Theoretical Study On The Relationship Between High - Order Harmonic Radiation Efficiency And Driving Laser Wavelength Of Oriented Molecule

In recent years, high-order harmonic generation (HHG) is a hot issue in strong laser-matter interaction. First, using HHG, one can obtain attosecond pulses which is important in science and technology. Secondly, the HHG can also be used to achieve the molecular orbital imaging in an ultrafast time scale, which is important in understanding the chemical reaction.Generally, one expects that the HHG plateau can be expanding fully so that a shorter attosecond pulse can be obtained. In addition, one also expects that the HHG efficiency (the ratio of the integrated HHG yields versus the power of the laser field) is high so that a stronger attosecond pulse can be acquired. The method for increasing the HHG efficiency has attracted great interests recently. The dependence of the HHG efficiency on the laser wavelength has been studied both experimentally and theoretically. The experimental measurements have shown that the scaling law for the wavelength-dependent HHG efficiency is λ-(6.3±1.1). The numerical simulations, based on the numerical solution of the time-dependent Schrodinger equation, have shown a scaling law similar to the experimental one. The analytical treatments, based on strong-field approximations, show a scaling law which differs somewhat from the experimental and numerical results. Present studies on the HHG efficiency mainly focus on atoms. The relevant studies on molecules are relatively less. For molecules which possess more degrees of freedom, the situation is more complex and some new effects emerge, such as the orientation effect and two-center interference effect. In this paper, we will study the wavelength dependence of the HHG efficiency from aligned molecules in detail.In chapter 3 in this paper, we investigate the HHG from aligned molecules Hj with varying the laser wavelength from 400 nm to 2300 nm. Our simulations show that the wavelength dependence of the integrated HHG yield is closely associated with the molecular alignment. Our analyses reveal that the dynamics of the rescattering electron responsible for the HHG differs significantly for different laser wavelengthes. The interplay of the electronic dynamics and the molecular structure plays a dominating role in this association. A simple model is proposed to describe these wavelength-orientation-related HHG phenomena. The wavelength scaling of the HHG yield relating to different electron trajectories for the aligned molecule is also addressed.In chapter 4 in this paper, we study the efficiency of HHG from CO2 molecule exposed to strong laser fields with different laser wavelengths and different orientation angles numerically. Through calculating the HHG spectra parallel and perpendicular to the laser polarization, we show that the efficiency of perpendicular harmonics can be higher than or comparable with the parallel one at the relatively little and middle orientation angles in some wavelength regions. At larger angles, the efficiency of perpendicular harmonics is generally lower than the parallel one. Further analyses show that the structure of the CO2 molecule plays an important role in the HHG efficiency and this role is also related to the laser wavelength. Our results suggest that for the complicated molecule, the perpendicular harmonics should be considered in the molecular orbital tomography experiments.