Isolated Attosecond Pulse Generated By A Model Helium Atom Exposed To The Combined Field

The development of attosecond technology is one of the most significant achievement in thefield of ultrafast optics. Not only have it been greatly promoting the spectroscopic and themeasurement of time resolution, realizing the dream of steering and tracing of the electron dynamicprocesses in atoms and molecules. The scientists were looking for some efficient methods toachieve shorter and shorter attosecond pulses over the past decade. Since the high-order harmonicspectrum can be extended to the soft X-ray region and exhibit a supercontinuous structure, itbecome a preferred source for the generation of attosecond pulses in the experiment. Such ultrashortattosecond pulses can be produced from the harmonics of gas-phase atom with different techniques,including polarization gating, two-colour gating and ionization gating. Theoretically, the attosecondpulse can be generated by superposing several harmonics of atoms exposed to an intense laser fieldsby solving the time-dependent Schr dinger equation.The major investigations are listed as follows:we proposed an efficient method to generate an ultrashort attosecond pulse when a model He is exposed anintense few-cycle chirped laser pulse and a half cycle pulse. By solving the time-dependent Schr dinger equationnumerically with split-operator method.We theoretically study the high-order harmonics generation (HHG) of amodel He atom exposed to a combined field of1600nm and800nm laser pulses. By making different rise rate andfall rate with time for the combined filed during mainly radiated HHG, it presents the different characteristics ofhigh-order harmonics generation from the short electron trajectories and the long electron trajectories. Bysuperimposing several harmonics radiated from He atom driven by the combined field, we could obtain shorterattosecond pulses. Finally, by adjusting the time delay between two laser pulses, we can effectively suppress thecontribution of the short electron trajectories to HHG, and obtain an isolated attosecond pulse with duration33.7as.