Doctor of Philosophy (PhD)
Physics and Astronomy
We present a theoretical study of attosecond transient absorption in gases and high harmonic generation in solids. In this thesis, different features in the attosecond transient absorption laser-dressed spectrum are studied, including features near bright (dipole allowed) atomic states and features that appear approximately one or two laser photons away from the bright states due to the laser-induced coupling to dark (non-dipole allowed) states. The calculations are carried out using both numerical and analytic solutions to the time-dependent Schr\"odinger equation (TDSE). From these solutions, several models of time-dependent absorption and electron dynamics at the femtosecond time scale are presented. We discuss transient absorption in two different regimes: a resonant regime when the dressing field resonantly couples bright and dark states; and a non-resonant regime when the dressed states are detuned. Moreover, a Floquet picture of the transient absorption process is presented, in which the different features in the absorption spectrum are explained as the Floquet states induced by the strong probe pulse. We demonstrate that this Floquet picture applies even though dressing field lasts only a few cycles. We also present a theoretical study of high harmonic generation (HHG) by Bloch electrons in a model transparent solid. This model applies to the recent experiments in ZnO. We solve the TDSE using a velocity gauge numerical method and the resulting harmonic spectrum exhibits a plateau due to the coupling of the valence band to the first conduction band. The energy cutoff of the plateau scales linearly with field strength, which agrees with the ZnO experiment. To facilitate the analysis of the time-frequency characteristics of the emitted harmonics, the TDSE is also solved in the so-called Houston basis which allows the separation of the interband and intraband contributions to the time-dependent current. The interband and intraband transitions are shown to correspond to diabatic and adiabatic dynamics of the system, respectively. The contributions from interband and intraband display very different time-frequency characteristics, which can potentially be used as an experimental signature of the intraband and interband dynamics.
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Wu, Mengxi, "Attosecond Transient Absorption in Gases and High Harmonic Generation in Solids" (2015). LSU Doctoral Dissertations. 3321.