Femtosecond pulse damage thresholds of dielectric coatings in vacuum

Abstract

At 10 -7 Torr, the multiple femtosecond pulse damage threshold, F(∞), 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 F(∞) 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 F(∞) 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.

Analysis of laser damage tests on coatings designed for broad bandwidth high reflection of femtosecond pulses

“…On the other hand, the HfO 2 and SiO 2 IBS single layers of one of the above studies 8,9 did exhibit evidence of water absorption and, as we will show, the IAD TiO 2 ∕SiO 2 BBHR coatings of this paper also exhibit spectral shifts due to absorption of water. We have observed a trend with our coatings that such spectral shifts increase in amount as the number of coating layers or their thickness increases, which is consistent since more or thicker layers can absorb more water.…”

“…[8][9][10] This holds promise that the lack of vacuumambient LIDT differences of our in-house study will also apply to the BBHR coatings of this paper. Nevertheless, it is important to be aware that results of any LIDT tests in ambient environments may differ from those of tests in vacuum.…”

“…Two studies were for pulses in the fs regime and specific to single layers of HfO 2 and SiO 2 deposited by ion-beam sputtering (IBS), with 50 fs LIDT test laser pulses of 800-nm center wavelength and 1-kHz pulse repetition rate (PRR). 8,9 These LIDT tests with 50 fs pulses showed that 1-on-1 LIDTs remained the same between vacuum and ambient conditions. The multiple-pulse LIDTs in vacuum proved, however, to be much less than the 1-on-1 LIDTs in comparison to their counterparts at atmospheric pressure.…”

“…This probably would not be the case for multiple-pulse LIDTs or for lower density/ more porous, non-IAD coatings, especially if the ambient conditions are humid, since these vacuum-ambient LIDT differences correlate strongly with the absence (in vacuum) or presence (under ambient conditions) of water vapor and absorption of water by the coatings. [8][9][10] It is not clear from these studies whether the latter vacuum-ambient LIDT differences depend only on water vapor being part of the ambient environment. This could mean that they may not be as strong or even evident at all when the ambient environment is dry (0% RH).…”

“…Several specific studies [8][9][10][11] have explained vacuum-ambient LIDT differences primarily in terms of the amount of water absorbed from an ambient environment by a coating, especially one that is less dense/more porous. Two studies were for pulses in the fs regime and specific to single layers of HfO 2 and SiO 2 deposited by ion-beam sputtering (IBS), with 50 fs LIDT test laser pulses of 800-nm center wavelength and 1-kHz pulse repetition rate (PRR).…”