Role of deposited energy density and impact ionization in the processes of femtosecond laser-matter interaction in solids: scaling from visible to mid-IR

摘要

The deposited energy density (DED) serves as a key parameter in the process of the femtosecond laser pulse energydelivery into the bulk of transparent dielectrics. The laser-induced micromodification can be created if the value of DEDexceeds a certain threshold, which is specific for each material and does not depend on the laser wavelength. In thiscontribution, we present a comprehensive study of the DED evolution with the driving pulse energy and wavelengthunder femtosecond microstructuring of transparent dielectrics. To precisely determine the laser impact area we appliedfor the first time a real-time diagnostic of microplasma based on third harmonic generation. This technique givessubmicron spatial resolution and is extremely sensitive to the free electron density (about 10~(-5) of the critical electrondensity). We found out that the threshold DED equals to approximately 2.5 kJ/cm~3 for fused silica and roughlycorresponds to excess of glass transition temperature. The highest DED is achieved for the shortest wavelength (620 nm)and equals to 16 kJ/cm~3.