Electrons in two-dimensional hexagonal materials have an extra degree of freedom, the valley pseudospin, that can be used to encode and process quantum information. Valley-selective excitations, governed by the circularly polarized light resonant with the material’s bandgap, are the foundation of valleytronics. It is often assumed that achieving valley selective excitation in pristine graphene with all-optical means is not possible due to the inversion symmetry of the system. Here, we demonstrate that both valley-selective excitation and valley-selective high-harmonic generation can be achieved in pristine graphene by using a combination of two counter-rotating circularly polarized fields, the fundamental and its second harmonic. Controlling the relative phase between the two colors allows us to select the valleys where the electron–hole pairs and higher-order harmonics are generated. We also describe an all-optical method for measuring valley polarization in graphene with a weak probe pulse. This work offers a robust recipe to write and read valley-selective electron excitations in materials with zero bandgap and zero Berry curvature.

中文翻译：

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原始石墨烯中的光诱导谷电子学

二维六边形材料中的电子具有额外的自由度，即伪自旋谷，可用于编码和处理量子信息。山谷选择性激发，是由与材料的带隙共振的圆偏振光控制的，是山谷电子学的基础。通常认为由于系统的反对称性，不可能用全光学手段在原始石墨烯中实现谷选择性激发。在这里，我们证明了通过结合两个反向旋转的圆极化场，基频及其二次谐波，可以在原始石墨烯中实现谷选择性激发和谷选择性高谐波生成。通过控制两种颜色之间的相对相位，我们可以选择产生电子空穴对和高次谐波的波谷。我们还描述了一种使用弱探针脉冲测量石墨烯中谷底极化的全光学方法。这项工作为在具有零带隙和零贝里曲率的材料中写入和读取谷选择性电子激发提供了可靠的方法。