【作者】 陈高；

【导师】 朱颀人；

【作者基本信息】 吉林大学 ， 原子与分子物理， 2005， 博士

【摘要】 在本论文中,我们利用一束组合激光脉冲辐照联合两原子模型系统,在大幅度地展宽高次谐波平台的同时,又有选择性地提高了特定的几次谐波的效率,其具体内容主要由以下三部分组成。第一部分,在分析单原子高次谐波产生机制的基础上,给出了扩展谐波发射平台的新途径,即设法充分利用那些超过截止频率的高能电子,在此基础上,给出了采用联合两原子模型作为研究对象的原因,随后计算了该模型在不同阱间距下的谐波发射谱,从而实证了利用该模型确实可以大幅度地扩展谐波发射平台。最后分别从定性以及定量的角度对平台展宽部分的产生机制给出了合理的分析。第二部分,在简要分析了影响谐波效率的几个主要因素的基础上,强调了电离产额在提高谐波效率方面的绝对重要性。针对由联合原子模型得到的延展的谐波平台低效的特点,给出了提高谐波转换效率的一个新方案:考虑到在单一低频脉冲作用下联合原子模型的电离产额较低,设想通过采用一束组合脉冲来代替单一的低频脉冲,以控制电离产额在一个较短的时间内达到合适的数值,结果我们实现了延展平台的选频提高,并进而从三步模型的角度给出了合理的解释。最后为了验证我们给出的分析具有合理性,又从波包动力学的观点加以阐述和说明。第三部分,通过采用联合两原子模型与组合脉冲相互作用,可以有选择性地提高延展的谐波平台上的特定的谐波群,由于这些谐波已经进入了X 射线波段,因此通过对这个谐波群中的一次谐波或几次谐波做小波变换,我们得到了不仅具有较高的强度,而且脉宽仅有几十阿秒的超短阿秒脉冲。更多还原

