FEMTOSECOND PULSE INDUCED DAMAGE IN DIELECTRIC THIN-FILM STRUCTURES

摘要

Laser induced breakdown of a high-quality mirror consisting of alternating λ/4 layers of Ta_2 O_5 and SiO_2 and a single 500 nm thin film of Ta_2O_5 were studied with amplified and unamplified femtosecond pulses. The experimental data can be fitted with a model taking into account multiphoton absorption, impact ionization, and local intensity enhancements due to interference effects in the films. The results indicate that state of the art, high-quality thin films show a damage behavior that is similar to bulk materials. Defects and impurities play a negligible role. Incubation effects are found to reduce the damage threshold when the coatings are damaged with multiple pulses from a femtosecond oscillator. Time-resolved pump-probe reflection and transmission experiments indicate a decay of the excited electron plasma with characteristic time constants of 4 ps, 60 ps, and 700 ps.