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. With this
high average power measuring hitherto uninvestigated optical
transition frequencies in the XUV, such as the 1S-2S transition in
singly charged helium ions may become a reality.