When solids are exposed to intense laser fields whose forces are comparable to the binding forces of valence electrons in crystals, nonlinear optics of solids advances to the nonperturbative or extreme nonlinear regime. A hallmark effect of extreme nonlinear optics is the emission of high-order harmonics of the laser from the bulk of materials. The discovery and detailed study of this phenomenon over the course of the past decade has offered a broad range of possibilities and seen the dawn of a new field of extreme solid-state photonics. In particular, it opens the way to previously inaccessible spectral ranges, as well as the development of novel solid-state spectroscopy and microscopy techniques that enable detailed probing of the electronic structure of solids. Here we review recent experimental and theoretical advances in this emerging area of study, and highlight the potential future directions and applications.

当固体暴露在其力与晶体中价电子的结合力相当的强激光场中时，固体的非线性光学会发展到非微扰或极端非线性状态。极端非线性光学的一个标志性效应是从大量材料中发射激光的高次谐波。在过去十年中，这一现象的发现和详细研究提供了广泛的可能性，并见证了极端固态光子学新领域的曙光。特别是，它开辟了通往以前无法访问的光谱范围的道路，以及开发新的固态光谱和显微技术，从而能够详细探测固体的电子结构。