High harmonic generation by short laser pulses: time-frequency behavior and applications to attophysics

Abstract

High harmonic generation (HHG) is a process in which noble gas atoms excited by an intense laser field at frequency ω1 emit radiation of higher frequencies that are odd integer multiples of ω1. Driven by an infrared laser, high harmonic radiation can span from the optical into the extreme ultraviolet (XUV) frequency range. In this thesis, I study HHG using short laser pulses by solving the time-dependent Schrödinger equation (TDSE). In particular, I will focus on calculating and controlling the time-dependent instantaneous frequency (chirp) of high harmonics. The harmonic chirp is a direct consequence of the atom-field interaction during HHG, and its behavior in the time-frequency domain reveals the fundamental physics in the strong laser field. In addition, controlling the harmonic chirp is essential for practical applications such as when high harmonic radiation is used as a seed for soft X-ray lasers. Moreover, I will discuss how to find the ionization probability of an atom exposed to few-cycle laser pulses. This is important in order to understand the propagation effects on high harmonics as their driving pulse goes through a rapidly ionizing medium. Finally, I will consider high harmonics as a source of attosecond pulses. Such short pulses (∼10-18 seconds) could induce and probe the dynamics of electrons in atoms and solids. In the last part of my thesis, I will examine some examples of attosecond electron dynamics.