We introduce the principle and applications of one‐photon absorption (OPA) and two‐photon absorption (TPA) controlled by external electric fields. The physical mechanism of OPA and TPA are firstly introduced, which can visually promote thoroughly understanding of principle and physical analysis. Secondly, the applications of different molecules in OPA and TPA with and without external electric field are introduced in detail. The effect of the external electric field on the charge transfer during the absorption process is also exemplified. Furthermore, the external electric field on the molecular orbital wave function is visualized through the charge transfer process in the excited state transitions. The purpose of this review is to deepen the understanding of the types of charge transfer under linear and non‐linear absorption in different systems.

中文翻译：

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一，二光子吸收：物理原理与应用

介绍了由外部电场控制的一光子吸收（OPA）和二光子吸收（TPA）的原理和应用。首先介绍了OPA和TPA的物理机理，可以在视觉上促进对原理和物理分析的彻底理解。其次，详细介绍了不同分子在有无外电场的情况下在OPA和TPA中的应用。还举例说明了吸收过程中外部电场对电荷转移的影响。此外，在激发态跃迁中，通过电荷转移过程可以看到分子轨道波函数上的外部电场。本文的目的是加深对不同系统中线性和非线性吸收下电荷转移类型的理解。