Femtosecond electron and X-ray source developments for time-resolved applications

This manuscript summarizes the different technological developments and scientific activities in the field of ultrafast laser and electron physics which I have carried out as part of my PhD thesis. The topics addressed in my thesis range from the development of a novel electron gun to the construction and operation of a coherent X-ray laser, which I used for the study of ultrafast magnetization dynamics. The work was carried out within the frame of SwissFEL at the Paul Scherrer Institute. Electron sources with a very good beam quality are a prerequisite for the realization of compact free electron lasers. In the first part of my work I will describe a novel electron cathode, which is based on a micro-structured surface. The cathode offers a quantum efficiency 2 orders of magnitude larger than what is achieved with conventional metallic cathodes. Then I will describe a table-top soft x-ray source based on HHG where I have played a major role in the construction and commissioning. The generated attosecond pulse train from this source has been characterized by means of a newly developed Terahertz streak camera. Furthermore, first attempts have been made to extend the the spectral range offered by HHG to significantly higher photon energies (up to 400 eV) using an intense mid-infrared laser. Such a source will enable new science applications in near future. Finally, the ultrashort x-ray pulses were used to study ultrafast magnetic dynamics in Cobalt thin films. For this purpose two different techniques, namely XMCD and X-ray magneto-optical effect were implemented and tested at the HHG beamline and first studies on ultrafast magnetization were successfully performed

Complete characterisation of attosecond SXR pulses generated by MIR laser sources

Attosecond streaking with broadband SXR continua leads to contributions from multiple overlapping lines in the photoelectron spectrum. The Volkov-transform generalized projection algorithm (VTGPA) is generalised to include all contributing photoelectron bands (multi-line VTGPA) for the reconstruction of ultra-broadband SXR continua. We further investigate the influence of the collection angle of photoelectron detectors on attosecond streaking spectrograms and show full reconstruction for angle-integrated streaking traces. Also, the effects of the photoionization dipole matrix elements on the reconstruction are demonstrated

Complete characterisation of attosecond SXR pulses generated by MIR laser sources

Attosecond streaking with broadband SXR continua leads to contributions from multiple overlapping lines in the photoelectron spectrum. The Volkov-transform generalized projection algorithm (VTGPA) is generalised to include all contributing photoelectron bands (multi-line VTGPA) for the reconstruction of ultra-broadband SXR continua. We further investigate the influence of the collection angle of photoelectron detectors on attosecond streaking spectrograms and show full reconstruction for angle-integrated streaking traces. Also, the effects of the photoionization dipole matrix elements on the reconstruction are demonstrated

Streaking of 43-attosecond soft-X-ray pulses generated by a passively CEP-stable mid-infrared driver

ISSN:1094-408

Energy scaling of carrier-envelope-phase-stable sub-two-cycle pulses at 1.76 µm from hollow-core-fiber compression to 1.9 mJ

We present the energy scaling of a sub-two-cycle (10.4 fs) carrier-envelope-phase-stable light source centered at 1.76 µm to 1.9 mJ pulse energy. The light source is based on an optimized spectral-broadening scheme in a hollow-core fiber and a consecutive pulse compression with bulk material. This is, to our knowledge, the highest pulse energy reported to date from this type of sources. We demonstrate the application of this improved source to the generation of bright water-window soft-X-ray high harmonics. Combined with the short pulse duration, this source paves the way to the attosecond time-resolved water-window spectroscopy of complex molecules in aqueous solutions.ISSN:1094-408

Apparatus for attosecond transient-absorption spectroscopy in the water-window soft-X-ray region

We present an apparatus for attosecond transient-absorption spectroscopy (ATAS) featuring soft-X-ray (SXR) supercontinua that extend beyond 450 eV. This instrument combines an attosecond table-top high-harmonic light source with mid-infrared (mid-IR) pulses, both driven by 1.7-1.9 mJ, sub-11 fs pulses centered at 1.76 [Formula: see text]m. A remarkably low timing jitter of [Formula: see text] 20 as is achieved through active stabilization of the pump and probe arms of the instrument. A temporal resolution of better than 400 as is demonstrated through ATAS measurements at the argon L[Formula: see text]-edges. A spectral resolving power of 1490 is demonstrated through simultaneous absorption measurements at the sulfur L[Formula: see text]- and carbon K-edges of OCS. Coupled with its high SXR photon flux, this instrument paves the way to attosecond time-resolved spectroscopy of organic molecules in the gas phase or in aqueous solutions, as well as thin films of advanced materials. Such measurements will advance the studies of complex systems to the electronic time scale.ISSN:2045-232

Few-femtosecond electronic and structural rearrangements of CH 4 + driven by the Jahn–Teller effect

The Jahn–Teller effect (JTE) is central to the understanding of the physical and chemical properties of a broad variety of molecules and materials. Whereas the manifestations of the JTE in stationary properties of matter are relatively well studied, the study of JTE-induced dynamics is still in its infancy, largely owing to its ultrafast and non-adiabatic nature. For example, the time scales reported for the distortion of CH 4 + from the initial T d geometry to a nominal C 2 v relaxed structure range from 1.85 fs over 10 ± 2 fs to 20 ± 7 fs. Here, by combining element-specific attosecond transient-absorption spectroscopy and quantum-dynamics simulations, we show that the initial electronic relaxation occurs within 5 fs and that the subsequent nuclear dynamics are dominated by the Q2 scissoring and Q1 symmetric stretching modes, which dephase in 41 ± 10 fs and 13 ± 3 fs, respectively. Significant structural relaxation is found to take place only along the e-symmetry Q2 mode. These results demonstrate that CH 4 + created by ionization of CH 4 is best thought of as a highly fluxional species that possesses a long-time-averaged vibrational distribution centered around a D 2 d structure. The methods demonstrated in our work provide guidelines for the understanding of Jahn–Teller driven non-adiabatic dynamics in other more complex systems

Sub-7-femtosecond conical-intersection dynamics probed at the carbon K-edge

Conical intersections allow electronically excited molecules to return to their electronic ground state. Here, we observe the fastest electronic relaxation dynamics measured to date by extending attosecond transient-absorption spectroscopy (ATAS) to the carbon K-edge. We selectively launch wave packets in the two lowest electronic states (D0 and D1) of C2H4+. The electronic D1 → D0 relaxation takes place with a short time constant of 6.8 ± 0.2 femtoseconds. The electronic-state switching is directly visualized in ATAS owing to a spectral separation of the D1 and D0 bands caused by electron correlation. Multidimensional structural dynamics of the molecule are simultaneously observed. Our results demonstrate the capability to resolve the fastest electronic and structural dynamics in the broad class of organic molecules. They show that electronic relaxation in the prototypical organic chromophore can take place within less than a single vibrational period.ISSN:0036-8075ISSN:1095-920