Generating isolated attosecond pulse by a dual optical gating scheme

Based on the theory of strong field approximation, we simulate the high harmonic and attosecond pulse generation from helium atom irradiated by a dual optical gating pulse composed of an orthogonally polarized field and a second harmonic pulse. The orthogonally polarized field consists of two linearly polarized pulses with a certain time delay, orthogonally polarized directions and 7 fs pulse duration. If the pulse that occurs first is called the driving pulse, the delayed pulse is called the gating pulse. The dual optical gating pulse is realized by adding a linearly polarized second harmonic pulse in the direction of the driving pulse of an orthogonally polarized field. It is found if the time delay between the driving pulse and the gating pulse and the phase difference between the second harmonic pulse and the driving pulse are reasonably adjusted, the high order harmonic generation spectra with higher efficiency and supercontinuum bandwidth can be obtained. After the inverse Fourier transform, an isolated short pulse with 124 as pulse duration can be obtained. Compared with the double optical gating scheme proposed by Chang et al., this experimentally relatively easy-to-operate scheme, on the one hand, overcomes the restrictions on the pulse width and intensity of the incident drive pulse, and on the other hand, avoids the atomic pre-ionization caused by the laser cycles before the polarization gate which is not conducive to the harmonic phase matching.