Hybrid-material-based mirror coatings for picosecond laser applications.

• Four hybrid materials (HfO 2 /Al 2 O 3 nanolaminate, HfO 2 /SiO 2 nanolaminate, HfO 2 -Al 2 O 3 mixture, and HfO 2 -SiO 2 mixture coatings) are investigated. • Hybrid materials show suppressed crystallization and reduced surface roughness, enabling tunable bandgaps and optical constants. • Hybrid-material-based mirror coatings are investigated and mixture-based mirror coatings have the highest LIDT. • The damage initiation mechanism of the mirror coating varies around 3 ps for laser pulses ranging from 0.5 to 8 ps.. Four hybrid materials (HfO 2 /Al 2 O 3 nanolaminate, HfO 2 /SiO 2 nanolaminate, HfO 2 –Al 2 O 3 mixture, and HfO 2 –SiO 2 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 HfO 2 –Al 2 O 3 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 HfO 2 –Al 2 O 3 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. [ABSTRACT FROM AUTHOR]