【Abstract】 As a result of the rapid development of ultra-short and ultra-intense laser technology, the peak electric field strength of lasers has reached or exceeded the coulomb field strength seen by the electron in the ground state of atomic hydrogen. The application of such intense laser fields to the atoms leaded to the discovery of a number of novel strong-field phenomena that can’t be explained by traditional perturbation theories. The high-order harmonic generation (HHG) is one of those. It is well known that the interaction of the intense laser pulse with atoms, molecules, solids and clusters can lead to HHG of the laser frequency. In the presence of linearly polarized laser fields, a common characteristic of HHG power spectra has been confirmed by a variety of experiments: it generally shows a marked drop in intensity for the first few harmonics, then levels off forming a plateau region in which the harmonic intensity remains approximately constant up to a rather sharp cutoff. The plateau structures of HHG power spectra make HHG possible sources of radiation in x-ray and XUV regions as well as a promising means of generating laser pulses of attosecond scale. Due to these potential applications, it has become a focus of study and attention in recent years. In order to realize these prospects the unremitting pursuit in HHG studies is to simultaneously increase conversion efficiencies and reach shorter wavelengths. That is to say, it is necessary to bring the HHG in control to satisfy one’s needs. The mechanism of HHG should be investigated in order to control it. Recently, the semiclassical “three-step”model was accepted widely in the understanding of the harmonic generation: the electron is first ionized due to tunneling through the potential barrier formed by the laser field and the ionic field or due to absorbing one or several photons, and then oscillates in the laser field. When the laser field reverses direction, part of ionized electrons come back to the vicinity of their parent ions and transit back to the bound states. At the same time, the system emits high energy photons. The photon energy is the sum of the kinetic energy that electrons acquire from the light field and the ionization potential of the electrons in the atom. According to the classical calculation, when the ionized electron comes back to the parent ion, its maximal kinetic energy is 3.2U p, so that the maximal photon energy is Ec utoff = I p + 3.2Up. Here I p is the ionization potential and U pis the ponderomotive energy. From the cutoff law, we notice that for a single atom model, taking the atom or ion with larger ionization potential as target is the one of most direct way in extending the harmonic plateau. In fact, the maximal cutoff position wascurrently achieved from the interaction of intense laser pulse with He atom with the largest ionization potential among all atoms. Using highly charged ions, which have much higher ionization potentials, has been proposed to further extend cutoff position. Unfortunately, the small ionization yield leads to a poor harmonic efficiency. Another direct way is to increase ponderomotive energy. To realize the aim, it is necessary to raise the intensity of the laser pulse when laser wavelength is kept constant. However, an atom will be depleted completely when the laser intensity rises up to a certain threshold amount, so that the corresponding harmonic emission process also terminates. To overcome current difficulty, one turns to short pulse lasers, which provides the advantage that the depletion of atom is avoided when relatively stronger lasers are employed. Experimentally, the energetic photons of 500ev can be generated using the laser pulses with duration of 5-7fs, which is close to one optical cycle. Therefore, the HHG plateau can no longer be extended through the further shortening the pulse duration. The above analysis shows that the existing methods are limited in extending the width of the plateau. To obtain more energetic photons, new approaches must be suggested. In this thesis, we propose a scheme, by which not only the width of the plateau is extended on a large scale, more importantly, the efficiency of the HHG power spectrum is enhanced selectively. Firstly, a united two-atom model is employed to realize the extension of the plateau width. We know from the analysisof single-atom harmonic spectra that there are plenty of ionized electrons whose kinetic energies exceed 3.2U p in the process of the laser-atom interaction and that they can’t converted into high-energy photons owing to their positions far away from parent-ions. We suppose if these electrons are provided with another recombination object, so that more energetic electrons can participate in the recombination process, then the plateau width will be extended on a large scale. So we design a united two-atom model to simulate the actual plasma environment, in which the atom-separation is optimized. Such a plasma condition will occur in some certain period in relevant plasma expansion process, when an intense laser irradiates on clusters or solid surface. To demonstrate the physical feasibility of our project, by using the Crank-Nicolson time-propagation method, we systematically investigate the HHG power spectrum of the one-dimensional united two-atom model. It is observed that the harmonic plateau takes on the complex multi-plateau structure. Specifically speaking, besides cutoff position I p + 3.2Up there are still three plateaus in the HHG spectrum, where cutoff positions are in turn I p + 5.6Up, I p + 7.0Up, and I p + 8.5Up for the inter-nuclear separation x0 = πα0 / 2( α0is the quiver radius).In the following we give detailed analysis of the generation mechanism of these plateaus, both qualitative and quantitatively. Due to the existence of two collision-centers in an untied two-atom system, the ionized electron driven by the laserelectric field does not only recombine with it, but also recombine with the other ion. Here we would like to emphasize a basic fact: when ionized electrons meet an ion, only part of them can recombine with the ion; the rest of them will either elastically collide with the ion and go back to the other ion or pass by the ion in the 1D situation. It is the existence of those complex factors that gives rise to the complex new structures of the HHG spectrum. In order to quantitatively examine the above ideas, we analyzed with the famous three-step model the contributions of the four main mechanisms to the HHG power spectrum. It is found that the maximal kinetic energies acquired by the electron at the moment of recombination is in accordance with the cut-off positions of the plateau. This confirms our analysis of the HHG mechanism in the united two-atom model. Secondly, it is observed from the united-atom HHG power spectrum that although the width of the plateau is extended on a large scale, the efficiency of the extended harmonics is not as much ideal. To enhance the height of the plateau, we employed the combined laser pulse, which is composed of a low-frequency femtosecond pulse and a high-frequency attosecond pulse. We would like to stress that the high-frequency laser sources used here should be easily obtained, for example, with the aid of the VUV-FELs at HASYLAB, where photon energies in excess of 200 eV, at high intensities (1018 Wcm-2 and up), should soon become available.We systematically investigate the HHG power spectrum of the one-dimensional united two-atom model interacting with the combined pulse, by using the Crank-Nicolson time-propagation method. Here the high-frequency attosecond pulse is added around the zero-zone of the low-frequency oscillation near the peak of the driving pulse envelope. As a result, the harmonic efficiencies near the second cutoff position I p + 5.6Up are higher over four orders of magnitude than those in the driving pulse alone. The high frequency pulse in such a combined pulse irradiating on the united two-atom system ionizes each atom, in a large rate (but not to a too large ionization yield), mainly at a particular time-interval. When the ionized electron from an atom gets into the vicinity of the other atom and recombines with it, particular harmonics enhancement is achieved. Here the role of the low-frequency pulse is still to accomplish simple-man’s dynamics, that is, make ionized electrons to accelerate and to recombine with ion. In the above description, we analyzed the mechanism of frequencies-selected enhancement of the extended HHG plateau from the viewpoint of the three-step model. To demonstrate the validity of the above analysis, we also observed the motion of the wave-packet of the ionized electrons. We found that at the moment that the spatial wave-packet ionized by the high-frequency pulse gets into the vicinity of the other atom, the wave-packet is mainly composed of ionized electrons with kinetic energy 5.6U p. And that the moment is in accordance with the maximum recombination moment given by the wavelet transform for the更多还原