Study On The Single Attosecond Pulse Generation Driven By The Multi-Color Optimized Laser Fields

Attosecond pulses (1as=10^-18s) is the natural time scale for electronic transition and electronic motion in atoms.This is a key to uncover many scientific questions by using the real-time observation and control of the electronic dynamic behaviors on the attosecond time scale.Therefore,the study attosecond pulse generation is important significance that attracts substantial attention in ultrafast science and technology.In this thesis,we have studied high-order harmonic and the single attosecond pulse generation of atoms in the intense laser field by numerically solved the time-dependent Schr?dinger equation by means of using the psedospectral method for the time-dependent wave function.To obtain the optimized two-color laser field and three-color laser field for an isolated attosecond generation from He atom,we have combined the time-dependent Schr?dinger equation with the new unconstrained optimization algorithm(NEWUOA),and set the shortest single attosecond pulses as the objective function for optimizing the two-color and three-color fields.In our calculation,the initial wavelengths of the two-color combined laser field are 800nm and 400nm,the intensities are3.0×10¹⁴W?cm² and 0.8×10¹⁴W?cm²,respectively,and the pulse duration is 8fs,(full width at half maximum,FWHM).The wavelength and time-delay of the second color laser field are optimized as the optimization parameters.As a result,an isolated 25as pulse is obtained.The optimization of three-color laser field is divided into two schemes:one is that the wavelength and intensity of three laser fields are fixed.Namely,the initial wavelengths are 800nm,2400nm and 1600nm,and the intensities are 3.0×10¹⁴W?cm ²,0.8×10¹⁴W?cm ²,and 0.2×10¹⁴W?cm²,respectively.Next,the time-delay of the second between third laser field as the optimization parameters are optimized.As a result,an isolated 18as pulse is obtained.Another one is that the parameters of the first color laser field are fixed,the wavelengths of the second and third laser field,and the time-delay of the second between third laser field are optimized as the optimization parameters.As a result,an isolated 17as pulse is obtained.The results indicate that the scheme of the optimized three-color laser can obtain a shorter single attosecond pulse,but in this scheme,one of three-color laser wavelengths optimized is larger than 2000nm for extending the HHG cutoff,this scheme can cause the lower yield in HHG.To consider the effects of the medium propagation on an isolated attosecond pulse generation in the combined laser field,we have to solve the Maxwell equations for the macroscopic medium propagation.By optimizing the features of gas beam,such as the gas pressure,the gas long,the focus position,either the ultrashort or high intensity isolated attosecond pulse can be produced.Our results show that the macroscopic medium propagation is important to the HHG on axis rather than those HHG off axis.