02 81 zro 2 sio 2 45750 850 089003 43 hfo 2 sio 2 hr

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0.02 81 ZrO 2 SiO 2 45 750 850 0.89 0.03 43 HfO 2 SiO 2 HR 45 750 850 1.46 0.04 42 HfO 2 SiO 2 AR 45 380 420/ 750 850 1.99 0.05 48 a All of these optical components were manufactured by magnetron sputtering except for the ZrO 2 SiO 2 mirror, which was evaporated with ion assistance. All of the 45° AOI mirrors were suitable for s and p polarization with the experiments being performed in p polarization. Research Article Vol. 57, No. 2 / January 10 2018 / Applied Optics 341
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case of a 10-shot experiment than with 100,000 shots for most samples, with 20% difference being the highest value. The dependence on the number of pulses is always observable re- gardless of the material of the mirrors in this study. The error on each measurement point was derived from the error caused by the measured spot size and intensity fluctuations, and this error proved to be higher than the statistical error of five independent measurements on the sample in every case. Thereby, we found a similar tendency in damage behavior to previous experiments performed on single layers [ 6 ]. Even though the difference be- tween 10-shot and 100,000-shot damage is higher in Ref. [ 6 ] (some 30%) than in our case, the completely different damage definition (based on crater size) and experimental conditions and samples can account for this difference. Subsequently, we also plotted the damage thresholds in the case of 10 and 100,000 shots in Fig. 4 (full and open symbols) as a function of the maximum design field strength, i.e., the maximum standing wave field in the multilayer stack. We define the maximum field strength as the maximum standing- wave electric field occurring inside any of the high-index layers (with its value compared to the incident electric field). In our case, both optimized structures with TiO 2 SiO 2 stacks per- form much better than an industrial baseline TiO 2 SiO 2 HR without electric field optimization. Previously, we found that the damage threshold of such a HR is significantly lower than that of the current ones, being only 0.28 J cm 2 for the same test conditions [ 11 ]. The improvement can be attributed both to mirror design and advanced coating techniques. Figure 4 additionally shows that even though proper design and manufacturing can increase LIDT significantly, an even higher improvement is expected from the usage of high-bandgap materials, especially HfO 2 . If we compare mirrors involving HfO 2 , it can be seen that a pure dielectric mirror with 42 layers has a significantly higher LIDT than a metal dielectric hybrid one (having nine layers). This can be partly attributed to the lower maximum standing- wave field in the stack (Fig. 4 ). On the other hand, the hybrid Fig. 3. Dependence of LIDT on the number of laser pulses illumi- nating the mirror for eight different femtosecond laser mirrors consist- ing of different materials and design approaches. The material of the different mirrors is color-coded.
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  • Fall '19
  • APPLIED OPTICS, SPIE, Ultrashort pulse

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