【作者】 郭福明；

【导师】 朱颀人；

【作者基本信息】 吉林大学 ， 原子与分子物理， 2006， 硕士

【摘要】 本文主要研究了一维势模型原子在高、低频组合激光脉冲辐照下的高次谐波发射效率的提高问题,具体工作如下:第一,研究了一维情况下模型原子在强激光辐照下电离电子的动力学行为的SMT结果与量子计算结果的一致性问题。第二,利用SMT分析强场高次谐波发射的平台结构,并以此为基础提出提高高次谐波发射效率的有效方案。更多还原

【Abstract】 In recent years, with the rapid development of laser technology,especially the progress in femto-second laser pulses, the peak intensity of laserpulses has reached and even exceeded that of the Coulomb field in a hydrogenatom. Under the action of such strong pulses, a series of phenomena whichcan 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 stronglaser fields, high-order harmonics of the incident laser pulse frequencies canbe generated. A variety of experiments has demonstrated the followingcharacteristics of the HHG spectra: a sharp decline of the intensity for the firstfew harmonics, followed by a plateau consisting of many harmonics with theroughly same intensity and then an abrupt cutoff at a certain frequency. Theimportance of the HHG researches lies in the fact that it is not only apromising way of generating coherent light in the extreme ultraviolet andx-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 HHGplateau. The goal of this thesis is to promote the HHG plateau of an atommodeled as a one-dimensional P?schl-Teller potential with a combined pulse.Firstly, we will introduce the HHG mechanism. It has been generallyaccepted that the HHG process can be divided into three steps. Firstly, theelectron is ionized by the laser field through multi-photon ionization, tunnelionization or over-barrier ionization, depending on the strengths of the laserfields. Secondly, the ionized electron travels in the laser field. The electronsionized at different instants exhibit different behavior. Some of them will runfar away from the core and some others will probably be dragged back to thecore region. And thirdly, the electrons in the core region will in variousprobability recombine with the core and emit high energy photons. As theHHG is essentially a stimulated recombination process, the HHG efficiency isdecided by the following factors. Factor one, the ground state population atthe recombination instant. Higher recombination efficiency will be achieved ifthe ground state population is richer. Factor 2, the population on thecontinuum of which the energy difference with the ground state matches thephoton energy of a certain harmonic. The higher the population, the higher theHHG efficiency will be. Factor 3, the properties of the atom itself, such as thecoupling strength between the bound state and the continuum. The last factoris uncontrollable for a given atom. So to promote the HHG efficiency, it isnecessary to enhance the ground state population at the recombination instantand simultaneously population of the continuum near the core.Based on the above analysis, we will firstly consider the HHG from asingle atom irradiated by a monochromatic laser pulse. We know easily fromthe “simple man’s theory” (SMT) that when an atom is irradiated by amonochromatic pulse, not all of the ionized electron will come back to the ioncore. Driven by the laser pulse, some of them will drift away from the coreand therefore lose the chance of recombination. So this portion of electrons isof no contribution to the HHG. For the electrons that can come back to thecore region, their kinetic energy is of great difference due to their differentionization instant and only part of them has very high kinetic energy. That isto say, only this part of electrons is able to contribute to the HHG. In summary,limited by the electric field’s form of the monochromatic laser, only part ofthe ionized electrons can contribute to the HHG (especially the higher orderharmonics). So, under the precondition that there is sufficient population onthe ground state, the population on the high continuum state can not be highenough at the recombination instant. As a result, the HHG can not be quiteefficient.To overcome the above described shortcoming, we proposed to employthe combined pulse composed of a low frequency pulse and a high-frequencyone with very thin pulse width. Our method is described as follows: we turnedon a high-frequency pulse when the phase of electric field of thelow-frequency pulse reaches 0.6π + nπ.The laser field parameters areadjusted such that the atom is not or rarely ionized under the actionthe low-frequency pulse alone, while in the time domain thathigh-frequency field exist, the ionization product reaches about50%. Thus, 50% electrons are ionized at the short time regioncentralized about the high-frequency pulse. According to the SMT,driven by the low-frequency field, all of these electrons will comeback to the core taking with them very high kinetic energy. Soprovided that the ground state is sufficiently populated, thepopulation on the high continuum is remarkably enhanced comparedwith the case where only a single monochromatic laser pulse is employed.Therefore, the HHG efficiency is promoted remarkably.更多还原