Nanosecond dual-wavelength irradiation effects on laser-induced damage in hafnia coatings

Nanosecond dual-wavelength laser-induced damage was investigated at 1064 nm (1 ω ) and 355 nm (3 ω ) on ion-beam-sputtered hafnia single-layer coatings. Single-wavelength 1-on-1 tests were first conducted to establish reference onset fluences. Dual-wavelength tests combined a primary beam with variable fluence at one wavelength and a background, secondary wavelength beam with fixed fluences set to several percentages of the corresponding onset fluence. When 1 ω was used as the background irradiation, the perceived damage resistance at 3 ω decreased gradually. In contrast, 3 ω background irradiation at as little as 10% of the onset fluence led to an abrupt ≈50% reduction in the perceived 1 ω damage resistance, followed by modest changes upon further increases in 3 ω background fluences. Scanning electron microscopy and conversion ratios indicate that 3 ω light is the primary driver of damage onset in the case of dual wavelength irradiation. A two-step precursor generation and activation model reproduces the 3 ω behavior (1 ω as background), but not the strong initial 1 ω reduction (3 ω as background), suggesting additional 3 ω -induced precursors that saturate at low fluence.