利用脉宽10 fs偏振控制脉冲获得孤立阿秒脉冲

Isolated attosecond pulse generation from polarization gating pulse with 10 fs duration

利用强场近似理论开展了具有较长脉宽的偏振控制脉冲与氦原子相互作用产生高次谐波和阿秒脉冲发射的理论研究.研究发现,当具有10 fs脉冲宽度的偏振控制脉冲被用作驱动脉冲时,只要恰当地调整两束反向旋转圆偏振脉冲峰值之间的时间延迟和强度比,即使不附加二次谐波脉冲,仍然可以得到效率较高且规则分布的高次谐波平台结构,傅里叶变换后得到了175 as的孤立短脉冲.该方案一方面通过调整两束脉冲峰值之间时间延迟突破了传统偏振控制方案中要求偏振门宽度为半个光学周期的限制,另一方面通过调整两束脉冲峰值之间的强度比避免了偏振门前端多个光学周期电场引起气体介质电离不利于谐波相位匹配的弊端.

Isolated attosecond pulses make it possible to study and control the ultrafast electron processes in atoms and molecules. High order harmonic generation (HHG) is the most promising way to generate such pulses, which is benefited from the broad plateau structure of the typical HHG spectrum. In previous HHG studies on the polarization gating pulse with longer pulse duration, one needs to dramatically increase the separation in time between the two counter-rotating circularly-polarized pulses to generate the nearly-linear half-cycle “polarization gate”. This leads to a low harmonic conversion efficiency because the field outside the polarization gate is much stronger than inside the polarization gate. In this paper, by using Lewenstein model, we theoretically simulate the high-order harmonic generation from helium atom subjected to the polarization gating pulse with 10 fs pulse duration. It is found that high-order harmonic spectra each with a higher efficiency and regular structure can still be obtained by reasonably adjusting the delay time ratio and the amplitude ratio of electric fields between the two counter-rotating pulses. Further, a single 175 as pulse in the time domain is obtained by Fourier transforming the 80th order harmonics into the 172nd order harmonic without compensating for the harmonic chirp. This scheme has two main advantages. First, the adjustment of the polarization gate width from half optical cycle into nearly one cycle ensures higher intensity of the synthesized electric field inside the polarization gate. Second, the suitable adjustment of the amplitude ratio between two electric fields ensures the low ionization probability before the polarization gate, and thus further fulfills the harmonic phase matching condition in the process of the propagation.