Influence of Na-related defects on DUV nonlinear absorption in CaF2: nanosecond versus femtosecond laser pulses

Ch. Mühlig; H. Stafast; W. Triebel; Th. Zeuner; Ch. Karras; M. Letz

doi:10.1117/12.836354

31 December 2009 Influence of Na-related defects on DUV nonlinear absorption in CaF₂: nanosecond versus femtosecond laser pulses

Ch. Mühlig, H. Stafast, W. Triebel, Th. Zeuner, Ch. Karras, M. Letz

Author Affiliations +

Proceedings Volume 7504, Laser-Induced Damage in Optical Materials: 2009; 75040I (2009) https://doi.org/10.1117/12.836354
Event: Laser Damage Symposium XLI: Annual Symposium on Optical Materials for High Power Lasers, 2009, Boulder, Colorado, United States

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 α^st(H) are proportional to H and rationalized by effective 1- and 2-photon absorption coefficients α^eff and β^eff, respectively. The α^eff and β^eff values increase with the Na content of the CaF₂ samples as identified by the fluorescence of Na related M_Na centers at 740 nm. This relation is simulated by a complex rate equation model describing the ArF laser induced M_Na generation and annealing. M_Na 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 F_Na to form M_Na centers. M_Na annealing occurs by its photo dissociation into a pair of F and F_Na centers. Comparative transmission measurements with DUV femtosecond pulses are done using the fourth harmonic of a Ti:Safs- laser at 197 nm. The resulting β_eff values virtually show no dependence on the MNa center concentration. Furthermore, the absolute β_eff 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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