Nonlinear mechanisms in photoacoustics—Powerful tools in photoacoustic imaging

Abstract

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.

Breaking Acoustic Limit of Optical Focusing Using Photoacoustic‐Guided Wavefront Shaping

“…More details regarding the saturation properties of other absorbing species are summarized in ref. [81, 91].…”

“…There are some conditions where the linear correlation between PA signal and optical fluence no longer holds true and nonlinear dependence occurs. [ 81–85 ] Different nonlinear PA mechanisms are summarized in ref. [81] Herein, we are particularly interested in the ascending power nonlinear dependence (, α > 1), because the occurrence of which leads to the improvement of focusing efficiency.…”

“…[ 81–85 ] Different nonlinear PA mechanisms are summarized in ref. [81] Herein, we are particularly interested in the ascending power nonlinear dependence (, α > 1), because the occurrence of which leads to the improvement of focusing efficiency. More specifically, the ascending power on the fluence (, > 1) allows light to be focused into a single optical speckle, thereby breaking the acoustic‐diffraction limit of conventional linear PA‐guided focusing.…”

Optothermal properties of plasmonic inorganic nanoparticles for photoacoustic applications

“…Deviations from the expected signal behaviour derive mainly from i) the time scale of the processes involved in signal generation and ii) non-linear response in relation to laser fluence [ 60 , 61 ]. The time course of photoexcitation and acoustic signal generation is schematically presented in Fig.…”

“…Besides theoretical descriptions of the photoinduced thermal processes, numerical simulations have been employed to study PNPs as photoacoustic signal generators [ 64 ]. The above-described approaches are valid until non-linearities arise in the laser / PNP interaction, thereby influencing the photoacoustic signal [ 61 ]. This possibility has been described both theoretically [ 65 , 66 ] and experimentally [ 58 , 60 , [67] , [68] , [69] ].…”