| As the laser technology developed recently, the research has attracted much attention on the laser fields interacted with material. Attosecond science can well explore electron dynamics in material, and High-order Harmonic Generation(HHG) is an efficient way to gain attosecond(as) pulses. A HHG spectrum has a general characteristic: at the first few orders the harmonic intensities decrease, later, the intensities keep nearly unchanged at the broad plateau, and then the harmonic intensity decrease steeply, the turning point in the process is called cutoff. The harmonic generation can be depicted by semi-classical three-step model: ionization of an electron; acceleration in the laser field; recombination with parent ion. In the past few years, many efforts have been paid to broaden the bandwidth of HHG by controlling the ionization process, including the employment of high-energy IR field, multicolor fields, Terahertz(THz) field, static field, spatially inhomogeneous field and chirped field. Furthermore, many methods have been investigated to enhance intensity of HHG, including the coherent superposition of ground state and excited states in atoms, adding an ultraviolet(UV) pulse. In this article, we theoretically investigate the HHG and attosecond pulse generation, and the research work includes three parts which are presented as follows:First, we theoretically do the research on the HHG of H atom in an 800 nm fundamental laser field combined with a THz field by solution the time-dependent schr?dinger equation(TDSE) accurately with time-dependent generalized pseudospectral(TDGPS) method. The result shows that the plateau of high-order harmonic is extended in the presence of THz field, which can be explained by the corresponding ionization probability. As a result, the optimizing combined field allows electrons to acquire more kinetic energy from the laser field, leading to the extension of the HHG plateau, and an isolated 133 as pulse can be generated.Second, we investigate the high harmonic generation(HHG) from 3D H atom in three kinds of inhomogeneous fields. The HHG spectra shows that compared with two-color homogeneous field case, the position of the harmonic cutoff extended from 80 th to 167 th, 167 th and 185 th in three kinds two-color inhomogeneous laser fields, respectively. The corresponding time frequency is also presented to explain the difference between three kinds of two-color inhomogeneous laser fields, the result shows that the energy and harmonic order of maximum peak in polynomial-type case is higher than that of linearly-type case. The ionization probability in polynomial-type case is a little larger than the cases of linearly-type cases. The electron wavepacket as functions of time is also calculated to further illustrate this phenomenon. There are larger electrons moves farther and gain high kinetic energy in the continuum and emits more energetic photons in recombination process with polynomial-type case than that with linearly-type cases. Finally, by superposing a series of properly selected harmonics, the generation of isolated attosecond pulses can be obtained straightforward without any phase compensation and the shortest of which is 64 as.Third, we have theoretically investigated the HHG and the attosecond pulse generation of H atom in the chirped fundamental field combined with a UV pulse. The result shows that when a 256 nm UV pulse is added, its photon energy of E 256 nm = 5.15 e V is close to that of the two-photon transition between the ground and excited states of the H atom, the harmonic spectrum presents a double-plateau structure with two cutoffs of the 120 th order and the 260 th order, the intensity of the second plateau is about six orders higher than that of chirped field only case. When we choose a 128 nm UV pulse instead, its photon energy of E128 nm = 10.3e V is close to that of the one-photon transition between the ground and excited states of the H atom, the two plateaus merge into one plateau and a broadband supercontinuum can be achieved. We also investigate the corresponding HHG from H atom initially prepared in the 1st excited state in the chirped fundamental field only, the HHG is quite similar to that initially prepared in the ground state in the combination of a chirped fundamental field and a 128 nm UV pulse case. The harmonic generation mechanism can be depicted by semi-classical three-step model and time-frequency analysis. In addition, the ionization probability and electron wavepacket as functions of time are also calculated to further illustrate this phenomenon. Furthermore, we also discuss the influence of time delay between the chirped fundamental field and the 128 nm UV pulse on HHG process. By superposing the harmonics in the range of 200th-260 th order, an isolated attosecond pulse with a duration of about 64 as can be generated. |
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