Femtosecond pulse damage thresholds of dielectric coatings in vacuum

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The dielectric breakdown behavior of dielectric coatings in studied for different ambient gas pressures with femtosecond laser pulses. At 10(-7) Torr, the multiple femtosecond pulse damage threshold, Fm, is about 10% of the single pulse damage fluence F(1) for hafnia and silica films compared to about 65% and 50%, respectively, at 630 Torr. In contrast, the single-pulse damage threshold is pressure independent. The decrease of Fm with decreasing air pressure correlates with the water vapor and oxygen content of the ambient gas with the former having the greater effect. The decrease in Fm is likely associated with an accumulation of defects derived from oxygen deficiency, for example vacancies. From atmospheric air pressure to pressures of ~3x10(-6) Torr, the damage "crater" starts deterministically at the center of the beam and grows in diameter as the fluence increases. At pressure below 3x10(-6) Torr, damage is initiated at random "sites" within the exposed area in hafnia films, while the damage morphology remains deterministic in silica films. A possible explanation is that absorbing centers are created at predisposed sample sites in hafnia, for example at boundaries between crystallites, or crystalline and amorphous phases.