Ultrasound Assisted Cavitation around Pulsed Laser Illuminated Nanoparticles

The laser generation of vapor bubbles around plasmonic nanoparticles can be enhanced through the application of an ultrasound field; a technique referred to as photoacoustic cavitation. The combination of light and sound allows for bubble formation at lower laser fluence and peak negative ultrasound pressure than can be achieved using either modality alone. The growth and collapse of these bubbles leads to local mechanical disruption and acoustic emission, and can potentially be used to induce and monitor tissue therapy. The objective of this thesis is to understand the physics of nanoparticle-mediated photoacoustic cavitation, develop approaches to minimize the cavitation threshold, and explore applications of this new phenomenon. In this work, the theory of photoacoustic cavitation is developed. Modeling of the transient thermal fields around laser-heated nanoparticles provides guidelines for choosing the optimal nanoparticle size and shape in order to minimize the nucleation threshold laser fluence. The model for bubble dynamics offers a useful tool to predict the inertial cavitation threshold and bubble oscillation characteristics. A detailed ex vivo experimental study is performed on photoacoustic cavitation generated by simultaneously exposing gold nanorods and nanospheres to laser light and focused ultrasound. The growth and collapse of microbubbles around nanoparticles generates strong acoustic emission, which is used for detecting nanoparticles in dilute solutions and imaging nanoparticle distributions within scattering media. The probability of producing cavitation events is found as a function of laser fluence, applied ultrasound pressure and nanoparticle concentration. The cavitation threshold fluences for both nanorods and nanospheres are found to drastically reduce in the presence of an ultrasound field. The threshold fluence for nanorods is lower than any previously reported for any type of nanoparticle in water in the absence of an applied ultrasound field. The results indicate that photoacoustic cavitation can be produced at depth in biological tissue without exceeding the safety limits for ultrasound or laser irradiation at the tissue surface