distribution of atomic fragments resulting from the 400 nm-induced dissociation was measured with a velocity map imaging spectrometer and used to monitor the internuclear distance as the molecule dissociated. By employing the simultaneously recorded in-house timing electro-optical sampling data, the time resolution of the final results could be improved to 300 fs, compared to the inherent 500 fs time-jitter of the FEL pulse. Before dissociation, the Br-2 molecules were transiently 'fixed in space' using laser-induced alignment. In addition, similar alignment techniques were used on CO2 molecules to allow the measurement of the photoelectron angular distribution (PAD) directly in the molecular frame (MF). Our results on MFPADs in aligned CO2 molecules, together with our investigation of the dissociation dynamics of the Br-2 molecules with CEI, show that information about the evolving molecular structure and electronic geometry can be retrieved from such experiments, therefore paving the way towards the study of complex non-adiabatic dynamics in molecules through XUV time-resolved photoion and photoelectron spectroscopy.

https://iopscience.iop.org/article/10.1088/0953-4075/46/16/164029/pdf

Towards imaging of ultrafast molecular dynamics using FELs

Two-photon excitation of a single-photon forbidden Auger resonance has been observed and investigated using the intense extreme ultraviolet radiation from the free electron laser in Hamburg. At the wavelength 26.9 nm (46 eV) two photons promoted a 3d core electron to the outer 4d shell. The subsequent Auger decay, as well as several nonlinear above threshold ionization processes, were studied by electron spectroscopy. The experimental data are in excellent agreement with theoretical predictions and analysis of the underlying multiphoton processes.

/pdf/two-photon-excitation-and-relaxation-of-the-3d-4d-resonance-4eeqyxwgw3.pdf

Two-photon excitation and relaxation of the 3d-4d resonance in atomic Kr.

We describe an experimental system designed for single-shot photoelectron spectroscopy on free atoms and molecules at the Free Electron Laser in Hamburg (FLASH at DESY). The combination of the extreme ultra-violet (EUV) Free Electron Laser and a temporally synchronized optical fs laser (Ti:Sapphire) enables a variety of two-color pump-probe experiments. The spectral, temporal and spatial characteristics of both the EUV FEL and the optical laser pulses, the experimental procedure to control their overlap as well as the performance of an electron spectrometer used to obtain single-shot photoelectron spectra are discussed. As an illustration of the capabilities of this set-up, some results on two-photon two-color ionization of rare gases are presented.

/pdf/an-experiment-for-two-color-photoionization-using-high-4nem7re4i8.pdf

An experiment for two-color photoionization using high intensity extreme-UV free electron and near-IR laser pulses

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An experiment for two-color photoionization using high intensity extreme-UV free electron and near-IR laser pulses