A computational analysis of nanoparticle-mediated optical breakdown

A theoretical model of the optical breakdown phenomena during picosecond and femtosecond laser pulse exposure with gold nanoparticles in water was developed. The model provides new and valuable insight into the dependence of the optical breakdown on the wavelength, morphology and environment in the vicinity of the nanoparticles. The developed model was successfully validated against experimental data, which also revealed some insights to the criterion for optical breakdown. Three studies were performed using the model. In the first study, the effects of the dielectric environment on the optical extinction spectra of individual bare and silica-coated gold nanorods were examined. The experimental extinction spectra of an individual gold nanorod was compared to a calculation from a numerical model that included environmental features present in the measurements and the morphology of the corresponding nanorod measured by transmission electron microscopy. The combination of these experimental and theoretical tools permitted a detailed interpretation of the optical properties of an individual gold nanorod. In the second study, a strongly coupled finite element model of nanoparticle-mediated optical breakdown phenomena was developed. This model was used to theoretically study a 6 ps laser pulse interaction with uncoupled and plasmon coupled gold nanoparticles. The study showed how the one-dimensional assembly of nanoparticles affects the optical breakdown threshold of its surroundings. The optical breakdown threshold had a stronger dependence on the optical near-field enhancement than on the volume of the nanostructure or its absorption cross-section. Finally, a model was developed to study the wavelength dependence of the threshold of gold nanorod-mediated optical breakdown during picosecond and femtosecond near infrared optical pulses. This study showed that the wavelength dependence in the picosecond regime is governed solely by the changes of the nanorod’s optical properties. On the other hand, the optical breakdown during femtosecond pulse exposures was found to depend on the multiphoton ionization and its wavelength dependence when, Eratio, the ratio of the maximum electric field from the outside to the inside of the nanorod was greater than 7. The developed model and conducted research deepens the understanding of the nanoparticlemediated optical breakdown in water and updates the theoretical formulation of the process with the latest findings, which leads to advancing this technology further.