|Title||Generation, Temporal Characterization and Applications of Femtosecond-/ Attosecond Extreme Ultraviolet Pulses|
|Year of Publication||2009|
The work of this thesis is arranged into three parts: (A) Generation and temporal characterization of extreme ultraviolet (EUV) attosecond pulses. In this work I present the generation and first tempo- ral characterization of sub-optical cycle EUV radiation generated in a noble-gas filled hollow-core waveguide. Two regimes of EUV radiation were characterized, ranging from 200 attoseconds to ~ 1 femtosecond in duration. The first regime that was character- ized distinguishes itself from EUV radiation generated by other methods by its narrow (~ 1 eV) spectral width, its simple energy tunability and its temporal confinement to ~ 1 femtosecond. In the second regime, single isolated pulses of 200 attoseconds du- ration (and accordingly larger bandwidth) were generated. In both regimes dynamic phase-matching effects create an extremely short time window within which efficient nonlinear conversion is possible, while it is suppressed outside this window. Temporal characterization of the generated EUV pulses was approached by two-color pump-probe photoelectron spectroscopy. Therefore an efficient photoelectron spectrometer was set up, detecting electrons in a 2π collection angle. For the interpretation of the experi- mental data, an analytical model as well as an iterative algorithm were developed, to allow extraction of complex EUV waveforms. The demonstrated radiation will allow for time-resolved studies of the fastest processes in molecules and condensed matter, while at the same time ensuring adequate energy resolution for addressing individual electronic states.
(B) Application of a COLTRIMS reaction microscope in combination with femtosecond EUV pulses to questions in molecular physics. The combina- tion of the sensitive detection capabilities of a COLTRIMS reaction microscope with the high time resolution of pump-probe experiments using femtosecond extreme-ultraviolet pulses makes it possible to answer very fundamental open questions in molecular physics such as the dependence of molecular photoionization on the molecular orientation. To this end, molecules were impulsively aligned by means of femtosecond pump pulses. The excited molecular rotational wavepacket experiences revivals that continue long after the pump pulse has left. During such revivals, the molecular axes change their orientation from parallel to perpendicular with respect to the polarization of the pump pulse within a few hundred femtoseconds. Therefore photoionization by femtosecond EUV pulses with variable delay during a revival is equivalent to photoionization of molecules with varying orientation. With this novel method the orientational depen- dence of single-photon photoionization of N2 and CO2 into non-dissociating channels, as well as for a long-lived state of CO+2 was measured for the first time.
(C) Carrier-envelope-phase (CEP) stabilization of a femtosecond chirped pulse amplifier. Lastly, CEP stabilization of intense laser pulses from a chirped pulse amplifier was realized. For amplifier systems containing a pulse stretcher and com- pressor based on diffraction gratings, an increased susceptibility to CEP fluctuations had been expected prior to this work. Therefore the pulse stretcher and compressor system were examined separately in a first step, and the introduced CEP fluctuations were found to be insignificant. The CEP stability of the full amplifier system was then characterized, and excellent long-term stability was shown. With this work a scaling of the energy of CEP-stabilized pulses into the joule range becomes possible. In contrast to absolute frequency measurements with low-energy femtosecond pulses which require stability in the frequency range, the interest in stabilized high-energy pulses is to con- trol the entire electric field of intense laser pulses in the time domain with attosecond precision, to allow full control of the dynamics of electrons in intense light fields.