Modeling generation and characterization of attosecond pulses

Generation of high-order harmonics has emerged as a powerful technique for the generation of broadband coherent radiation in the EUV regime. This has lead to the development of table-top EUV sources that can produce attosecond pulses. These pulses can serve as a probe to resolve atomic attosecond dynamics and image atomic orbitals and molecular motion. Due to high spatial and temporal coherence, highorder harmonic radiation can also be used to seed free electron lasers, which allow the generation of high-intensity X-ray radiation that can be used for imaging biomolecules. Since the first observation of high-order harmonics, effort has been made to accurately model both the generation and the characterization of attosecond pulses. Work on the modeling of high harmonic generation can be divided into two parts: (a) description of the interaction between the JR pulse and atoms that leads to emission of attosecond pulses (the single atom response) and (b) modeling of the propagation of attosecond pulses by accounting for macroscopic phase matching effects. In this work, we will focus on the single atom response which can be calculated either by numerically solving the time dependent Schr6dinger equation (TDSE) or through the semi-classical three step model (TSM). In Chapter 2, the theory of light-atom interaction will be reviewed with the focus on the calculation of the dipole trasition matrix element (DTME) in the strong field formalism. It will be shown that the choice of the basis states Volkov states and Coulomb Volkov states to describe electrons in the continuum is crucial to the accuracy of DTME calculation. In Chapter 3, the TSM will be derived from the Schr6dinger equation by using the saddle point approximation. Through this derivation, the quantum mechanical laser-atom interaction is reduced to a semi-classical model comprising of ionization, propagation and recombination . The numerical scheme for solving the TDSE will be discussed. It will then be used to demonstrate the generation of isolated attosecond pulses from non-sinusoidal sub-cycle pulses. The results of ADK and non-adiabatic ionization models will be compared with that from numerical TDSE, and then used to calculate the harmonic spectra in the tunneling