Frequencies-Selected Enhancement Of The Extended High-Order Harmonic Plateau From A United Two-Atom System Irradiated By A Combined Pulse

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...