Influence of Na-related defects on ArF laser absorption in CaF2

Abstract

ArF laser pulse transmission through commercial high purity CaF2 is determined by measuring the energy of each pulse before and behind the sample up to an incident fluence H of 10 mJ/cm2. The steady state transmission of ArF laser pulses decreases with increasing fluence. The related absorption coefficients αst(H) are proportional to H and rationalized by effective 1- and 2-photon absorption coefficients 2.4×10−4 cm−1≤αeff≤16.8×10−4 cm−1 and 1.7×10−9 cm W−1≤βeff≤9.3×10−9 cm W−1, 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 rate equation model describing the ArF laser induced MNa generation in the dark periods between the laser pulses and their annealing during laser irradiation. MNa generation starts with intrinsic 2-photon absorption in CaF2, yielding self-trapped excitons (STE). These pairs of F and H centers move 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.