Previously, the strong field processes in solids have always been explained by the single-active-electron (SAE) model with a frozen core excluding the fluctuation of background electrons. In this work, we demonstrate the strong field induced dynamic core polarization effect and propose a model for revealing its role in high harmonic generation (HHG) from solids. We show that the polarized core induces an additional polarization current beyond the SAE model based on the frozen cores. It gives a new mechanism for HHG and leads to new anisotropic structures, which are experimentally observed with MgO. Our experiments indicate that the influences of dynamic core polarization on HHG are obvious for both linearly and elliptically polarized laser fields. Our work establishes the bridge between the HHG and the dynamic changes of the effective many-electron interaction in solids, which paves the way to probe the ultrafast electron dynamics.

中文翻译：

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固体产生高谐波的动态核极化：以MgO晶体为例

以前，固体中的强场过程总是用单活性电子（SAE）模型解释的，该模型具有冻结的核，不包括背景电子的波动。在这项工作中，我们演示了强磁场引起的动态纤芯极化效应，并提出了一个模型来揭示其在固体高次谐波生成（HHG）中的作用。我们显示，极化的磁芯在基于冻结磁芯的SAE模型之外会引发额外的极化电流。它为HHG提供了新的机理，并导致了新的各向异性结构，这些结构在MgO的实验中得到了观察。我们的实验表明，对于线性和椭圆偏振激光场，动态纤芯偏振对HHG的影响都是显而易见的。