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

Temporal characterization of individual harmonics of an attosecond pulse train by THz streaking

We report on the global temporal pulse characteristics of individual harmonics in an attosecond pulse train by means of photo-electron streaking in a strong low-frequency transient. The scheme allows direct retrieval of pulse durations and first order chirp of individual harmonics without the need of temporal scanning. The measurements were performed using an intense THz field generated by tilted phase front technique in LiNbO_3 . Pulse properties for harmonics of order 23, 25 and 27 show that the individual pulse durations and linear chirp are decreasing by the harmonic order

Quantum efficiency of technical metal photocathodes under laser irradiation of various wavelengths

Quantum efficiency studies for various laser wavelengths and various technical metal surfaces were carried out in a dedicated unbaked vacuum chamber in the absence of a significant electrical field. Copper, magnesium, aluminum, and aluminum-lithium photocathodes were irradiated by two different high power, high repetition rate, laser systems. We have observed an emission of electrons for photon energies below the work function of the material. This is explained by multiple photon absorption by the photocathode. We have not observed any degradation of the QE for these materials, but an improvement when irradiating them over a long period of time. This is contrary to observations made in RF photogun

Field-driven femtosecond magnetization dynamics induced by ultrastrong coupling to THz transients

Controlling ultrafast magnetization dynamics by a femtosecond laser is attracting interest both in fundamental science and industry because of the potential to achieve magnetic domain switching at ever advanced speed. Here we report experiments illustrating the ultrastrong and fully coherent light-matter coupling of a high-field single-cycle THz transient to the magnetization vector in a ferromagnetic thin film. We could visualize magnetization dynamics which occur on a timescale of the THz laser cycle and two orders of magnitude faster than the natural precession response of electrons to an external magnetic field, given by the Larmor frequency. We show that for one particular scattering geometry the strong coherent optical coupling can be described within the framework of a renormalized Landau Lifshitz equation. In addition to fundamentally new insights to ultrafast magnetization dynamics the coherent interaction allows for retrieving the complex time-frequency magnetic properties and points out new opportunities in data storage technology towards significantly higher storage speed.Comment: 25 page

Towards high brightness electron beams from multifilamentary Nb3Sn wire photocathode

In order to find electron sources of low emittance and high quantum efficiency, single tip cathodes with a microstructured surface are investigated. Emission currents up to 310 A were obtained, by combining a 2 ns, 50 kV accelerating voltage pulse with a 266 nm wavelength, picosecond (sigma(t) = 6.2 ps) laser delivering a few mu J pulse energy. The multifilamentary cylindric Nb3Sn tip with a typical diameter of 0.8 mm provides quantum efficiencies up to 0.5%. The microstructured needle has also been tested in a combined diode-rf electron gun with 500 kV, 250 ns pulsed bias voltage as a first step towards reducing emittance-spoiling space-charge forces

Large Charge Extraction from Metallic Multifilamentary Nb3Sn Photocathode

The current density limit for photoemission from metals was measured in an rf photogun to be below 10(9) A/m(2). We have achieved 1.6 x 10(11) A/m(2) by photofield emission from a new type of photocathode made from a metallic-composite, multifilamentary Nb3Sn wire driven by a 266 nm picosecond laser pulse and a 2 ns, 50 kV accelerating voltage. This cathode has a micrometer arrayed structure with tens of thousands of Nb/Nb3Sn filaments embedded in a bronze matrix. Our measurements revealed the existence of a new electron emission regime at high laser fluence (100 mJ/cm(2)). We have extracted stably, and without any surface ablation, up to 4800 pC of charge. This corresponds to 0.9% quantum efficiency, 100 times larger than what is measured from conventional metallic photocathodes. The unexpected large and stable charge extraction cannot be explained by the 3-step model. Thanks to the small emitting area, the measured emittance (0.6 mm.mrad) is low in spite of the high current density and space charge effects. This cathode will be of benefit for many applications based on short and bright electron bunches

Towards high brightness electron beams from multifilamentary Nb_{3}Sn wire photocathode

In order to find electron sources of low emittance and high quantum efficiency, single tip cathodes with a microstructured surface are investigated. Emission currents up to 310 A were obtained, by combining a 2 ns, 50 kV accelerating voltage pulse with a 266 nm wavelength, picosecond (σ_{t}=6.2  ps) laser delivering a few μJ pulse energy. The multifilamentary cylindric Nb_{3}Sn tip with a typical diameter of 0.8 mm provides quantum efficiencies up to 0.5%. The microstructured needle has also been tested in a combined diode-rf electron gun with 500 kV, 250 ns pulsed bias voltage as a first step towards reducing emittance-spoiling space-charge forces

Quantum efficiency of technical metal photocathodes under laser irradiation of various wavelengths

Quantum efficiency studies for various laser wavelengths and various technical metal surfaces were carried out in a dedicated unbaked vacuum chamber in the absence of a significant electrical field. Copper, magnesium, aluminum, and aluminum-lithium photocathodes were irradiated by two different high power, high repetition rate, laser systems. We have observed an emission of electrons for photon energies below the work function of the material. This is explained by multiple photon absorption by the photocathode. We have not observed any degradation of the QE for these materials, but an improvement when irradiating them over a long period of time. This is contrary to observations made in RF photoguns

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