Phase-stabilized Ultrashort Laser Systems for Spectroscopy

The investigation of laser-matter interactions calls for ever shorter pulses as new
effects can thus be explored. With laser pulses consisting of only a few cycles of the
electric field, the phase of these electric field oscillations becomes important for many
applications.
In this thesis ultrafast laser sources are presented that provide few-cycle laser pulses
with controlled evolution of the electric field waveform. Firstly, a technique for phasestabilizing
ultra-broadband oscillators is discussed. With a simple setup it improves the
reproducibility of the phase by an order of magnitude compared to previously existing
methods.
In a further step, such a phase-stabilized oscillator was integrated into a chirped-pulse
amplifier. The preservation of phase-stability during amplification is ensured by
secondary phase detection. The phase-stabilized intense laser pulses from this system
were employed in a series of experiments that studied strong-field phenomena in a
time-resolved manner. For instance, the laser-induced tunneling of electrons from
atoms was studied on a sub-femtosecond timescale.
Additional evidence for the reproducibility of the electric field waveform of the laser
pulses is presented here: individual signatures of the electric field half-cycles were
found in photoelectron spectra from above-threshold ionization.
Frequency conversion of intense laser pulses by high-order harmonic generation is a
common way of producing coherent light in the extreme ultraviolet (XUV) spectral
region. Many attempts have been made to increase the low efficiency of this nonlinear
process, e.g. by quasi phase-matching. Here, high-harmonic generation from solid
surfaces under grazing incidence instead from a gas target is studied as higher
efficiencies are expected in this configuration.
Another approach to increasing the efficiency of high-harmonic generation is the
placing of the gas target in an enhancement resonator. Additionally, the production of
XUV photons happens at the full repetition rate of the seeding laser, i.e. in the region
of several tens to hundreds of megahertz. This high repetition rate enables the use of
the XUV light for high-precision optical frequency metrology with the frequency comb
technique. With such an arrangement, harmonics up to 15th order were produced. A
build-up cavity that stacks femtosecond laser pulses in a coherent manner to produce
intra-cavity pulse energies of more than ten microjoules at a repetition rate of ten
megahertz is presented here...