Research On The Interaction Mechanism Of Femtosecond Laser With Aluminum Nanostructure And Surrounding Medium Environment

The mechanism of femtosecond laser induced optical breakdown?LIOB?in medium environment and deposited substrate mediated by different morphology of aluminum nanostructures and Al/SiO₂ core/shell nanostructures has been studied in this dissertation.The femtosecond LIOB physical model includes:the electromagnetic?EM?field model for the description of near-field enhancement of nanoparticles,the two-temperature model?TTM?for the description of electron and lattice temperature of nanoparticles,the plasma model for the description of the variation of electron density in a medium environment?substrate,surrounding medium?,and the heat transfer?HT?for the description of the heat transfer of nanoparticles towards the surrounding medium and the absorption of heat by the plasma.These four physical field models are fully coupled in calculation.The near-field enhancement,laser breakdown threshold,electron and lattice temperature of nanoparticles,the evolution of electron density in medium,and the medium heat transfer for different forms of aluminum nanoparticles and Al/SiO₂ core/shell structure nanoparticles in different environments?vacuum,water,water film in air?have been investigated.The main contents and results are as follows:?1?In vacuum,the near-field enhancement of monomer,dimer,trimer and nanorods of aluminum nanostructures,the femtosecond laser breakdown threshold for ionization of the deposited substrate,the evolution of electron density of the substrate,the evolution of electron and lattice temperature of aluminum nanoparticles and nanorods were calculated during the breakdown process in substrate.The near-field enhancement ability of monomer,dimer and trimer of nanoparticles with the same diameter is gradually increased.Compared with dimer and trimer,the near-field enhancement ability of aluminum nanorods with the same aspect ratio decreased.For the same morphology of nanoparticles,both extinction cross section and maximum electric field enhancement reach peak value at the same resonance wavelength.For different deposited substrates,the location and effect of near-field enhancement are also different,and the effect of near-field enhancement is obviously improved by aluminum substrates.In vacuum,the lattice temperatures for all types of aluminum nanoparticles and nanorods at each laser breakdown threshold in this research are lower than the melting point?933K?.However,the polymeric aluminum nanoparticles?dimer,trimer?have more advantages in mediating deposited substrate breakdown,due to their strong near-field enhancement ability.?2?The process of femtosecond laser breakdown in water,mediated by different types of Al/SiO₂ core/shell nanoparticles and nanorods was calculated.The near-field enhancement and extinction cross-section of different types of Al/SiO₂ core/shell nanoparticles and nanorods,the laser energy threshold when the plasma electron density reaches the saturation density,and the evolution of lattice temperature,water electron density and ambient medium temperature under this energy threshold have been calculated.The results show that the near-field enhancement ability of Al/SiO₂ core/shell nanoparticles monomer,dimer and trimer increases gradually.In a laser pulse acting period?4t_p?,once the plasma is formed?within 2t_p time?,it's starting to absorb amount of laser energy.So,the temperature of the plasma in the vicinity of nanoparticles increases rapidly.Unlike monomer and nanorods,the lattice temperature of dimer and trimer is lower than the melting point at each laser energy threshold.?3?The process of femtosecond laser breakdown in water film and SiO₂ substrate,mediated by different types of Al/SiO₂ core/shell nanoparticles and nanorods coated with water film in air was calculated.The near-field enhancement of nanoparticles,the laser energy threshold when the plasma electron density reaches the saturation density,the evolution of lattice temperature of aluminum nanoparticles and nanorods were investigated.It is found that the near-field enhancement ability of aluminum nanoparticles trimer is higher than dimer with the same nanoparticle diameter and water film thickness.For dimer and trimer with the same diameter,the near-field enhancement increases with the increase of water film thickness,as the thickness of water film varies in the range of 10⁵0 nm.Different from nanorods,the lattice temperature of dimer and trimer is lower than melting point.Compared with Al/SiO₂core/shell nanostructures in the whole water field,the near-field enhancement of Al/SiO₂core/shell nanostructures coated with nano water film in air environment is enhanced,which indicates that the change of local medium environment directly affects the near-field enhancement of nanostructures.?4?In the study of thermal ablation model,a femtosecond laser thermal ablation model was established with temperature-dependent dynamic reflectivity R and absorption coefficient?.With rapid temperature increase in electrons,both R and?could drastically decrease leading to much more laser energy deposition in the surface and higher electron and lattice temperatures than that described by the constant R and?at room temperature,leading to a bigger ablation depth,which is closer to the experimental results from femtosecond laser ablation with high power and long wavelength?>600 nm?.The hot-electron-blast force in aluminum substrates is calculated by using the thermal-elastic-force equilibrium equation and hyperbolic two-temperature model,and the propagation process of shock wave in the substrates is simulated by employing solid mechanics.With the continuous propagation of shock wave to the substrate,the oscillation of time-dependent displacement and stress appeared in the substrate.The results reveal that the near-field enhancement ability of nanostructures can be improved by assembling nanoparticles?dimer and trimer?and changing the morphology of nanostructures,or the local medium environment of nanostructures,which can effectively reduce the femtosecond laser energy threshold required for breakdown in the medium,mediated by aluminum nanostructures and Al/SiO₂ core/shell nanostructures.In the above medium environments,the lattice temperature of dimer and trimer structure of aluminum nanoparticles or Al/SiO₂ core/shell nanoparticles is always lower than the melting point at corresponding laser breakdown threshold,which indicates that the polymeric aluminum nanoparticles have more advantages in maintaining the morphology of the particles and reducing the loss during the LIOB in deposited substrate and surrounding medium.The research work in this study can predict the breakdown threshold of femtosecond laser,the lattice temperature of nanoparticles,the occurrence time of LIOB and so on.It is of great significance in the frontier industrial processing fields,such as:femtosecond laser-induced nanohole formation on substrate surface,femtosecond laser-induced cavitation in water,femtosecond laser biomedical application,etc.