Study On The Isolated Attosecond Pulse Generation From The Orthogonally Polarized Two-color Fields

Attosecond pulses,which can perform real-time measurement and control of electronic dynamics,is the key to solve many current scientific problems.At present,high-order harmonic spectroscopy is the preferred approach to synthesize attosecond pulses,and the generation of attosecond pulses through high-order harmonics has been a research hotspot in the field of intense field physics in recent years.In this paper,orthogonally polarized two-color field with perpendicular polarization direction and certain time delay is adopted to obtain an effective high-order harmonic emission spectrum,realize the selection of a single quantum trajectory,and finally obtain the effective isolated attosecond pulse.The main research content is as follows:1.The high-order harmonic emission and isolated attosecond pulse generation from helium atoms irradiated by orthogonally polarized two-color field,which is composed of 4 fs/800 nm driving pulse and 8 fs/400 nm second harmonic gating pulse,are simulated numerically by Levenstein strong field approximation model.It is found that when the parameters of two pulses are selected properly,the harmonic spectrum with supercontinuous structure from 120 to 180 order can be obtained.Compared with the harmonic spectrum from a single driving pulse under the same conditions,the supercontinuous plateau region is broadened and the amplitude of its oscillations is significantly reduced.Through the analysis of electron trajectories,it is found that the harmonic platform with the supercontinuum structure mainly comes from the contribution of a single short orbit electron.An isolated attosecond pulse with a pulse width of 54 as is obtained by superimposing supercontinuous harmonics.In addition,we detail the reasons for the significant differences in radiation intensity and spectral shape of the high harmonic spectra in the direction of the driving pulse electric field and in the direction of the gating pulse electric field by applying electron motion trajectory analysis.2.The influence of carrier envelope phase,relative phase,electric field intensity and pulse width on high order harmonic spectrum and attosecond pulse generation in orthogonally polarized two-color field scheme are discussed.The results show that the carrier envelope phases of the two linearly polarized pulses can be varied in a small range around 0 to obtain a supercontinuous harmonic emission spectrum,and the selection of relative phase between two pulses is not strict,i.e.,the isolated attosecond pulses with short pulse widths can be obtained within the variation range of 0.3 π.The electric field intensity of the driving pulse laser has a significant effect on the high-order harmonic emission spectrum,while the change of the gating pulse electric field intensity has little influence on the high-order harmonic emission spectrum.The isolated attosecond pulse with pulse width below 60 as can be obtained within the range of the given gating pulse electric field intensity.Finally,we simulate the influence of the pulse widths of the driving pulse and the gating pulse on the high order harmonic spectra.It is found that the change in pulse width of the gating pulse has basically no effect on the high harmonic spectrum,while the change in pulse width of the driving pulse does not affect the cutoff position of harmonic spectra,but the width of the supercontinuous plateau shortens as the pulse width increases,and the oscillation amplitude increases as the pulse width increases.