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Characterization of the laser induced damage threshold of mirrors in ...

Characterization of the laser induced damage threshold of mirrors in ...

Figure 4.3: Measurements

Figure 4.3: Measurements conducted on a 100nm thick gold layer deposited on a fused silica substrate. All lines represent one measurement, all five measurements have been done on the same sample. The damage sites have a distance from each other of 1mm. pulse duration is expected.[73, 74] Therefore we measured a damage threshold different by almost a factor of two from the literature value. Due to the same issues as in the fused silica measurement this can not be regarded as clear falsification of our measurements. We measured on an old and in parts damaged commercial Thorlabs PO1 protected silver mirror a damage threshold of approximately 0.24 J cm 2 . Layertec states a damage threshold of 0.38 J cm 2 for their fs-optimized silver mirrors for 150 fs pulses at a center wavelength of 800 nm.[75] During the evaluation of the silver mirror measurements we learnt that silver mirrors usually have a dielectric overcoating to prevent the oxidation of the silver surface. These overcoatings can be of different nature depending on manufacturer and application and may lead to different damage thresholds. We also don’t know the actual thickness of the silver layer, which is problematic as the damage threshold is layer thickness dependent.[38] So its unclear whether or not the two values are comparable at all. 4.3 Problems with the measurement In the beginning of chapter 2 we presented a measurement of a Boron Carbide coated fused silica substrate (figure 2.1), it shows an approximate damage threshold of 0.04 J cm2 . After taking this data, we did a set of measurements where we focused pulses with lower fluences compared to the before measured threshold fluence on the sample. After 34

we examined the irradiated spots with a Zeiss Axio Imager microscope we found alterations at fluences significantly lower than the threshold peak fluence. Figure 4.4 shows one of those pictures taken with the microscope. Figure 4.4: A below threshold irradiated site on a Boron Carbide single layer on fused silica substrate. The threshold fluence has been measured to be around 0.04 J cm2 . The position has been irradiated for about 10 minutes at the displayed fluence at 500 Hz. Similar measurements done on a single Silicon Carbide layer on fused silica substrate and on various dielectric mirrors showed the same result. We expect this effect to be caused by nonlinear color-center generation, which has been observed in wide bandgap materials and silicate glasses irradiated with femtosecond infrared pulses [55]. Further investigations on this matter need to be done. 35

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