Many nonlinear effects have been discovered and developed in photoacoustic imaging. These nonlinear mechanisms have been explored for different utilizations, such as enhancing imaging contrast, measuring tissue temperature, achieving super-resolution imaging, enabling functional imaging, and extracting important physical parameters. This review aims to introduce different nonlinear mechanisms in photoacoustics, underline the fundamental principles, highlight their representative applications, and outline the occurrence conditions and applicable range of each nonlinear mechanism. Furthermore, this review thoroughly discusses the nonlinearity rule concerning how the mathematical structure of the nonlinear dependence is correlated to its practical applications. This summarization is useful for identifying and guiding the potential applications of nonlinearity based on their mathematical expressions, and is helpful for new nonlinear mechanism discovery or implementation in the future, which facilitates further breakthroughs in nonlinear photoacoustics.

光声成像中已经发现并发展了许多非线性效应。这些非线性机制已被探索用于不同的用途，例如增强成像对比度、测量组织温度、实现超分辨率成像、实现功能成像以及提取重要的物理参数。本文旨在介绍光声学中不同的非线性机制，强调其基本原理，突出其代表性应用，并概述每种非线性机制的发生条件和适用范围。此外，这篇综述深入讨论了非线性规则，涉及非线性相关性的数学结构如何与其实际应用相关。这一总结有助于基于非线性的数学表达式来识别和指导非线性的潜在应用，并有助于未来新的非线性机制的发现或实现，从而促进非线性光声学的进一步突破。