Modelisation de l'interaction entre une impulsion laser ultrabreve et une nanostructure plasmonique en milieu aqueux

There is still a limited understanding of the basic mechanisms and processes triggered by laser irradiation that lead to the damaging of the surrounding structures. In particular, there exists for the moment no complete modelling of the process that includes all potential sources of cellular damage, including temperature rise, plasma generation, pressure wave emission and vapor bubble formation. This thesis presents such a modelling, with the cell environment emulated by a much simpler aqueous medium. Water is used as its optical, mechanical and thermal characteristic are similar enough to the cell environment to enable the transposition of the results and tendencies observed in water to this medium.;Modelling consists in a continuous medium based approach that uses coupled partial differential equations to simulate the nanostructure-water system's response to an ultrafast laser pulse. The time-dependant field distribution in the system is simulated using classical electromagnetic theory, while the temperature in the nanostructure is calculated using a two-temperature model. Generation and heating of a plasma in the nanostructure's vicinity from photoionization and impact ionization in the near-field, as well as from photothermal emission from the nanostructure at the metal interface are also included. This plasma locally modifies the refractive index, which alters the simulated optical response of the system. Energy relaxation from this plasma as well as heat conduction from the nanostructure's lattice transfer energy to the water, which dynamics and thermodynamics evolutions are simulated from hydrodynamic and energy conservation equations.;Modelling is used for analyzing the interaction of a near-infrared ultrafast laser pulse (45 fs, 800 nm, 200 mJ/cm2) with two distinct systems : a 100nm gold nanoparticle and a 10x41 nm2 gold nanorod. The gold nanoparticle shows a plasmon resonance at a ∼550 nm wavelength, and is thus slightly off-resonance at the 800 nm irradiation wavelength. In opposition, the gold nanorod shows a plasmon resonance directly at 800 nm and is thus in-resonance at the irradiation wavelength. The system of tightly coupled time-dependant non-linear partial differential equations is solved in 3D using a finite-element method.;The modelling shows the generation of vapor bubble around both systems, which maximal diameter and growth time are validated from spectroscopic measurements. Results show an important production of plasma in the nanostructure's near field, which fast relaxation heats rapidly the surrounding water, leading to the emission of strong pressure wave and to the formation of a vapor bubble. Simulations further demonstrate the crucial importance of the plasma in the cavitation mechanism. For both the nanoparticle and the nanorod, results show that most of the laser energy is indeed absorbed in the plasma surrounding the nanostructure rather than in the nanostructure itself. In particular, the energy absorbed in the nanoparticle is shown to be insufficient to induce the formation of a vapor bubble at 200 mJ/cm2, whereas such a bubble is experimentally observed. This is a major result of the thesis, as such a plasma-mediated plasmonic enhanced cavitation mechanism has never been reported in the literature. The existence of this plasma-mediated mechanism that does not rely on the particle extreme heating to initiate cavitation has important consequences on cell nanosurgery applications.;Finally, based on the same modelling, we develop a simple and efficient method to evaluate the potential of specific nanostructures and irradiation parameters for cell nanosurgery. We introduce a performance factor based on minimizing the energy density deposited in the nanostructure to avoid fragmentation, while maximizing the overall energy deposited in the system, to enhance the production of vapor bubbles. Results predict off-resonance 175 nm gold nanoparticles to be best suited for near-infrared ultrafast laser cell nanosurgery using 45 fs pulses at a wavelength of 800 nm. They also confirm the low interest of in-resonance gold nanorods for this type of process. (Abstract shortened by UMI.).