Coherent Attosecond Photo-electron Spectroscopy

Since the introduction of the concept of an attosecond streak camera as a tool for resolving attosecond dynamics, this technique has evolved from the proof of principle experiments [Kienberger et al., Nature 427, 817 (2004), Goulielmakis et al., Science 305, 1267 (2004)] to a level where it is able to capture otherwise inaccessible information on the timing of photoemission from atoms, molecules and solids. This evolution echoes that of femtochemistry [S. Mukamel, Principles of Nonlinear Optics and Spectroscopy, Oxford University Press, New York, USA, 1995], where the first proof of principle experiments only served to confirm what was already known from steady state spectroscopy. Nowadays the pump probe schemes used in femtochemistry are routinely utilized to unravel complex chemical reactions, and even to control the outcome of chemical reactions via selective excitation of reaction pathways [Brixner and Gerber, ChemPhysChem 4, 418 (2003)]. Similarly, the attosecond streak camera is used to resolve the emission delay and ordering (sequence) of different electron emission processes, which is currently not possible with other attosecond methods. The objective of this project is to utilize the attosecond streak camera to investigate ever more complex and multistage emission processes, in systems where after the absorption of one soft-x-ray photon, several electrons can be emitted. Two main model systems are krypton and xenon atoms. The bandwidth of our attosecond pulses is such that there is a spectral overlap between several different electron wavepackets that may be emitted from krypton and xenon. This spectral overlap offers an opportunity for robust phase retrieval, and will offer a unique insight in more complex photoionization processes like the shake-up mechanism and Auger decay. Furthermore, with an appropriate choice of soft-x-ray photon energy, ultrafast single photon ionization in the strong-field regime can be studied. While the scaling of multiphoton strong field ionization is widely studied and the Keldysh formalism [L.V. Keldysh, Sov. Phys. JETP 20, 1307 (1965)] describing this process is widely accepted, single photon ionization in the strong field regime is barely studied but is fundamentally equally interesting. Resonantly excited krypton atoms are an excellent candidate for such studies, especially since the energy gap from single to double ionization is so large that it is possible to exclude double ionization by the infrared field in the experiment. Moreover, the analysis of the soft-x- ray photoelectron spectrum of krypton offers ample opportunities to resolve the timing and phase of the emitted resonant and non-resonant Auger electrons, which allows for a background free study of the single photon ionization of the resonantly excited krypton.

In the framework of this project we have shown that strong field interactions influence the decay properties of highly excited auto-ionizing atoms and ions. Furthermore, electrons emitted in the Auger decay retain phase information of the excitation pulses, and can interfere with electrons emitted via direct photo-ionization. However, in order to gain more insight into the complex role of the streaking laser field, it is necessary to perform additional measurements using different experimental techniques. We also analysed the photo-ionization delay of electrons emitted from xenon atoms. We measured the photo-ionization delay of electrons emitted from different sub-shells, with and without additional excitation. It was found that electron emission accompanied by further excitation of the parent ion (the so-called shake-up ionization) leads to the fastest emission of the electrons. The PI also worked on the development of novel femtosecond Yb-fiber sources and their applications to nonlinear imaging. - Record pulse parameters were achieved from a single-mode Yb-fiber oscillator using fiber-based intracavity dispersion control. - A completely monolithic Yb-fiber chirped pulse amplifier, with a spliced-on hollow-core photonic bandgap fiber compressor matching the group velocity dispersion of the stretcher consisting of standard single mode fiber and a short length of specially designed dispersion compensating fiber. - Sub-100-fs pulses were generated from an all polarization maintaining Yb-fiber oscillator with intracavity dispersion control using a polarization maintaining higher order mode fiber. - The polarization maintaining Yb-fiber oscillator combined with a fully polarization maintaining single mode fiber amplifier was used to demonstrate its use for in-vivo nonlinear optical microscopy - Using a simplified version of the completely monolithic Yb-fiber chirped pulse amplifier with spliced-on hollow-core photonic bandgap fiber, we demonstrated the significance of matching the stretcher and compressor in all-fiber systems applied to nonlinear optical microscopy.