Laser-induced Acoustic Transient by Facilitated Heat Transfer in Nanocomposite Materials for Cavitation and its Applications.

Abstract

Owing to their exceptional ability to convert light to sound, polymer-based nanocomposites have been used for producing high-amplitude focused sound waves (positive amplitude: > 50 MPa, focal spot: < 100 micrometers) by pulsed laser beam irradiation. This laser-induced focused sound shows great promise in non-invasive micro-surgery. To realize this promise, scalable, robust, and highly efficient light-to-sound (photoacoustic) converters are called for. This thesis work addresses these issues while focusing on three main topics: (1) how the polymeric composites can be designed on the basis of a fundamental study on the light-to-sound conversion process; (2) how the single, short pressure pulses behave and interact with matters; and (3) how those two understanding can contribute to designing optoacoustic converters applicable for targeted therapies. More specifically, photoacoustic generation in a metal-based composite consisting of a thin metal layer and the adjacent polymer layers is investigated, with an emphasis on heat transfer and optical resonance. This study proposes a design principle for developing an efficient photoacoustic transmitter. Also, enhanced out-coupling of generated photoacoustic waves, otherwise cancelled under a certain condition, is examined for further understanding of photoacoustic generation. Moreover, focusing of a short photoacoustic pulse (< 100 ns) leads to an interesting observation of anomaly acoustic phase shift, which could benefit the understanding of the Gouy phase shift. Then, investigation of the interaction between a focused photoacoustic pulse and impedance-mismatched boundaries follows; at a sound hard interface, controlled generation of single micro-bubbles is demonstrated; at a sound soft interface, nozzle-free liquid micro-jetting. Lastly, we demonstrate free-field cavitation and its therapeutic applications by using an optimized optoacoustic transmitter.