Stepwise two-photon absorption (2PA) is one of the nonlinear photoresponses where two photons are sequentially absorbed to proceed a photophysical or photochemical reaction. The stepwise 2PA can selectively produce a higher excited state with relatively high spatial selectivity, and therefore, it can be an efficient tool to explore and realize nonlinear photoresponsive materials beyond Kasha’s rule. Moreover, if a photogenerated chemical species is used as an intermediate state of the stepwise 2PA, the power threshold to promote the stepwise 2PA can be greatly reduced compared with that of a simultaneous 2PA, and conventional LEDs and even sunlight could be used to induce the stepwise 2PA. Because of these advantages, the stepwise 2PA has been widely studied more than a half century ago not only for fundamental physics and chemistry but also for applications to optical memory, holography, and artificial photosynthesis. In this review, we overview various types of stepwise 2PA processes from the viewpoints of history, fundamentals, and applications. Especially we focus on the stepwise 2PA processes related to photochromic reactions because the combination of photochromic reactions and stepwise 2PA processes has realized various potentially-applicable nonlinear photoresponsive materials and found several anomalous phenomena beyond the one-photon photophysical and photochemical reactions. The stepwise 2PA processes are mainly divided into two categories involving an electronic excited state or a photogenerated transient species as the intermediates states of the stepwise 2PA. These two types of the stepwise 2PAs are explained respectively, and finally, the summary and outlook are presented.

逐步两光子吸收（2PA）是非线性光响应之一，其中依次吸收两个光子以进行光物理或光化学反应。逐步2PA可以选择性地产生具有较高空间选择性的较高激发态，因此，它可以成为探索和实现超出Kasha规则的非线性光响应材料的有效工具。此外，如果将光生化学物质用作阶梯式2PA的中间状态，则与同时进行的2PA相比，促进阶梯式2PA的功率阈值可以大大降低，并且可以使用常规的LED甚至太阳光来诱导阶梯式2PA。逐步2PA。由于这些优点，逐步的2PA在半个多世纪以前就得到了广泛的研究，不仅用于基础物理和化学，而且还用于光学存储，全息术和人工光合作用。在这篇综述中，我们从历史，基本原理和应用的角度概述了各种类型的2PA逐步过程。尤其是，我们专注于与光致变色反应有关的逐步2PA过程，因为光致变色反应和逐步2PA过程的组合已经实现了各种潜在适用的非线性光响应材料，并且发现了除单光子光物理和光化学反应之外的几种异常现象。逐步2PA过程主要分为两类，涉及电子激发态或光生瞬态物质作为逐步2PA的中间态。分别说明了这两种类型的逐步2PA，最后给出了总结和展望。