The highest damage resis tance for oxides is obtained

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the nanosecond regime). The highest damage resis- tance for oxides is obtained for silica, and the value is the same for materials in thin film or bulk form. In the case of Al 2 O 3 , we also observe the same value for thin film and bulk form. For fluoride materials with high optical bandgaps (CaF 2 , MgF 2 ) a thresh- old 20% higher than silica is observed for bulk ma- terials (for the best samples). Such high values are not observed on the tested samples for fluoride thin film and progress are required to obtain film mate- rials with higher damage thresholds. 3.C. Binary mixtures Composite or mixture films can be produced by the simultaneous deposition of several materials with the main deposition technologies used in optical coating production [42]. These films have interest for their intrinsic properties and for the design of multilayer stack. Indeed by incorporating a small amount of a material in another one it is possi-
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7 ble to change the microstructural properties of the film and reduce the residual stress or the roughness (for instance SiO 2 in ZrO 2 [6, 38], SiO 2 in TiO 2 [39], SiO 2 in Gd 2 O 3 [40] or Ta 2 O 5 in SiO 2 [41]) or improve the optical homogeneity[42]. Other point of interests of such materials for high power fem- tosecond applications is the possibility to tune the optical constants and the ability to produce contin- uous spatial variations of the refractive index. With an adapted design based on an optimization of the electric field distribution, the use of mixture ma- terials can lead to subsequent enhancement of the LIDT of multilayer systems compared to the use of pure-material systems [13]. Such approaches are based on the precise knowledge of the damage be- havior of the films as a function of their composition and the ability to predict this behavior. The LIDT of mixture coatings in the femtosecond regime has however not been the object of extensive studies and we report on figures 8, 9 and 10 the results of different experimental studies on the laser damage resistance that we have conducted on binary mix- tures. Results are expressed as the laser damage resistance evolution as a function of the bandgap (Fig. 8), the material content i.e. percentage of low index material in the mixture (Fig. 9) and the refractive index (Fig. 10). Fig. 8. Internal LIDT of single layers of mixture mate- rials tested under single irradiation at 500 fs and 1030 nm, as a function of the measured absorption gaps. The labels (Nb 2 O 5 - SiO 2 for instance) are an indication of the composition of the film (co-deposition of silica and niobia) but the films could be off-stoechiometric. The dotted lines are guides for the eyes and they link the samples of the same set of deposition. The black line correspond to the tendency observed for simple materi- als (Eq. 3). By comparing the evolution of LIDT versus op- tical bandgap for the case of mixture coatings with the case of simple materials (Fig. 8) we can ob- serve a very different behavior. The linear evolu- tion is not followed by the mixture coatings and a more complex behavior is evidenced on this fig- ure. Starting from the high index material (Nb
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  • Fall '19
  • refractive index, bandgap, Ultrashort pulse, LIDTs

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