Efficient nonlinear optical interactions are essential for many applications in modern photonics. However, they typically require intense laser sources and long interaction lengths, requirements that often render nonlinear optics incompatible with new nanophotonic architectures in integrated optics and metasurface devices. Obtaining materials with stronger nonlinear properties is a crucial step towards applications that require lower powers and smaller footprints. Recently, a new class of materials with a vanishing permittivity, known as epsilon-near-zero (ENZ) materials, has been reported to exhibit unprecedented ultrafast nonlinear efficiencies within sub-wavelength propagation lengths. In this Review, we survey the work that has been performed on ENZ materials and the related near-zero-index materials, focusing on the observation of various nonlinear phenomena (such as intensity-dependent refraction, four-wave mixing and harmonic generation), the identification of unique field-enhancement mechanisms and the study of non-equilibrium dynamics. Degenerately doped semiconductors (such as tin-doped indium oxide and aluminium-doped zinc oxide) are particularly promising candidates for ENZ-enhanced nonlinear optical applications. We conclude by pointing towards possible future research directions, such as the search for ENZ materials with low optical losses and the elucidation of the mechanisms underlying nonlinear enhancements.

对于现代光子学中的许多应用来说，有效的非线性光学相互作用是必不可少的。然而，它们通常需要强烈的激光源和较长的相互作用长度，这些要求常常使非线性光学器件与集成光学器件和超表面装置中的新纳米光子体系结构不兼容。获得具有更强非线性特性的材料是朝着要求更低功率和更小占位面积的应用迈出的关键一步。近来，已经报道了一种新型的具有消失的介电常数的材料，称为ε-接近零（ENZ）材料，在亚波长传播长度内展现出空前的超快非线性效率。在这篇评论中，我们调查了在ENZ材料和相关的接近零指数的材料上所做的工作，着重于观察各种非线性现象（例如强度相关的折射，四波混合和谐波产生），识别独特的场增强机制以及研究非平衡动力学。简并掺杂的半导体（例如，锡掺杂的氧化铟和铝掺杂的氧化锌）对于ENZ增强的非线性光学应用特别有希望。最后，我们指出了可能的未来研究方向，例如寻找光损耗低的ENZ材料以及阐明非线性增强的机理。简并掺杂的半导体（例如，锡掺杂的氧化铟和铝掺杂的氧化锌）对于ENZ增强的非线性光学应用特别有希望。最后，我们指出了可能的未来研究方向，例如寻找光损耗低的ENZ材料以及阐明非线性增强的机理。简并掺杂的半导体（例如，锡掺杂的氧化铟和铝掺杂的氧化锌）对于ENZ增强的非线性光学应用特别有希望。最后，我们指出了可能的未来研究方向，例如寻找光损耗低的ENZ材料以及阐明非线性增强的机理。