Recent advances in the availability of intense, carrier-envelope phase-stabilized few-cycle lasers have led to the active study of the control of electrons using a light field in atoms, molecules, and solids. The field-driven ultrafast current in solids by strong fields is of far-reaching importance in view of ultrafast devices. Recent ab initio time-dependent density functional theory calculations [Wachter et al., Phys. Rev. Lett. 113, 087401 (2014) [CrossRef] ] predict that the crystal anisotropy manifests itself as the phase shift between induced currents along different crystal axes. The present work observes such a phase shift, clearly demonstrating that the electric current induced by a strong light field in an anisotropic crystal is sensitive to the orientation. A series of experiments has been carried out with few-cycle laser fields polarized parallel to the axes of quartz, $\hat c$ and $\hat a$, respectively. Owing to the anisotropic atomic composition in the crystalline lattice, the transition to the tunneling regime takes place at lower intensity along the $\hat a$ axis than along the $\hat c$ axis. This implies that at a given tailored intensity, the tunneling transition occurs along the $\hat a$ but not along the $\hat c$ axis (still in the multiphoton regime). Hence, the currents induced by the two different mechanisms lead to an unequal accumulative phase, thus the nonzero phase shift. This work promotes an understanding of the strong field response of solids at the atomic level and in the subcycle time scale.

中文翻译：

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晶体各向异性对介电晶体中光场驱动电流的影响

高强度，载流子包络相稳定的几周期激光器的可用性方面的最新进展已导致人们积极研究利用原子，分子和固体中的光场来控制电子。鉴于超快设备，由强场以场驱动的固体中的超快电流具有深远的重要性。最近的从头算时间相关的密度泛函理论计算[Wachter等。，物理 牧师 113，087401（2014）[交叉引用] ]预测晶体各向异性表现为沿不同晶体轴的感应电流之间的相移。本工作观察到这种相移，清楚地表明各向异性晶体中强光场感应的电流对取向敏感。已经进行了一系列实验，分别对平行于石英轴$ \ hat c $和$ \ hat a $偏振的几个周期的激光场进行了极化。由于晶格中的各向异性原子组成，向隧穿区域的过渡沿$ \ hat a $轴的强度低于沿$ \ hat c $的强度轴。这意味着在给定的定制强度下，隧穿跃迁沿着$ \ hat a $发生，而不沿着$ \ hat c $轴发生（仍然在多光子状态下）。因此，由两种不同机制感应的电流导致不相等的累积相位，从而导致非零相移。这项工作促进了人们对固体在原子级和子周期时间尺度上的强场响应的理解。