The measured properties of the samples refractive

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the measured properties of the samples (refractive indices, thickness, optical bandgap) and other data can be found in the appendix. A summary of the samples properties is given in figure 1, as a plot of refractive index versus optical bandgap. In the case of bulk materials the data can be fitted with the following expression (black line on figure 1): n 2 = 1 + 12 . 5 E g (1) with E g the bandgap in eV.This expression that is
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3 Fig. 1. Refractive indices at 1030nm plotted as a func- tion of optical bandgap values for the different dielectric and semi-conductor samples under study. Data have been obtained from spectrophometric measurements. The black line is a fit of bulk data. The dotted gray line is a plot of the Moss relation [18] and the gray line is a plot of the Herve and Vandamme relation [20] . obtained for a large range of bandgap values can be compared to other gap-refractive index relations based on simple physical models [19]. For instance the relationship proposed by Moss for the case of semi-conductor material [18] is n 4 E g = 95 eV . When applied to our measurements, the evolution of refractive index versus bandgap predicted by the Moss rule (gray dotted line on Fig.1) compares well to our experiments up to 5eV and then over- estimates the data for higher bandgap. At the opposite the model of Herve and Vandamme [20] ( n 2 = 1+13 . 6 / ( Eg +3 . 47)) is accurate for high band gap materials and shows strong deviations with ex- periments for semi-conductors. In the intermediate range between semi-conductors and high bandgap materials (fluorides) there is some dispersion of re- fractive index values, particularly for mixture ma- terials, and the two previously cited relations can be considered as the low and high boundary limits. 2.B. Measurements of single-shot laser damage resistance The single shot laser damage resistance of the sam- ples has been measured at 1030nm with a commer- cial diode-pumped ytterbium amplified laser oper- ating in the sub-picosecond regime ( 500fs) with the experiment and procedures described in details in Ref. [7]. All the samples have been tested at nor- mal incidence, with the surface to be tested facing the incoming beam (front face testing), in linear polarization with a beam diameter in the range of 50 - 60 μm at 1/e depending on the test campaign. The oscillator, amplifier and compressor of the laser source have been realigned and optimized several times during the acquisition of the data reported in this paper. This implies that the measurements were not done in exactly the same conditions. Par- ticularly, the pulse duration and spot size were not the same for all the tested samples. The effective spot size as defined in the ISO standard [21] being used for the fluence determination, the spot size variations are implicitly taken into account. The pulse duration however has varied between 375fs and 600fs and a correction of the results is neces- sary to analyze the results. Therefore in order to compare the results on the same basis (arbitrary set to 500fs), we applied the scaling law determined by Mero et al. in Ref. [3]:
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  • Fall '19
  • refractive index, bandgap, Ultrashort pulse, LIDTs

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