Simultaneous Extension And Enhancement Of The HHG Plateau By Using Combined Laser Pulse Irradiating On A United Two-Atom System

The interaction of intense laser pulses with atoms, molecules, clusters, and solids can lead to high-order harmonic generation (HHG) of the laser frequency, as a consequence of highly nonlinear dynamics. A general characteristic of atomic HHG spectra has been demonstrated by a variety of experiments: 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 I p + 3.2Up ( I pis the ionization potential of the atom and U pis the pondermotive energy). The plateau structure of HHG power spectra makes HHG a promising way to generate coherent radiations in the "water window"(4.4-2.3nm) and to produce attosecond (asec) x-ray pulses. Due to the exciting application prospects of HHG, researchers are interested in HHG since it is first observed in laboratory at 1987. In order to realize these applications the extension and enhancement of the harmonic plateau become urgent. In this thesis, we use a combined laser pulse composed of a fundamental Ti:sapphire laser and its 33rd harmonic (H33) irradiating on a united two-atom system, and the width and the height of the harmonic plateau are simultaneously extended and enhanced by a large scale. By analyzing the mechanism of HHG from a single atom in detail, we found that the ionized electron in laser field can obtain energies far exceeding3.2U P, but these electrons with high energies have no chance to recombine with the atom because their positions are far away from the nuclear. Compared to the single atom model, the united two-atom model can provide new collision-center for these electrons, which means that the ionized electron can not only recombine with the parent atom but also with the other atom. So it is possible to extend the harmonic plateau by adopting the united two-atom model. As far as the reality of this united two-atom model is concerned, we can consider a plasma expansion process (e.g., when an intense laser pulse irradiates on clusters or solid surfaces), in which the atom-separation will reach the designed value in a certain period. We present the HHG power spectra from the united two-atom system at different inter-nuclear separations, and it is observed that the harmonic plateau is remarkably widened compared with the single atom case. Furthermore, it is found that the larger the separation, the wider the harmonic plateau, but the lower the relevant efficiency; and vice versa. Therefore, we choose a suitable separation πα0 2 ( a0 = E0 /ω2is the quiver radius, where E 0 and ωare the laser electric field amplitude and the central frequency) to ensure the remarkable extension of the plateau and the higher efficiency of the extended plateau. The mechanism of the extension can well be explained with the "three-step"model considering the elastic scattering of the electrons with respect to the atoms: the harmonics in the extended region are produced by the recombination of the ionized electrons directly with the neighboring atom, or the recombination ofthe ionized electrons involving the elastic reflection above the non-parent atom with the parent atom. However, the harmonic emission efficiency of the extended plateau is not ideal, so that it is necessary for the sufficient applications of HHG to enhance the efficiency of these harmonics. On the basis of analyzing the factors affecting the conversion efficiency, we suggest to employ the combined pulse to enhance the efficiency of the HHG. The plateau is heightened in excess of six orders of magnitude by using the combined pulse compared with the case of the fundamental pulse alone. It is found that, the ionization is by and large accomplished by the following process: through a single H33 photon resonance transition an appropriate amount of electrons are first populated to an excited state, and then the electrons are easily ionized by the fundamental pulse. Compared with the case of the fundamental pulse alone, the ionization yield is remarkably enhanced through this way, and the enhancement of the harmonic plateau can be achieved here. In the usual situation (i.e., when ionization probability is not too large) it is the case that the larger the ionization, the higher the harmonic efficiency. Nevertheless, the validity of this law is limited. It is found that the reverse trend may set in when the ground state population is more and more depleted: the larger the ionization, the lower the harmonic efficiency. Why does such a counter-intuitive behavior appear? The reason for this lies in the fact that the recombination probability becomes smaller and smaller due to the stimulated property of the recombination (SR) in laser fields, and so is the harmonic efficiency. Therefore, as far as the harmonic efficiency enhancement is concerned, it is necessary to raise the ionization to an appropriate level. In the case of our combined pulse, we can control the ionization by adjusting thepeak intensity of the H33 pulse. The broad plateau characteristic of HHG power spectra makes HHG a preferred source of generating the asec pulses. The generation of asec pulses is very important and it opens a brand new chapter in ultrafast spectroscopy. For example, asec pulses will allow one to trace electronic motion and relaxation (such as inner-shell dynamics) in atoms and molecules. Recently, first experimental results on the production and measurement of a 650asec pulse were reported. This asec pulse is in the soft-X-ray spectral range and was obtained by using 7-fs laser pulse irradiating neon generating HHG. In this thesis, a single X-ray asec pulse could also be obtained from HHG , which is produced by adopting a long 800nm pulse (9.4fs) and a short H33 pulse (1fs) irradiating the united two-atom system. The central wavelength and the duration of this asec pulse is 5.84nm (H137), 300asec. By observing the ionization yield as a function of time in this case, it is found that the ionization occurs in principle at the half optical cycle near the peak of the fundamental pulse envelope. It means that we can control the ionization because the H33 pulse is basically set in this interval (of course, this result also benefits from the little ionization yield in the case of the fundamental pulse alone). Therefore, the harmonic emission could be limited to this interval or the next half optical period. As far as H137 is concerned, it is generated by the recombination of ionized electrons with the neighboring atom for once, which means that the emission of H137 is concentrated on a very short time scale, consequently, a single rather than a train of X-ray asec pulse is produced in this case. Otherwise, when we change the peak electric amplitude I h of H33 and keep the amplitude of the fundamental pulse constant, we find that there is a range of I h where the intensity of the...