Multiscale Multiphysics Simulation Model of Laser-Induced Ultrasonic Energy Conversion
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With rapid advancement on pushing the resolution and strong optical contrast at desired imaging depth of laser-induced ultrasound, compact and miniature photoacoustic imaging and diagnosis systems are emerging as a promising functional imaging technology for modern biomedical applications. In this context, a structured absorber medium exposed by laser excitation increases their temperature and launches a pressure wave that propagates as ultrasound emission through the subject under inspection. In contrast to direct laser excitation of the testing objects, this structured absorber allows generation of ultrasound emission at higher central frequency and minimizing the variance of ultrasound intensity injected in biological tissue, therefore enabling more accurate and reliable quantitative measurements. However, due to the complex interaction of photothermal conversion and photoacoustic waves involved, the signal pathway of laser-induced ultrasound emission is not fully established. There is a critical need to elaborate the energy conversion process spanning from electronic scale, atomistic scale, to microscale, in order to properly control the laser dose and design structured absorbers. To unravel the hidden physical pictures, we propose a multiscale multiphysics simulation model for capturing the interconnected process of laser-induced ultrasonic energy conversion. As the first and foremost task, we focus on the optical, plasmonic, and thermal responses of metallic and semiconductor particles to laser irradiation. Understanding of this process unveils the energy conversion and heat transport during and after photoexcitation of these materials. The developed simulation model is used to illustrate and evaluate the advantages of the widespread carbon materials-based laser-induced ultrasonic transducers, while also aiding in the comprehension of the process of light-matter interaction. As a result, our findings of this research can lead to quantifiable metrics and design guidelines of novel high-performance laser-induced ultrasonic transducers, ultimately benefiting modern biomedical diagnosis.