Microstructure Characterization And Laser Damage Behavior Of SiO₂ Multilayer Optical Film

Currently,the performance and lifetime of fused silica based devices limit the output power of high-energy laser in the inertial confinement fusion?ICF?.It is a great challenge to improve the laser transmission efficiency and prolong the lifetime of optical components.However,the development of thin film based optical components provides an effective way to solve this problem.The laser transmission efficiency can be greatly improved through coating different kinds of optical films on the surface optical components.Nevertheless,the nonnegligible decrease of damage threshold,induced by the optical thin film,will seriously hinder the further development of the laser power and laser energy.Therefore,great attention has been paid to prepare optical films with high laser-induced damage threshold?LIDT?.According to the reports,the damage threshold of optical thin film depends greatly on its microstructure.In this thesis,we focus on the synthesis of SiO₂ colloids with different microstructures and their influences on the damage threshold.The microstructures of different SiO₂ colloids have been characterized and their influences on the damage threshold of optical film have been analyzed.The main contents of this thesis are as follow:SiO₂ colloid has been prepared through sol-gel method,including acid sol-gel method,alkali sol-gel method,propylene oxide?PO?modified alkali sol-gel method and methyltriethoxysilane?MTES?modified alkali sol-gel method.Four kinds of optical thin films have been prepared finally through mixture of colloid prepared by these four methods.In order to investigate the morphology and defects as well as functional groups,our samples were characterized with atomic force microscope?AFM?,scanning electron microscope?SEM?,transmission electron microscope?TEM?,Fourier transform infrared spectroscopy?FTIR?and nuclear magnetic resonance?NMR?.The size distribution of colloidal particles can be optimized through the mixture of acid sol-gel method and alkali sol-gel method.However,it will also lead to the enlargement of the particle size so that it is more different to disperse the particles in the solution.Samples prepared through PO modified sol-gel method exhibit smaller size and higher crystallinity without forming porous structure.However,functional groups will remain on the surface of the thin film.The AFM results of single-layer and multi-layer films reveal that the roughness of the film increases with the increase of the layer number.However,contrary to the roughness,the porosity and particle size of the film decrease with the increase of the layer number.The lattice tensile of substrate increases first and then decreases with the increase of layer number,indicating that the bonding strength between bilayer film and substrate is strongest.The dependences of damage threshold on layer number is analyzed in detail in this thesis.The LIDTs of single-layer and double-layer films based optical components have been tested under ultraviolet?UV?laser?355nm?irradiation.The experimental results show that the laser damage resistance of the film coated optical element decrease with the increase of layer number.On the one hand,the porous structure leads to higher possibility of generation of energy deposition for double-layer film.On the other hand,there are more voids and dislocations on double-layer film,which will also reduce the damage threshold.The influence of laser conditioning is also investigated in this thesis.The LIDTs of thin films will be promoted as a whole after the laser conditioning.The energy deposition,generated in the porous structure under the irradiation of low-energy laser,will further improve the structure so as to promote the anti-laser damage performance of the films.Furthermore,the energy deposition will fix the voids and dislocations on the surface of the film under the irradiation of low-energy laser,which hinders the inducement of laser damage.