High harmonic generation from combined femtosecond laser fields

In this thesis I introduce experiments that explore the effect of different combinations of femtosecond laser fields on the high harmonic generation (HHG) process and its ap- plications to probe ultrafast dynamics. Combining a fundamental driving field with a secondary field allows for control over the HHG process in several ways. The ionisation step of the process can be altered significantly by introducing a secondary field. The elec- tron trajectories in the continuum can be controlled and manipulated by the secondary field. Further, the secondary field can be used to drive a population transfer between different states of the cation. In a first experiment, we used the fundamental pulses in combination with copies of themselves (ω + ω experiment) to synthesise a field with a new temporal and spectral structure. Using this synthesised field to drive the HHG process results in a shift of the central frequency of the high-order harmonics. We demonstrated very fine control over the harmonic energy with a setup which is significantly simpler and more cost effective than more commonly used setups, and compatible with very high pulse energy sources. In a second experiment, the fundamental field is combined with its second harmonic (ω + 2ω experiment). The addition of the second frequency results in a harmonic spec- trum consisting of odd and even harmonics, as the symmetry of the combined generating field is broken. We observed below- and above-threshold harmonics in Kr and CO2 and investigated the effect of the secondary field on the harmonic generation as a function of the time delay between the fundamental and its second harmonic. We observed harmon- ics generated by the secondary field only, a delayed onset of some even harmonic orders depending on the delay and interesting modulations of the harmonic yield depending on the delay. This experiment served as a proof of principle for the setup used for the more demanding third experiment. The third experiment combined the fundamental with its third harmonic (ω + 3ω exper- iment). The idea of this experiment is to drive the HHG process with the fundamental field, whilst the third harmonic drives a population transfer between two cation states in CO2. If the population transfer is timed right, the recombination probability for the short and long trajectory contributions can be controlled separately. Since each trajectory rep- resents a specific ionisation and excursion time, any changes of the state of the system are encoded in the interferences between the contributions of the short and long trajecto- ries. This makes so-called quantum path interferences (QPI), interferences between the short and long trajectory contributions, a useful observable to probe femtosecond and attosecond dynamics.Open Acces