光声显微成像通过光吸收激发信号、探测声信号进行成像，结合了光学对比与超声成像的优势，可以实现对组织光吸收特性高灵敏成像，并兼顾分辨率和穿透深度。但传统的光声显微以线性光吸收为对比度来源，而很多物质的吸收谱相似，难以区分。非线性光声显微引入了更丰富的对比度来源，但非线性过程的信号通常较弱，限制了其运用。
  本文首先对光声显微成像的各个环节中涉及到的非线性效应进行了调查研究。梳理了各种非线性效应出现的条件、非线性信号的特点、运用非线性信号成像的策略及其成像效果。
  然后在量子电动力学框架下推导双光子吸收的吸收率。从理论上分析出，在激光频率接近中间能级频率时会发生共振增强，使双光子吸收截面显著增大，且用两束不同频率的光束进行激发时更容易产生明显的共振增强。
  最后通过实验探测了两个频率不同的光束进行激发时的光声信号，对理论分析进行了初步验证。

  Photoacoustic microscopy imaging (PAM) generates signal through optical absorption and acoustically detection. Combining the advantages of optical contrast and ultrasonic imaging, PAM can achieve high sensitivity of optical absorption characteristics, as well as great resolution and penetration depth. However, traditional PAM generally employs linear light absorption as the source of contrast, and the absorption spectra of many substances are so similar that they are difficult to distinguish. Nonlinear photoacoustic microscopy introduces richer contrast sources, but the signal of nonlinear processes is usually weak, which limits its application. 
  In this paper, the nonlinear effects involved in each link of photoacoustic microscopy imaging were investigated and studied. The occurring condition of these nonlinear effects, the characteristics of nonlinear signals, the imaging strategies and their imaging results were sorted out. 
  Then the absorption rate of two-photon absorption (TPA) is deduced under the framework of quantum electrodynamics. The theoretical analysis shows that the resonance enhancement occurs when the laser frequency is close to the intermediate level frequency, which makes the two-photon absorption significantly
increase, and it is easier to get obvious resonance enhancement when use two beams of different frequencies to generate TPA. 
  Finally, the two-photon photoacoustic signal of two beams with different frequencies is detected experimentally, and the theoretical analysis is preliminatively verified.