Pulsed damage thresholds of flouride and oxide thin films from 0.26 μm to 1.06 μm

The pulsed laser-induced damage threshold for CaF 2 , MgF 2 , ThF 4 , MgO, Al 2 O 3 , TiO 2 , SiO 2 , HfO 2 , and ZrO 2 thin films was measured. The measurements were made at 1.06 μm, 0.53 μm and 0.26 μm with pulse lengths of -5 and -15 ns (measured at 1.06 μm). Different film thicknesses of each material were damaged at each wavelength and pulse length. The film thicknesses were 1λ, ½ λ, 1/4 λ, 1/6 λ, and 1/8 λ at 1.06 μm. It was found that the pulse length dependence of breakdown fit with a [pulse length] 1 /2 curve for most of the materials. Exceptions were found in some oxide coatings at 1.06 μm where the breakdown energy density was essentially independent of the pulse length. The damage threshold of the flouride and SiO 2 films increased with a decrease in film thickness. This did not correlate with changes in the E field inside the thin films. (The oxide films did not exhibit as pronounced a thickness dependence.) The breakdown threshold decreased with decreasing wavelength and correlated inversely with the absorption coefficient, although the correlation was not linear. There was a marked change in the damage morphonogy of the materials as wavelength was decreased. At 0.26 μm all the oxide material damaged in a uniform region while the flourides damaged as isolated pits. The morphology, pulse length and film thickness trends are discussed in terms of impurity, avalanche ionization, and multiphoton damage models.