Material ejection and layer peeling-off in HfO2/SiO2 thin-film beam splitters induced by 1ω and 3ω lasers

Nanosecond pulse laser-induced damage is associated with nodular defects and nano-precursors in hafnium oxide (HfO2)/silica (SiO2) multilayer films. In this study, the dynamic evolution of material ejection and layer peeling-off in HfO2/SiO2 thin-film beam splitters used for third harmonic separation were examined by time-resolved shadowgraph technique under the fundamental frequency of 1064 nm (1ω) and third harmonics 355 nm (3ω) laser beam, respectively. The initial material ejection and subsequent peeling-off of layers were studied to clarify most thin films damage events caused by 1ω and 3ω lasers. The peeling-off of films under 1ω laser was mainly caused by thermomechanical effect. In contrast, the peeling-off of films under 3ω laser was mainly affected by laser-induced mechanical damage. The different transmission forms of layers peeled off from the films revealed the influence of mechanical strength and thermal strength on damage progression. The Taylor–Sedov theoretical model was used to analyze the propagation characteristic shock waves after nanosecond laser irradiation of films. In summary, identification of propagation forms and speed of peeling-off layers provides helpful insights into the damage mechanisms during nanosecond laser-induced damage of optical thin films.