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31 December 2009 Influence of Na-related defects on DUV nonlinear absorption in CaF2: nanosecond versus femtosecond laser pulses
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Abstract
The influence of Na stabilized F and M centers on the DUV absorption behavior of CaF2 is comparatively studied for nanosecond and femtosecond laser pulses by in-situ transmission and laser induced fluorescence measurements. For 193 nm nanosecond pulses the steady state transmission of ArF laser pulses through CaF2 is measured in dependence on the incident fluence H ≤ 10 mJ cm-2 pulse-1. The related absorption coefficients αst(H) are proportional to H and rationalized by effective 1- and 2-photon absorption coefficients αeff and βeff, respectively. The αeff and βeff values increase with the Na content of the CaF2 samples as identified by the fluorescence of Na related MNa centers at 740 nm. This relation is simulated by a complex rate equation model describing the ArF laser induced MNa generation and annealing. MNa generation starts with intrinsic 2-photon absorption in CaF2 yielding self-trapped excitons (STE). These pairs of F and H centers can separate upon thermal activation and the F centers combine with FNa to form MNa centers. MNa annealing occurs by its photo dissociation into a pair of F and FNa centers. Comparative transmission measurements with DUV femtosecond pulses are done using the fourth harmonic of a Ti:Safs- laser at 197 nm. The resulting βeff values virtually show no dependence on the MNa center concentration. Furthermore, the absolute βeff values are lower by a factor of three compared to those obtained for nanosecond pulses. This is explained by additional two-step absorption for nanosecond pulses after formation of self-trapped excitons (STE).
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Ch. Mühlig, H. Stafast, W. Triebel, Th. Zeuner, Ch. Karras, and M. Letz "Influence of Na-related defects on DUV nonlinear absorption in CaF2: nanosecond versus femtosecond laser pulses", Proc. SPIE 7504, Laser-Induced Damage in Optical Materials: 2009, 75040I (31 December 2009); https://doi.org/10.1117/12.836354
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