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When atoms and molecules are excited by ultrashort laser pulses, highly nonlinear strong-field processes like above-threshold ionization and high harmonic generation occur. By analyzing the emitted light and electron signals, the atomic and molecular structures and ultrafast dynamics can be detected with a combination of Angstrom spatial resolution and sub-femtosecond temporal resolution, which provides a powerful tool to study the basic structures and physical processes in the microscopic world. The molecular orbital tomography (MOT) developed since 2004 enables one to image the wavefunction of the molecular orbital itself, which will help people gain deeper insight into the chemical reactions. In this paper, the theory of MOT will be introduced, and the progresses of MOT in the past ten years will be reviewed.

The propagation of high-order harmonics with attosecond ionization gate is simulated with three-dimensional propagation equation. After 0.5 mm propagation distance， the short trajectories in the harmonics， which are generated by two 5 fs laser pulses centered at 800 and 400 nm， are selected and a highly efficient smooth supercontinuum with the bandwidth of about 80 eV is obtained in the plateau， because of the phase matching. Simulation results of different propagation distances show the broadband supercontinuum can last steadily. A highly efficient pure isolated attosecond pulse with the duration of about 135 as can be generated by directly filtering out of the harmonics from 60—90 eV in the supercontinuum， and a single 50 as pulse can be generated， which facilitates the experimental implementation for the isolated attosecond pulse generation.

A new method to generate isolated broadband attosecond pulses with stable pulse duration is proposed. By adopting a few-cycle driving pulse combined with its second harmonic field, of which the polarization angle is π/3 to the driving pulse, the short path is well selected and a smooth supercontinuum with the bandwidth of about 50 eV is observed. For the intensity of the second harmonic field higher than 1014 W/cm2, the intensity variation of the second harmonic field can hardly affect the location and the bandwidth of the generated supercontinuum. Pure isolated attosecond pulses with the duration of about 100 as can be generated by directly filtering out a supercontinuum from 85—125 eV, which facilitates the experimental implement for the isolated attosecond pulse generation.

A new method is proposed to generate extreme ultraviolet supercontinuum. The polarization gating at 1600nm is employed to intensify the ellipticity dependence of high harmonic signal， and the addition of a second harmonic field is used for modulating the ionization ratio. Calculations show that a fundamental pulse up to 6 optical cycles (32.4fs) in duration can be applied to confine the efficient high harmonic generation to half an optical cycle. This feature allows us to produce an extreme ultraviolet supercontinuum with the bandwidth of about 280eV which supports generating a Fourier-transform-limited pulse of 10as. By selecting one part of the supercontinuum， a 100as pulse is obtained directly. In addition， the intensity utilizing efficiency of fundamental pulses can be increased by making the delay between the two pulses which compose the polarization gating smaller，when the modulated polarization gating is employed.