Valence electrons contribute a small fraction of the total electron density of materials, but they determine their essential chemical, electronic and optical properties. Strong laser fields can probe electrons in valence orbitals 1 – 3 and their dynamics 4 – 6 in the gas phase. Previous laser studies of solids have associated high-harmonic emission 7 – 12 with the spatial arrangement of atoms in the crystal lattice 13 , 14 and have used terahertz fields to probe interatomic potential forces 15 . Yet the direct, picometre-scale imaging of valence electrons in solids has remained challenging. Here we show that intense optical fields interacting with crystalline solids could enable the imaging of valence electrons at the picometre scale. An intense laser field with a strength that is comparable to the fields keeping the valence electrons bound in crystals can induce quasi-free electron motion. The harmonics of the laser field emerging from the nonlinear scattering of the valence electrons by the crystal potential contain the critical information that enables picometre-scale, real-space mapping of the valence electron structure. We used high harmonics to reconstruct images of the valence potential and electron density in crystalline magnesium fluoride and calcium fluoride with a spatial resolution of about 26 picometres. Picometre-scale imaging of valence electrons could enable direct probing of the chemical, electronic, optical and topological properties of materials. Laser-generated high-harmonic emission is used to image the valence potential and electron density in magnesium fluoride and calcium fluoride at the picometre scale, enabling direct probing of material properties.

中文翻译：

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固体中价电子的激光显微观察

价电子只占材料总电子密度的一小部分，但它们决定了材料的基本化学、电子和光学性质。强激光场可以探测气相中价轨道 1 – 3 中的电子及其动力学 4 – 6。先前的固体激光研究将高次谐波发射 7 – 12 与晶格中原子的空间排列 13、14 联系起来，并使用太赫兹场来探测原子间势力 15 。然而，固体中价电子的直接、皮米级成像仍然具有挑战性。在这里，我们表明，与晶体固体相互作用的强光场可以实现皮米尺度的价电子成像。强激光场的强度与保持价电子束缚在晶体中的场相当，可以诱导准自由电子运动。晶体势对价电子的非线性散射产生的激光场谐波包含能够实现价电子结构的皮米级实空间映射的关键信息。我们使用高次谐波重建晶体氟化镁和氟化钙中的价势和电子密度图像，空间分辨率约为 26 皮米。价电子的皮米级成像可以直接探测材料的化学、电子、光学和拓扑特性。激光产生的高次谐波发射用于在皮米尺度上对氟化镁和氟化钙中的价势和电子密度进行成像，从而能够直接探测材料特性。