Theoretical Exploration Of High-order Harmonic Generation From Atoms And Small Molecules

High-order harmonic generation (HHG) is a nonlinear phenomenaresulting from the interaction of intense laser fields with the atomsand small molecules. As a tool to generate extreme ultraviolet (XUV)light source and even soft X radiation source, HHG has attractedmore and more attention during the last two decades. To generatean intense short attosecond pulse, how to extend the plateau andimprove the intensity of the HHG is the most important issue.Additionally, HHG also plays a significant role in the detection ofultrafast processes with its potential application. Aiming at theabove hot issue, in this article, we not only optimize the HHG inatoms, but also investigate the molecular high-order harmonicgeneration, from which the molecular structure as well as themovement of electrons in the laser field can be traced.The main contents of our works are:(1) Extending the plateau ofHHG spectrum. A scheme to extend the plateau of high-order harmonic spectrum was proposed by using three-color synthesizedlaser field, and the results show that the width can be expendedeffectively. Additionally, the role of the control pulse in the HHG isalso studied. By means of the classical returning kinetic-energymaps and the time-frequency distributions, the physical origin of theextending can be revealed.(2) Improving the intensity of the HHG.An effective method to enhance the intensity of the HHG is toprepare the initial state as a coherent superposition of the groundstate and excited state. The results show that the emissionefficiency of the continuous harmonics is enhanced approximatelyeight orders of magnitude by preparing the initial state as acoherent superposition of the ground state and the first excitedstate. Moreover, when the initial state prepared as the coherentsuperposition of the ground state and the Rydberg state, the HHGare also investigated.(3) Symmetric molecular high-order harmonicgeneration. For H2+molecular ion in few cycle laser pulses,numerical simulation of the one-dimensional (1D) non-Born-Oppenheimer approximation time-dependent Schr dinger equationare performed to study the effect of laser-parameter on molecularhigh-order harmonic generation (HHG). The results show that thereare two minimums in each harmonic spectrum, and the minimum inthe low-order region is insensitive to the laser-parameter, which is associated with the electronic structure of the molecule; however,the minimum in the high-order region is significantly dependent onthe laser parameter, which stems from electronic dynamics inducedby the laser field.(4) Asymmetric molecular high-order harmonicgeneration (MHOHG). The MHOHG of HeH2+is investigated bynumerical integration of the1D non-Born-Oppenheimertime-dependent Schr dinger equation. The results show that theminimum in the MHOHG attributes to the interference of differentrecombination paths. Furthermore, the effects of the nuclearsignature, i.e., the initial vibration state and the isotopic effect onthe MHOHG are studied.