Study Of The Factors Affecting High-Order Harmonic Generation Efficiency From A One-Dimensional Model Atom

In recent years, with the rapid development of laser technologies, especially the progress in femto-second laser pulses, the peak intensity of laser pulses has reached and even exceeded that of the Coulomb field in a hydrogen atom. Under the action of such strong pulses, a series of phenomena which can no longer be interpreted by the conventional perturbation theory occurs. The High-order harmonic generation (HHG) is just one of them.As we know, when an atom, molecule or cluster interacts with strong laser fields, high-order harmonics of the incident laser pulse frequencies can be generated. A variety of experiments has demonstrated the following characteristics of the HHG spectra: a sharp decline of the intensity for the first few harmonics, followed by a plateau consisting of many harmonics with the roughly same intensity and then an abrupt cutoff at a certain frequency. The importance of the HHG researches lies in the fact that it is not only a promising way of generating coherent light in the extreme ultraviolet and x-ray region but also a means to produce attosecond x-ray pulses. Presently, the major focus of the HHG study is the extension and promotion of the HHG plateau. The main work of this thesis is to study the factors affecting HHG efficiency from a one-dimensional atom, in order to point out the way to promote the HHG plateau.Firstly, we will introduce the HHG mechanism. It has been generally accepted that the HHG process can be divided into three steps. Firstly, the electron is ionized by the laser field through multi-photon ionization, tunnel ionization or over-barrier ionization, depending on the strengths of the laser fields. Secondly, the ionized electron travels in the laser field. The electrons ionized at different instants exhibit different behavior. Some of them will run far away from the core and some others will probably be dragged back to the core region. And thirdly, the electrons in the core region will in various probabilities recombine with the core and emit high energy photons. As the HHG is essentially a stimulated recombination process, the HHG efficiency is decided by the following factors. Factor one, the ground state population at the recombination instant. Higher recombination efficiency will be achieved if the ground state population is richer. Factor 2, the population of the continuum near the core at the recombination instant, the energy difference between the continuum and the ground state matches the photon energy of a certain harmonic. The higher the population, the higher the HHG efficiency will be. Factor 3, the properties of the atom itself, that is the coupling strength between the bound state and the continuum. The last factor is uncontrollable for a given atom.The above theoretic analyses provide a direction for us to make efforts for an in-depth basic study of HHG efficiency. Firstly, under the single active electron approximation, we adopt an atom modeled as a one-dimensional P?schl-Teller potential, and using a Gaussian electron wave packet to take the place of the ionization wave packet in quasiclassical"three-step-model". We make this free wave packet travel freely or travel in the field to the core region, and make sure that it passes the core region. Comparing with the direct proportion relation between the photon emission efficiency and the product of the ground state population and the population of the continuum near the core at the recombination instant without laser electric field, we draw the conclusion by simulating the HHG under the ideal condition as follows: the HHG efficiency is directly proportional to the product of the ground state population and the population of the continuum near the core at the recombination instant.Given the atomic potential parameter and the product of the ground state population and the population of the continuum at initial instant without laser electric field, we analyze the dependency relationship between the photon emission efficiency and the electron energy to be recombined. It is shown that the higher the electron energy, the lower the photon emission efficiency. Simulating the HHG under the ideal condition similarly, we come to the conclusion that the HHG efficiency drops rapidly with the increase of harmonic order. By analyzing the variation of the square module for the coupling strength between the ground state and different energy continuum with the increase of continuum energy, we indicate that just the square module for coupling strength lead to the decline of the harmonic efficiency.In conclusion, to promote the HHG efficiency for a given atom, it is necessary to enhance the product of the ground state population and the population of the continuum near the core at the recombination instant.