The ultrafast excitation processes in femtosecond laser-induced damage in dielectric omnidirectional reflectors

A pump and probe system is developed, where the probe pulse duration τ is less than 60 fs while the pump pulse is stretched up to 150-670 fs. The time-resolved excitation processes and damage mechanisms in the omnidirectional reflectors SiO₂/TiO₂ and ZnS/MgF₂ are studied. It is found that as the pump pulse energy is higher than the threshold value, the reflectivity of the probe pulse decreases rapidly during the former half, rather than around the peak of the pump pulse. A coupled dynamic model based on the avalanche ionization (AI) theory is used to study the excitation processes in the sample and its inverse influences on the pump pulse. The results indicate that as pulse duration is longer than 150 fs, photoionization (PI) and AI both play important roles in the generation of conduction band electrons (CBEs); the CBE density generated via AI is higher than that via PI by a factor of 102-104. The theory explains well the experimental results about the ultrafast excitation processes and the threshold fluences.