Multi-shot laser damage effects on Ta₂O₅/SiO₂ gradient-index optical thin film

Ta2O5/SiO2 gradient-index optical thin films for 1064 nm have been prepared with Ta2O5 and SiO2 by ion beam sputtering (IBS). The optical property and laser-induced damage threshold (LIDT) of the coatings were investigated, respectively. Sidelobe ripples beside the stopband are suppressed by the gradient-index design, meanwhile the design achieves above 99.8% reflectivity at 1064 nm. The multi-shot LIDT decreases as the number of shots increases, but after 20 pulses, it remains almost unchanged all the way to 100 pulses. Typical damage micrographs demonstrate a significant dependence of the initiated damage on the diameter of nodular defect. Scanning electron microscope (SEM) and focused ion beam microscope (FIB) pictures of the nodular defects show that, ablation pits caused by the initiated nodular defect expanded to farther nodular defects under multi-shot pulses. Damage accumulation effects may be attributed to the ejection of farther nodular defects.