Study On The Isolated Attosecond Pulse From The Polarization Gating Scheme

The emergence of attosecond science has great value for many fields such as microphysics,biomedicine,new materials,chemistry,and information science.The high-order harmonic spectrum is currently the only effective means to obtain attosecond pulses in XUV and soft X-ray bands due to its characteristics of equal spacing and unique platform structure.High-order harmonic generation undering the polarization gating schemes is one of several mainstream schemes for generating isolated attosecond pulse.However,the disadvantage of this scheme is that the ionization of gas medium caused by circularly or elliptically polarized light before the half optical cycle of linear polarization is not conducive to the high-order harmonic generation.Based on this,the high-order harmonic generation and isolated attosecond pulse of helium atom under asymmetric polarization gating scheme are studied in this paper by using Lewinstein's strong field approximation model theory,and the advantages of asymmetric polarization gating scheme in the generation of isolated attosecond pulse are analyzed.The main contents of this paper are as follows:First,the high-order harmonic generation and the isolated attosecond pulses radiation of helium atoms are calculated under the asymmetric polarization gating pulses with a pulse width of 5.3 fs.It is found that when the time delay between the two pulses is taken as 5.3 fs,if the electric field intensity ratio of the front end and the back end of the pulse is less than 1,the atomic pre-ionization probability is smaller at the front end of the polarization gate,and the atomic ionization probability of the polarization gate is larger,so the high-order harmonic generation spectrum with a far cut-off position and higher harmonic efficiency can be obtained.Further research finds that in the range where the ratio of the intensity of the electric field at the front end and the back end of the pulse is less than 1,the smaller this ratio is,the larger the harmonic spectrum cut-off position is,but the harmonic spectrum conversion efficiency also decreases.Therefore,the appropriate intensity ratio between two pulses can be selected in the range of intensity ratio less than 1 according to the actual needs.We have also conducted the high-order harmonic generation and the isolated attosecond pulses radiation of helium atoms under asymmetric polarization control pulses with a pulse width of 10 fs.In the polarization gating scheme with a pulse width of 5.3 fs,if we want to use two beams of circularly polarized light rotating in opposite directions to obtain a half optical cycle polarization gate,the time delay between the two pulses is approximately equal to the pulse width of the incident drive pulse.However,for a long pulse duration of 10 fs,the time delay between two circularly polarized pulses should be 22.5 fs in order to ensure that the width of the polarization gate is half optical cycle.At this time,the field strength in the polarization gate is far lower than that outside,so it is difficult to get effective high-order harmonic generation.The study found that if the time delay between the two pulses is shortened to 15 fs,the width within the polarization gate is adjusted from half optical cycle to nearly one optical cycle.With the increase of polarization gate width,the electric field strength of the synthesized pulse in the polarization gate is increased to be close to the external field strength,which overcomes the disadvantage of low efficiency of the harmonic spectrum caused by the energy loss of the laser field.In addition,we find that even if the width of the polarization gate is close to an optical period,the first half of the optical cycle is responsible for the ionization of atoms in the harmonic generation process,and the first 1/4 of the optical cycle only contributes to the harmonic generation near the order 40.The platform and cut-off position of high-order harmonic generation still comes from the last quarter of the optical period,so the regular and continuous harmonic spectrum with high efficiency in the platform area is obtained,and the isolated short pulse of 175 as is obtained after Fourier transform.