Hybrid-material-based mirror coatings for picosecond laser applications

Four hybrid materials (HfO2/Al2O3 nanolaminate, HfO2/SiO2 nanolaminate, HfO2-Al2O3 mixture, and HfO2-SiO2 mixture) and four hybrid-material-based mirror coatings are prepared via electron-beam evaporation. The refractive index (n), optical bandgaps, and microstructural properties of the hybrid materials are investigated and compared with those of pure materials. Experimental results show that the hybrid materials suppress crystallization, reduce surface roughness, and provide tunable bandgaps and optical constants. Among the four hybrid materials with similar n values, the HfO2-Al2O3 mixture shows the largest bandgap. All the hybrid-material-based mirror coatings exhibit a higher laser-induced damage threshold (LIDT) at 8 picosecond (ps) than the conventional pure-material-based mirror coating. The HfO2-Al2O3 mixture-based mirror coating with a relative high LIDT is used for further study of the pulse width dependence of the LIDT, and shows superior LIDT over the conventional mirror coating at 0.5 ps, 1 ps and 3 ps. Typical damage morphological features induced at different pulse widths show that the damage-initiation mechanism of the mirror coating changes around 3 ps.