We present transient absorption spectra of an extreme ultraviolet attosecond pulse train in helium dressed by an 800 nm laser field with intensity ranging from $2times10^{12}$ W/cm$^2$ to $2times10^{14}$ W/cm$^2$. The energy range probed spans 16-42 eV, straddling the first ionisation energy of helium (24.59 eV). By changing the relative polarisation of the dressing field with respect to the attosecond pulse train polarisation we observe a large change in the modulation of the absorption reflecting the vectorial response to the dressing field. With parallel polarized dressing and probing fields, we observe significant modulations with periods of one half and one quarter of the dressing field period. With perpendicularly polarized dressing and probing fields, the modulations of the harmonics above the ionisation threshold are significantly suppressed. A full-dimensionality solution of the single-atom time-dependent Schr odinger equation obtained using the recently developed ab-initio time-dependent B-spline ADC method reproduce some of our observations.
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机译：我们展示了由800 nm激光场覆盖的氦中的极紫外阿秒脉冲串的瞬态吸收光谱，强度范围从$ 2×10 ^ {12} $ W / cm $ ^ 2 $到$ 2×10 ^ {14} $ W / cm $ ^ 2 $。探测到的能量范围跨度为16-42 eV，跨越了氦的第一电离能（24.59 eV）。通过改变修整场相对于阿秒脉冲串偏振的相对极化，我们观察到吸收调制的大变化反映了对修整场的矢量响应。对于平行极化的修整场和探测场，我们观察到显着的调制，其周期为修整场周期的一半和四分之一。使用垂直极化的修整场和探测场，可以显着抑制高于电离阈值的谐波调制。使用最近开发的从头开始与时间有关的B样条ADC方法获得的单原子与时间有关的Schr odinger方程的全维解再现了我们的一些观察结果。
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