Short Attosecond Pulse Trains at High Repetition Rates for Novel Pump-Probe and Coincidence Studies

Sammanfattning: This work aims at studying photoionization dynamics from atoms and surfaces at attose time scales. The work was based on development and applications of a high repetition rate High-order Harmonic Generation (HHG) light source that utilizes Optical Parametric Amplification (OPA) laser technology. The laser system delivers few-cycle pulses in the near infrared (IR) at a repetition rate of 200 kHz. Through the use of advanced pulse characterization techniques, the pulse quality was kept high in order to ensure efficient HHG at relatively low pulse energies. In the HHG process, a short extreme ultraviolet (XUV) light pulse of attosecond duration is produced with every half cycle of the driving field.When performing HHG with driving pulses in the few-cycle regime, a short attosecond pulse train consisting of only a handful pulses is generated. The spectral properties and temporal structure of the HHG radiation was explained in terms of interference between attosecond pulses. The number of pulses can be controlled with the Carrier-to-Envelope Phase (CEP). The short pulse trains were used together with a weak IR field for two-color photoionization, measured with a 3D momentum imaging spectrometer. The angle-resolved photoionization spectra are found to be asymmetric, and behave distinctly different depending primarily on the number of pulses that are used for ionization. In the case when two pulses are used, the electron peaks are shifted, but when the number is increased to three, additional peaks appear. This was explained using an attosecond time-slit interference model.The coincidence capabilities of the photoelectron spectrometer were utilized to measure single-photon double-ionization of helium for the first time with an HHG source. This challenging measurement combines the coincidence and imaging properties of the spectrometer with the efficient generation of high harmonics, and is only possible due to the high repetition rate of the source. The full Triple Differential Cross Section (TDCS) was obtained for a range of energies. Additionally, the IR light source was used for surface science applications, which benefit strongly from a high repetition rate. Light-wave driven currents in semiconductor materials were measured at a higher repetition rate and with longer pulses than reported previously. Plasmon dynamics in gold nanosponges and induced at the edges of thin layers of the Transition Metal Dichalcogenide (TMD) WSe2 were studied.