Studies of femtosecond laser induced damage of HfO 2 thin film in atmospheric and vacuum environments

Abstract

The single pulse femtosecond laser induced damage threshold (LIDT) of hafnia and silica films is not affected by the ambient gas pressure. In vacuum, the multiple pulse LIDT drops to ~10% (~10%) of its atmospheric value for hafnia (silica). The water vapor content of the ambient gas was found to control the change in the LIDT. The LIDT of bulk fused silica surfaces did now show any dependence on the ambient gas pressure. Hydrocarbons (toluene) did not change the multiple pulse LIDT for Hafnia films

Femtosecond pulse damage thresholds of dielectric coatings in vacuum

“…This drop can be explained with the occupation of native and laser induced defects during the pulse train and their reexcitation by subsequent pulses [11,12,16]. Values of F(∞) for transparent materials are commonly 50-80% of the single-pulse value [11,12,17]. At 3x10 -7 Torr, the measured values of F(S) are the same as for 630 Torr for S < 300, but drop off dramatically for larger number of pulses.…”

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

“…[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.…”

“…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).…”

“…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.…”

“…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).…”