Isolated pulse generation of He~+ ion in an intense chirped laser and its high frequency pulse

By solving the time-dependent Schrdinger equation numerically,theoretically we study the high-order harmonics of a model He+ion exposed to the combined field of an intense chirped laser pulse and its high frequency pulse,and analyze the characteristics of the attosecond pulse produced by the high-order harmonics(HHG).Due to the presence of the chirped pulse and the high frequency pulse in the case of the combined fields,it is shown that the plateau of the high-order harmonics is extended largely and the conversion efficiency of high-order harmonics is enhanced effectively.By superimposing some orders harmonics in different region of the second plateau,all of the pulses obtained are single attosecond ones,minimum pulse achieves 21 as.To better understand the physical reasons of HHG enhancement and attosecond pulse emission,we perform classical analysis and the time-frequency analysis.