Generation And Optimization Of Isolated Attosecond Pulses

The generation of attosecond pulses and development of correlative techniques enable the investigation of steering and tracing electronic motion in atoms, molecules and nanostructures, and open the door to attosecond physics. Currently, it is still the key of attosecond science to generate the isolated attoseocnd pulses with shorter duration and higher intensity. Consequently, it is one of the motivations to explore the new mechanisms for the generation of isolated attosecond pulses with shorter duration and higher intensity. On the other hand, the conventional schemes for generating isolated attosecond pulses require few-cycle driving pulses. Therefore, it is also one of the motivations to explore the new mechanisms for generating isolated attosecond pulses using multi-cycle driving pulses. Aim at the above questions, the main contents of this dissertation are given in the following:(1) The generation of the broadband water window supercontinuum with a ω+3ω/2multi-cycle two-color field is investigated. It is found that the two-color field can simultaneously control the acceleration and the ionization steps of the harmonic process. Then the broadband water window supercontinuum can be obtained.(2) The generation of the broadband water window supercontinuum with the combined field synthesized by a few-cycle two-color field and an orthogonally polarized laser pulse is investigated. It is found that the few-cycle two-color field can simultaneously control the acceleration and the ionization steps, and the orthogonally polarized control pulse can control the recombination step. Namely, this scheme can simultaneously control the ionization, acceleration and recombination steps of the harmonic process. Then the broadband water window supercontinuum and isolated attosecond pulses can be obtained.(3) The generation of the broadband supercontinuum with the combined field synthesized by a few-cycle two-color field and a27th harmonic pulse is investigated. By properly adding a27th harmonic pulse to resonantly excite the He+ion, the intensity of the supercontinuum can be enhanced. As a result, intense isolated attosecond pulses can be generated. (4) We investigate the generation of the high-order harmonics and attosecond pulses from the asymmetric molecules. It is shown that the ionization asymmetry of the asymmetric molecules can work well only when the electric field is small enough. In addition, the generation of the extreme ultraviolet supercontinnuum from asymmetric molecules driven by the combined field synthesized by a few-cycle laser pulse and a terahertz field is also studied. It is found that the terahertz field significantly increases the energy difference between photons in the first and second halves of the optical cycle at the centre of the laser pulse. Then the broadband supercontinuum can be produced.(5) The propagation effects of the broadband supercontinuum generated with a multi-cycle chirped and chirped-free two-color field are investigated. It is found that the long trajectory can be removed after propagation because of the different phase-matching conditions of the short and the long trajectories. In addition, we further introduce this scheme to the pre-excited medium. It is found that only the small initial population of4%can generate well phase-matching supercontinnum and intense isolated attosecond pulses.(6) We investigate the generation of the broadband supercontinuum with the two-color field synthesized by a long-wavelength driving pulse and a weak400-nm few-cycle pulse. With this scheme, the ionization can be confined within the half-cycle of the driving pulse. Thus the broadband supercontinuum in the plateau can be produced, and the intensity is improved by about2-3orders of magnitude. This scheme is also suitable for multi-cycle driving laser pulses. In addition, we also investigate the influence of the near-field filter to the far-field characteristics of the attosecond pulse.(7) The generation of the broadband supercontinuum with a modulated generalized polarization gating is studied. It is found that this scheme can simultaneously control the ionization, acceleration and recombination steps in the HHG process. Then an ultra-broadband supercontinuum covered by the spectral range from ultraviolet to water window x-ray can be obtained. This scheme is also suitable for multi-cycle driving laser pulses.(8) The propagation effects of the broadband supercontinuum generated with a modulated chirped polarization gating are investigated. It is found that the modulations on the single-atom supercontinuum can be eliminated after propagation. Then a macroscopic broadband supercontinuum can be produced. Moveover, the isolated attosecond pulse can still be generated for multi-cycle driving laser pulses.(9) The generation of the elliptical isolated attosecond pulses from the oriented CO gas medium driven by a linearly polarized few-cycle laser pulse is investigated. It is found that the largely elliptical supercontinuum can be produced when the CO molecule is properly oriented. Further investigation reveals that the nonsymmetric structure of the HOMO of the CO molecule is the major origin of the large ellipticity of the harmonics. Then the elliptical isolated attosecond pulses can be produced by filtering the largely elliptical supercontinuum.