Attosecond Streaking of a Backward Re-scattered Electron in an Ultrashort Laser Field

Abstract

Ultrafast atomic dynamics has become a topic of intense research with the technological advancement of high power femtosecond lasers. When an atom interacts with an intense laser field with an intensity of 1014-1015 W/cm2, an electron can be tunnel-ionized and accelerated in the laser field, and returned to the parent atom with the sign change of the field, making rescattering. The strong-field processes including tunneling, acceleration, and rescattering lead to rich physical phenomena such as above-threshold ionization, high harmonic generation, and frustrated tunneling ionization (FTI). In this thesis, we investigate the strong field processes in which the rescattering dynamics of the tunnel-ionized electron is involved. In the first part of the thesis, we demonstrate a method to characterize the waveform of a laser field using the rescattered electron. The momentum of the rescattering electron is altered by adding a weak laser field to the main driving laser field, resulting in the modulation of photoelectron spectra. This streaking experiment provides information on both the laser field and the ultrafast electron dynamics. The waveform of the laser field can be reconstructed from the modulation of the photoelectron spectra. In addition, the ionization time and the rescattering time of the rescattered electron can also be found in the photoelectron spectra. Thus, our attosecond streaking method, utilizing the rescattered electron, is highly useful to investigate the rescattering process. In the second part of the thesis, we present experimental studies on the properties of coherent extreme ultraviolet emission generated through FTI. Four different experiments were performed. Firstly, a pump-probe type experiment was performed in which the FTI emission is obtained using an intense driving laser field with a time-delayed probe pulse. The modulations of both the photon energy and the amplitude of the FTI emission measured as a function of the time delay were observed. Secondly, an FTI experiment was performed using an aligned molecule in which the dependence on the molecular alignment was observed. Thirdly, the interference of two FTI emissions was observed which confirms the high degree of coherence. Lastly, the phase-matching condition of the FTI emission is demonstrated using the spatial distribution of the FTI emission obtained at different target positions. These FTI experiments deepen our understanding of the ultrafast electron dynamics in the FTI process. In the last part of the thesis, we briefly discuss the progress on a liquid target development. We recorded the evolution of a laser-induced plasma with the temporal resolution of tens of fs using an interferometer. We present the progress of high-order harmonic generation using a liquid target and the difficulties to make a stable liquid target in a vacuum. High-order harmonic signal generated from the surface overdense plasma using the liquid target is presented where the harmonics is up to 11th order. The liquid target being developed will be used for an attosecond streaking experiment that requires continuous operation with a high repetition rate. The high-order harmonics generated from the liquid target can be a good candidate for developing a bright extreme ultraviolet light source.